Change search
Refine search result
12345 101 - 150 of 236
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 101.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andreassen, Karin
    Bjarnadóttir, Lilja Rün
    Dove, Dayton
    Dowdeswell, Julian A.
    England, John H.
    Funder, Svend
    Hogan, Kelly
    Ingólfsson, Ólafur
    Jennings, Anne
    Krog Larsen, Nikolaj
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Landvik, Jon Y.
    Mayer, Larry
    Mikkelsen, Naja
    Möller, Per
    Niessen, Frank
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    O'Regan, Matthew
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Polyak, Leonid
    Nørgaard-Pedersen, Niels
    Stein, Ruediger
    Arctic Ocean glacial history2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 92, p. 40-67Article in journal (Refereed)
    Abstract [en]

    While there are numerous hypotheses concerning glacial interglacial environmental and climatic regime shifts in the Arctic Ocean, a holistic view on the Northern Hemisphere's late Quaternary ice-sheet extent and their impact on ocean and sea-ice dynamics remains to be established. Here we aim to provide a step in this direction by presenting an overview of Arctic Ocean glacial history, based on the present state-of-the-art knowledge gained from field work and chronological studies, and with a specific focus on ice-sheet extent and environmental conditions during the Last Glacial Maximum (LGM). The maximum Quaternary extension of ice sheets is discussed and compared to LGM. We bring together recent results from the circum-Arctic continental margins and the deep central basin; extent of ice sheets and ice streams bordering the Arctic Ocean as well as evidence for ice shelves extending into the central deep basin. Discrepancies between new results and published LGM ice-sheet reconstructions in the high Arctic are highlighted and outstanding questions are identified. Finally, we address the ability to simulate the Arctic Ocean ice sheet complexes and their dynamics, including ice streams and ice shelves, using presently available ice-sheet models. Our review shows that while we are able to firmly reject some of the earlier hypotheses formulated to describe Arctic Ocean glacial conditions, we still lack information from key areas to compile the holistic Arctic Ocean glacial history.

  • 102. Jakobsson, Martin
    et al.
    Ingolfsson, Olafur
    Long, Antony J.
    Spielhagen, Robert F.
    The dynamic Arctic Introduction2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 92, no SI, p. 1-8Article in journal (Refereed)
    Abstract [en]

    Research campaigns over the last decade have yielded a growing stream of data that highlight the dynamic nature of Arctic cryosphere and climate change over a range of time scales. As a consequence, rather than seeing the Arctic as a near static environment in which large scale changes occur slowly, we now view the Arctic as a system that is typified by frequent, large and abrupt changes. The traditional focus on end members in the system - glacial versus interglacial periods - has been replaced by a new interest in understanding the patterns and causes of such dynamic change. Instead of interpreting changes almost exclusively as near linear responses to external forcing (e.g. orbitally-forced climate change), research is now concentrated on the importance of strong feedback mechanisms that in our palaeo-archives often border on chaotic behaviour. The last decade of research has revealed the importance of on-off switching of ice streams, strong feedbacks between sea level and ice sheets, spatial and temporal changes in ice shelves and perennial sea ice, as well as alterations in ice sheet dynamics caused by shifting centres of mass in multi-dome ice sheets. Recent advances in dating techniques and modelling have improved our understanding of leads and lags that exist in different Arctic systems, on their interactions and the driving mechanisms of change. Future Arctic research challenges include further emphases on rapid transitions and untangling the feedback mechanisms as well as the time scales they operate on. (C) 2014 The Authors. Published by Elsevier Ltd.

  • 103. Jakobsson, Martin
    et al.
    Ingólfsson, Ólafur
    Long, Antony J.
    Spielhagen, Robert F.
    The dynamic Arctic2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 92, p. 1-8Article in journal (Refereed)
    Abstract [en]

    Abstract Research campaigns over the last decade have yielded a growing stream of data that highlight the dynamic nature of Arctic cryosphere and climate change over a range of time scales. As a consequence, rather than seeing the Arctic as a near static environment in which large scale changes occur slowly, we now view the Arctic as a system that is typified by frequent, large and abrupt changes. The traditional focus on end members in the system – glacial versus interglacial periods – has been replaced by a new interest in understanding the patterns and causes of such dynamic change. Instead of interpreting changes almost exclusively as near linear responses to external forcing (e.g. orbitally-forced climate change), research is now concentrated on the importance of strong feedback mechanisms that in our palaeo-archives often border on chaotic behaviour. The last decade of research has revealed the importance of on-off switching of ice streams, strong feedbacks between sea level and ice sheets, spatial and temporal changes in ice shelves and perennial sea ice, as well as alterations in ice sheet dynamics caused by shifting centres of mass in multi-dome ice sheets. Recent advances in dating techniques and modelling have improved our understanding of leads and lags that exist in different Arctic systems, on their interactions and the driving mechanisms of change. Future Arctic research challenges include further emphases on rapid transitions and untangling the feedback mechanisms as well as the time scales they operate on.

  • 104.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Long, A.
    Ingólfsson, Ó.
    Kjaer, K. H.
    Spielhagen, R. F.
    New insights on Arctic Quaternary climate variability from palaeo-records and numerical modelling2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 25-26, p. 3349-3358Article in journal (Refereed)
    Abstract [en]

    Terrestrial and marine geological archives in the Arctic contain information on environmental change through Quaternary interglacial–glacial cycles. The Arctic Palaeoclimate and its Extremes (APEX) scientific network aims to better understand the magnitude and frequency of past Arctic climate variability, with focus on the “extreme” versus the “normal” conditions of the climate system. One important motivation for studying the amplitude of past natural environmental changes in the Arctic is to better understand the role of this region in a global perspective and provide base-line conditions against which to explore potential future changes in Arctic climate under scenarios of global warming. In this review we identify several areas that are distinct to the present programme and highlight some recent advances presented in this special issue concerning Arctic palaeo-records and natural variability, including spatial and temporal variability of the Greenland Ice Sheet, Arctic Ocean sediment stratigraphy, past ice shelves and marginal marine ice sheets, and the Cenozoic history of Arctic Ocean sea ice in general and Holocene oscillations in sea ice concentrations in particular. The combined sea ice data suggest that the seasonal Arctic sea ice cover was strongly reduced during most of the early Holocene and there appear to have been periods of ice free summers in the central Arctic Ocean. This has important consequences for our understanding of the recent trend of declining sea ice, and calls for further research on causal links between Arctic climate and sea ice.

  • 105.
    Jakobsson, Martin
    et al.
    Stockholm Univ, Dept Geol Sci, S-10691 Stockholm, Sweden..
    Long, Antony
    Univ Durham, Dept Geog, Durham DH1 3LE, England..
    Ingolfsson, Olafur
    Univ Iceland, Fac Earth Sci, IS-101 Reykjavik, Iceland..
    Kjaer, Kurt H.
    Univ Copenhagen, Nat Hist Museum, Ctr GeoGenet, DK-1350 Copenhagen, Denmark..
    Spielhagen, Robert F.
    IFM GEOMAR, Leihniz Inst Marine Sci, D-24148 Kiel, Germany.;Acad Sci Humanities & Literature, Mainz, Germany..
    New insights on Arctic Quaternary climate variability from palaeo-records and numerical modelling2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 25-26, p. 3349-3358Article in journal (Refereed)
    Abstract [en]

    Terrestrial and marine geological archives in the Arctic contain information on environmental change through Quaternary interglacial-glacial cycles. The Arctic Palaeoclimate and its Extremes (APEX) scientific network aims to better understand the magnitude and frequency of past Arctic climate variability, with focus on the "extreme" versus the "normal" conditions of the climate system. One important motivation for studying the amplitude of past natural environmental changes in the Arctic is to better understand the role of this region in a global perspective and provide base-line conditions against which to explore potential future changes in Arctic climate under scenarios of global warming. In this review we identify several areas that are distinct to the present programme and highlight some recent advances presented in this special issue concerning Arctic palaeo-records and natural variability, including spatial and temporal variability of the Greenland Ice Sheet, Arctic Ocean sediment stratigraphy, past ice shelves and marginal marine ice sheets, and the Cenozoic history of Arctic Ocean sea ice in general and Holocene oscillations in sea ice concentrations in particular. The combined sea ice data suggest that the seasonal Arctic sea ice cover was strongly reduced during most of the early Holocene and there appear to have been periods of ice free summers in the central Arctic Ocean. This has important consequences for our understanding of the recent trend of declining sea ice, and calls for further research on causal links between Arctic climate and sea ice. (C) 2010 Elsevier Ltd. All rights reserved.

  • 106.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Löwemark, Ludvig
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dowdeswell, J. A.
    Mayer, L.
    Polyak, L.
    Colleoni, Florence
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Anderson, L. G.
    Björk, G.
    Darby, D.
    Eriksson, Björn
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hanslik, Daniela
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hell, Benjamin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Marcussen, C.
    Sellén, Emma
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wallin, T.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    An Arctic Ocean ice shelf during MIS 6 constrained by new geophysical and geological data2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 25-26, p. 3505-3517Article in journal (Refereed)
    Abstract [en]

    The hypothesis of floating ice shelves covering the Arctic Ocean during glacial periods was developed in the 1970s. In its most extreme form, this theory involved a 1000 m thick continuous ice shelf covering the Arctic Ocean during Quaternary glacial maxima including the Last Glacial Maximum (LGM). While recent observations clearly demonstrate deep ice grounding events in the central Arctic Ocean, the ice shelf hypothesis has been difficult to evaluate due to a lack of information from key areas with severe sea ice conditions. Here we present new data from previously inaccessible, unmapped areas that constrain the spatial extent and timing of marine ice sheets during past glacials. These data include multibeam swath bathymetry and subbottom profiles portraying glaciogenic features on the Chukchi Borderland, southern Lomonosov Ridge north of Greenland, Morris Jesup Rise, and Yermak Plateau. Sediment cores from the mapped areas provide age constraints on the glaciogenic features. Combining these new geophysical and geological data with earlier results suggests that an especially extensive marine ice sheet complex, including an ice shelf, existed in the Amerasian Arctic Ocean during Marine Isotope Stage (MIS) 6. From a conceptual oceanographic model we speculate that the cold halocline of the Polar Surface Water may have extended to deeper water depths during MIS 6 inhibiting the warm Atlantic water from reaching the Amerasian Arctic Ocean and, thus, creating favorable conditions for ice shelf development. The hypothesis of a continuous 1000 m thick ice shelf is rejected because our mapping results show that several areas in the central Arctic Ocean substantially shallower than 1000 m water depth are free from glacial influence on the seafloor.

  • 107. Jennings, Anne
    et al.
    Andrews, John
    Pearce, Christof
    Aarhus University, Denmark.
    Wilson, Lindsay
    Ólfasdótttir, Sædís
    Detrital carbonate peaks on the Labrador shelf, a 13–7 ka template for freshwater forcing from the Hudson Strait outlet of the Laurentide Ice Sheet into the subpolar gyre2015In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 107, p. 62-80Article in journal (Refereed)
    Abstract [en]

    The Laurentide Ice Sheet (LIS) was a large, dynamic ice sheet in the early Holocene. The glacial events through Hudson Strait leading to its eventual demise are recorded in the well-dated Labrador shelf core, MD99-2236 from the Cartwright Saddle. We develop a detailed history of the timing of ice-sheet discharge events from the Hudson Strait outlet of the LIS during the Holocene using high-resolution detrital carbonate, ice rafted detritus (IRD), ή18O, and sediment color data. Eight detrital carbonate peaks (DCPs) associated with IRD peaks and light oxygen isotope events punctuate the MD99-2236 record between 11.5 and 8.0 ka. We use the stratigraphy of the DCPs developed from MD99-2236 to select the appropriate ΔR to calibrate the ages of recorded glacial events in Hudson Bay and Hudson Strait such that they match the DCPs in MD99-2236. We associate the eight DCPs with H0, Gold Cove advance, Noble Inlet advance, initial retreat of the Hudson Strait ice stream (HSIS) from Hudson Strait, opening of the Tyrrell Sea, and drainage of glacial lakes Agassiz and Ojibway. The opening of Foxe Channel and retreat of glacial ice from Foxe Basin are represented by a shoulder in the carbonate data. ΔR of 350 years applied to the radiocarbon ages constraining glacial events H0 through the opening of the Tyrell Sea provided the best match with the MD99-2236 DCPs; ΔR values and ages from the literature are used for the younger events. A very close age match was achieved between the 8.2 ka cold event in the Greenland ice cores, DCP7 (8.15 ka BP), and the drainage of glacial lakes Agassiz and Ojibway. Our stratigraphic comparison between the DCPs in MD99-2236 and the calibrated ages of Hudson Strait/Bay deglacial events shows that the retreat of the HSIS, the opening of the Tyrell Sea, and the catastrophic drainage of glacial lakes Agassiz and Ojibway at 8.2 ka are separate events that have been combined in previous estimates of the timing of the 8.2 ka event from marine records. SW Iceland shelf core MD99-2256 documents freshwater entrainment into the subpolar gyre from the Hudson Strait outlet via the Labrador, North Atlantic, and Irminger currents. The timing of freshwater release from the LIS Hudson Strait outlet in MD99-2236 matches evidence for freshwater forcing and LIS icebergs carrying foreign minerals to the SW Iceland shelf between 11.5 and 8.2 ka. The congruency of these records supports the conclusion of the entrainment of freshwater from the retreat of the LIS through Hudson Strait into the subpolar gyre and provides specific time periods when pulses of LIS freshwater were present to influence climate.

  • 108.
    Johnsen, Timothy F.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Olsen, Lars
    Murray, Andrew
    Osl ages in central Norway support a mis 2 interstadial (25 20 ka) and a dynamic Scandinavian ice sheet2012In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 44, p. 96-111Article in journal (Refereed)
    Abstract [en]

    Recent work has suggested that the Scandinavian ice sheet was much more dynamic than previously believed, and its western marine-based margin can provide an analogue to the rapid-paced fluctuations and deglaciation observed at the margins of the Antarctic and Greenland ice sheets. In this study we used a complimentary dating technique, OSL (Optically Stimulated Luminescence dating), to support the existence of the Trofors interstadial in central Norway; an ice-free period that existed from similar to 25 to 20 ka recorded at multiple sites throughout Norway (cf. Andoya interstadial) and that divides the Last Glacial Maximum (LGM) into two stadials. OSL signal component analysis was used to optimize data analysis, and internal (methodological) tests show the results to be of good quality. Both large and small aliquots gave consistent OSL ages (22.3 +/- 1.7 ka, n = 7) for sub-till glaciofluvial/fluvial sediments at the Langsmoen stratigraphic site, and an apparent old age (similar to 100 ka) for a poorly-bleached sample of glaciolacustrine sediment at the nearby stratigraphically-related Flora site. Eight radiocarbon ages of sediment from the Flora site gave consistent ages (20.9 +/- 1.6 cal ka BP) that overlap within 1 sigma with OSL ages from the nearby Langsmoen site. The similarity in age within and between these stratigraphically-related sites and using different geochronological techniques strongly suggests that this area was ice-free around similar to 21 or 22 ka. The existence of the Trofors interstadial along with other interstadials during the Middle and Late Weichselian (MIS 3 and MIS 2) indicates that not only the western margin, but the whole western part of the Scandinavian ice sheet, from the ice divide to the ice margin was very dynamic. These large changes in the ice margin and accompanying drawdown of the ice surface would have affected the eastern part of the ice sheet as well.

  • 109.
    Johnsen, Timothy
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Olsen, Lars
    Geological Survey of Norway, Trondheim.
    Murray, Andrew
    Nordic Laboratory for Luminescence Dating, Aarhus University, Risø National Laboratory.
    OSL ages in central Norway confirm a MIS 2 interstadial (25-20 ka) and a dynamic Scandinavian ice sheetIn: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457XArticle in journal (Other academic)
    Abstract [en]

    Recent work has suggested that the Scandinavian ice sheet was much more dynamic than previously believed, and its western marine-based margin can provide an analogue to the rapid-paced fluctuations and deglaciation observed at the margins of the Antarctic and Greenland ice sheets.

    In this study we used a complimentary dating technique, OSL (Optically Stimulated Luminescence dating), to confirm the existence of the Trofors interstadial in central Norway; an ice-free period that existed from ~25 to 20 ka recorded at multiple sites throughout Norway (cf. Andøya interstadial) and that divides the Last Glacial Maximum (LGM) into two stadials. OSL signal component analysis was used to optimize data analysis, and internal (methodological) tests show the results to be of good quality. Both large and small aliquots gave consistent OSL ages (22.3 ±1.7 ka, n = 7) for sub-till glaciofluvial/fluvial sediments at the Langsmoen stratigraphic site, and an apparent old age (~100 ka) for a poorly bleached sample of glaciolacustrine sediment at the nearby stratigraphically-related Flora site. Eight radiocarbon ages of sediment from the Flora site gave consistent ages (20.9 ±1.6 cal. ka BP) that overlap within 1σ with OSL ages from the nearby Langsmoen site. The similarity in age within and between these stratigraphically-related sites and using different geochronological techniques strongly suggests that this area was ice-free around ~21 or 22 ka. The existence of the Trofors interstadial along with other interstadials during the Middle and Late Weichselian (MIS 3 and MIS 2) indicates that not only the western margin, but the whole western part of the Scandinavian ice sheet, from the ice divide to the ice margin was very dynamic. These large changes in the ice margin and accompanying drawdown of the ice surface would have affected the eastern part of the ice sheet as well.

  • 110.
    Katrantsiotis, Christos
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. Navarino Environmental Observatory (NEO), Greece.
    Kylander, Malin E.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Smittenberg, Rienk
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Yamoah, Kweku K. A.
    Hättestrand, Martina
    Stockholm University, Faculty of Science, Department of Physical Geography. Navarino Environmental Observatory (NEO), Greece.
    Avramidis, Pavlos
    Strandberg, Nichola A.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences. Navarino Environmental Observatory (NEO), Greece.
    Eastern Mediterranean hydroclimate reconstruction over the last 3600 years based on sedimentary n-alkanes, their carbon and hydrogen isotope composition and XRF data from the Gialova Lagoon, SW Greece2018In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 194, p. 77-93Article in journal (Refereed)
    Abstract [en]

    Understanding past hydroclimate variability and related drivers is essential to improve climate forecasting capabilities especially in areas with high climatic sensitivity, such as the Mediterranean. This can be achieved by using a broad spectrum of high resolution, multiple proxy records which can also allow us to assess linkages between regional hydroclimate variability and shifts in the large-scale atmospheric patterns. Here, we present a multiproxy reconstruction of the central-eastern Mediterranean hydro climate changes over the last 3600 years based on a sediment core from the Gialova Lagoon, a shallow coastal ecosystem in SW Peloponnese, Greece. Our combined dataset consists of the distribution and compound-specific carbon and hydrogen isotope (delta C-13 and 8D) composition of n-alkanes, bulk organic matter properties and X-ray fluorescence (XRF) core scanning data. This approach was complemented with a semi-quantitative analysis of plant remains in the core. The results indicate a high contribution of local aquatic vegetation to organic matter. Large delta C-13 variations in predominantly aquatic plant-derived mid-chain alkanes (C23-23) mainly reflect changes in the aquatic plant abundance and their carbon source. Our data suggest that higher delta C-13(23-25) values (up to 19 parts per thousand) largely correspond to expansion of aquatic vegetation during wet and/or cold periods causing carbon-limiting conditions in the water and assimilation of isotopically-enriched bicarbonate by the plants. The 8D records of the individual n-alkanes (C-17 to C-31) exhibit a nearly identical pattern to each other, which implies that they all reflect changes in the source water isotope composition, driven by hydroclimate variability. In addition, the 8D profiles are consistent with the XRF data with both proxies being driven by a common hydroclimate signal. We observe two major shifts from dry and/or warm periods at ca 3600-3000 cal BP and ca 17001300 cal BP to wet and/or cold episodes at ca 3000-2700 cal BP and ca 1300-900 cal BP. The period ca 700-200 cal BP is the wettest and/or coldest in our record and coeval with the Little Ice Age. The climatic fluctuation reported in this study can be explained by the relative dominance of high-latitude (e.g. North Atlantic Oscillation during winters) and the low-latitude atmospheric patterns (Intertropical convergence zone, Subtropical High and the effects of Asian monsoons during summers) which suggests an Atlantic-Mediterranean-Monsoon climate link in this area for the late Holocene.

  • 111. Kaufmann, Patrik
    et al.
    Fundel, Felix
    Fischer, Hubertus
    Bigler, Matthias
    Ruth, Urs
    Udisti, Roberto
    Hansson, Margareta
    de Angelis, Martine
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Barbante, Carlo
    Wolff, Eric W.
    Hutterli, Manuel
    Wagenbach, Dietmar
    Ammonium and non-sea salt sulfate in the EPICA ice cores as indicator of biological activity in the Southern Ocean2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 02-jan, p. 313-323Article in journal (Refereed)
    Abstract [en]

    Sulfate (SO42-) and ammonium (NH4+) flux records over the last 150,000 years from both Antarctic EPICA ice cores (European Project for Ice Coring in Antarctica) are presented. The ice core record from Dome C is influenced by the Indian sector of the Southern Ocean (SO), whereas Dronning Maud Land is facing the Atlantic sector. Generally, they reflect the past atmospheric aerosol load and, thus, potentially reveal the fingerprint of marine biogenic sources from the SO. The most important feature of both, the nssSO(4)(2-) as well as NH4+ flux records, is the absence of any significant glacial cycles, in contrary to the distinct transitions for mineral dust and sea salt aerosol over the last 150,000 years. This finding challenges the iron fertilization hypothesis on long time scales, as the significant changes in dust, e.g. from the last glacial maximum toward the Holocene have neither an impact on nssSO(4)(2-) nor on NH4+ fluxes found in interior Antarctica. The inter-site correlation of both species is weak, r(2) = 0.42 for the nssSO(4)(2-) flux and r(2) = 0.12 for the NH4+ flux respectively, emphasizing the local Source characteristics of biogenic aerosol from the SO. Millennial variability in NH4+ and nssSO(4)(2-) is within the uncertainty of our flux estimates. Correlation with mineral dust and sea ice derived sodium shows only a very weak influence of dust deposition on those insignificant changes in nssSO(4)(2-) flux for the Atlantic sector of the Southern Ocean, but also small transport changes or terrigeneous sulfate contributions may contribute to those variations at EDML.

  • 112.
    Kirchner, N.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Hutter, Kolumban
    Swiss Federal Institute of Technology.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Capabilities and limitations of numerical ice sheet models: a discussion for Earth-scientists and modelers2011In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 30, no 25-26, p. 3691-3704Article in journal (Refereed)
    Abstract [en]

    The simulation of dynamically coupled ice sheet, ice stream, and ice shelf-systems poses a challenge to most numerical ice sheet models. Here we review present ice sheet model limitations targeting a broader audience within Earth Sciences, also those with no specific background in numerical modeling, in order to facilitate cross-disciplinary communication between especially paleoglaciologists, marine and terrestrial geologists, and numerical modelers. The ‘zero order’(Shallow Ice Approximation, SIA)-,‘higher order’-, and‘full Stokes’ice sheet models are described conceptually and complemented by an outline of their derivations. We demonstrate that higher order models are required to simulate coupled ice sheetice shelf and ice sheet-ice stream systems, in particular if the results are aimed to complement spatial ice flow reconstructions based on higher resolution geological and geophysical data. The zero order SIA model limitations in capturing ice stream behavior are here illustrated by conceptual simulations of a glaciation on Svalbard. The limitations are obvious from the equations comprising a zero order model. However, under certain circumstances, simulation results may falsely give the impression that ice streams indeed are simulated with a zero order SIA model.

  • 113.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Ahlkrona, J.
    Gowan, Evan J.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Lötstedt, P.
    Lea, James M.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Noormets, R.
    von Sydow, L.
    Dowdeswell, J. A.
    Benham, T.
    Shallow ice approximation, second order shallow ice approximation, and full Stokes models: A discussion of their roles in palaeo-ice sheet modelling and development2016In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 135, p. 103-114Article, review/survey (Refereed)
    Abstract [en]

    Full Stokes ice sheet models provide the most accurate description of ice sheet flow, and can therefore be used to reduce existing uncertainties in predicting the contribution of ice sheets to future sea level rise on centennial time-scales. The level of accuracy at which millennial time-scale palaeo-ice sheet simulations resolve ice sheet flow lags the standards set by Full Stokes models, especially, when Shallow Ice Approximation (SIA) models are used. Most models used in paleo-ice sheet modeling were developed at a time when computer power was very limited, and rely on several assumptions. At the time there was no means of verifying the assumptions by other than mathematical arguments. However, with the computer power and refined Full Stokes models available today, it is possible to test these assumptions numerically. In this paper, we review (Ahlkrona et al., 2013a) where such tests were performed and inaccuracies in commonly used arguments were found. We also summarize (Ahlkrona et al., 2013b) where the implications of the inaccurate assumptions are analyzed for two paleo-models - the SIA and the SOSIA. We review these works without resorting to mathematical detail, in order to make them accessible to a wider audience with a general interest in palaeo-ice sheet modelling. Specifically, we discuss two implications of relevance for palaeo-ice sheet modelling. First, classical SIA models are less accurate than assumed in their original derivation. Secondly, and contrary to previous recommendations, the SOSIA model is ruled out as a practicable tool for palaeo-ice sheet simulations. We conclude with an outlook concerning the new Ice Sheet Coupled Approximation Level (ISCAL) method presented in Ahlkrona et al. (2016), that has the potential to match the accuracy standards of full Stokes model on palaeo-timescales of tens of thousands of years, and to become an alternative to hybrid models currently used in palaeo-ice sheet modelling. The method is applied to an ice sheet covering Svalbard.

  • 114.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Ahlkrona, Josefin
    Gowan, Evan J.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Lötstedt, P.
    Lea, James M.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Noormets, R.
    von Sydow, L.
    Dowdeswell, J. A.
    Benham, T.
    Shallow ice approximation, second order shallow ice approximation, and full Stokes models: A discussion of their roles in palaeo-ice sheet modelling and development2016In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 147, p. 136-147Article in journal (Refereed)
    Abstract [en]

    Full Stokes ice sheet models provide the most accurate description of ice sheet flow, and can therefore be used to reduce existing uncertainties in predicting the contribution of ice sheets to future sea level rise on centennial time-scales. The level of accuracy at which millennial time-scale palaeo-ice sheet simulations resolve ice sheet flow lags the standards set by Full Stokes models, especially, when Shallow Ice Approximation (SIA) models are used. Most models used in paleo-ice sheet modeling were developed at a time when computer power was very limited, and rely on several assumptions. At the time there was no means of verifying the assumptions by other than mathematical arguments. However, with the computer power and refined Full Stokes models available today, it is possible to test these assumptions numerically. In this paper, we review (Ahlkrona et al., 2013a) where such tests were performed and inaccuracies in commonly used arguments were found. We also summarize (Ahlkrona et al., 2013b) where the implications of the inaccurate assumptions are analyzed for two paleo-models - the SIA and the SOSIA. We review these works without resorting to mathematical detail, in order to make them accessible to a wider audience with a general interest in palaeo-ice sheet modelling. Specifically, we discuss two implications of relevance for palaeo-ice sheet modelling. First, classical SIA models are less accurate than assumed in their original derivation. Secondly, and contrary to previous recommendations, the SOSIA model is ruled out as a practicable tool for palaeo-ice sheet simulations. We conclude with an outlook concerning the new Ice Sheet Coupled Approximation Level (ISCAL) method presented in Ahlkrona et al. (2016), that has the potential to match the accuracy standards of full Stokes model on palaeo-timescales of tens of thousands of years, and to become an alternative to hybrid models currently used in palaeo-ice sheet modelling. The method is applied to an ice sheet covering Svalbard.

  • 115. Kirchner, Nina
    et al.
    Ahlkrona, Josefin
    Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Division of Scientific Computing. Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Numerical Analysis.
    Gowan, Evan J.
    Lötstedt, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Division of Scientific Computing. Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Numerical Analysis.
    Lea, James M.
    Noormets, Riko
    von Sydow, Lina
    Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Division of Scientific Computing. Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Numerical Analysis.
    Dowdeswell, Julian A.
    Benham, Toby
    Shallow ice approximation, second order shallow ice approximation, and full Stokes models: A discussion of their roles in palaeo-ice sheet modelling and development2016In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 135, p. 103-114Article, review/survey (Refereed)
  • 116.
    Kirchner, Nina
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Greve, Ralf
    Stroeven, Arjen P.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Heyman, Jakob
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Paleoglaciological reconstructions for the Tibetan Plateau during the last glacial cycle: evaluating numerical ice sheet simulations driven by GCM-ensembles2011In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 30, no 1-2, p. 248-267Article in journal (Refereed)
    Abstract [en]

    The Tibetan Plateau is a topographic feature of extraordinary dimension and has an important impact on regional and global climate. However, the glacial history of the Tibetan Plateau is more poorly constrained than that of most other formerly glaciated regions such as in North America and Eurasia. On the basis of some field evidence it has been hypothesized that the Tibetan Plateau was covered by an ice sheet during the Last Glacial Maximum (LGM). Abundant field- and chronological evidence for a predominance of local valley glaciation during the past 300,000 calendar years (that is, 300 kyr), coupled to an absence of glacial landforms and sediments in extensive areas of the plateau, now refute this concept. This, furthermore, calls into question previous ice sheet modeling attempts which generally arrive at ice volumes considerably larger than allowed for by field evidence. Surprisingly, the robustness of such numerical ice sheet model results has not been widely queried, despite potentially important climate ramifications. We simulated the growth and decay of ice on the Tibetan Plateau during the last 125 kyr in response to a large ensemble of climate forcings (90 members) derived from Global Circulation Models (GCMs), using a similar 3D thermomechanical ice sheet model as employed in previous studies. The numerical results include as extreme end members as an ice free Tibetan Plateau and a plateau-scale ice sheet comparable, in volume, to the contemporary Greenland ice sheet. We further demonstrate that numerical simulations that acceptably conform to published reconstructions of Quaternary ice extent on the Tibetan Plateau cannot be achieved with the employed stand-alone ice sheet model when merely forced by paleoclimates derived from currently available GCMs. Progress is, however, expected if future investigations employ ice sheet models with higher resolution, bidirectional ice sheet-atmosphere feedbacks, improved treatment of the surface mass balance, and regional climate data and climate reconstructions.

  • 117.
    Kirchner, Nina
    et al.
    Stockholm Univ, Dept Phys Geog & Quaternary Geol, S-10691 Stockholm, Sweden..
    Hutter, Kolumban
    Swiss Fed Inst Technol, Lab Hydraul Hydrol & Glaciol, CH-8092 Zurich, Switzerland..
    Jakobsson, Martin
    Stockholm Univ, Dept Geol Sci, S-10691 Stockholm, Sweden..
    Gyllencreutz, Richard
    Stockholm Univ, Dept Geol Sci, S-10691 Stockholm, Sweden..
    Capabilities and limitations of numerical ice sheet models: a discussion for Earth-scientists and modelers2011In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 30, no 25-26, p. 3691-3704Article in journal (Refereed)
    Abstract [en]

    The simulation of dynamically coupled ice sheet, ice stream, and ice shelf-systems poses a challenge to most numerical ice sheet models. Here we review present ice sheet model limitations targeting a broader audience within Earth Sciences, also those with no specific background in numerical modeling, in order to facilitate cross-disciplinary communication between especially paleoglaciologists, marine and terrestrial geologists, and numerical modelers. The 'zero order' (Shallow Ice Approximation, SIA)-, 'higher order'-, and 'full Stokes' ice sheet models are described conceptually and complemented by an outline of their derivations. We demonstrate that higher order models are required to simulate coupled ice sheet-ice shelf and ice sheet-ice stream systems, in particular if the results are aimed to complement spatial ice flow reconstructions based on higher resolution geological and geophysical data. The zero order SIA model limitations in capturing ice stream behavior are here illustrated by conceptual simulations of a glaciation on Svalbard. The limitations are obvious from the equations comprising a zero order model. However, under certain circumstances, simulation results may falsely give the impression that ice streams indeed are simulated with a zero order SIA model. (C) 2011 Elsevier Ltd. All rights reserved.

  • 118.
    Kirshner, Alexandra E.
    et al.
    Rice Univ, Dept Earth Sci, Houston, TX 77005 USA..
    Anderson, John B.
    Rice Univ, Dept Earth Sci, Houston, TX 77005 USA..
    Jakobsson, Martin
    Stockholm Univ, Dept Geol Sci, S-10691 Stockholm, Sweden..
    O'Regan, Matthew
    Cardiff Univ, Sch Earth & Ocean Sci, Cardiff, S Glam, Wales..
    Majewski, Wojciech
    Polish Acad Sci, Inst Paleobiol, PL-00818 Warsaw, Poland..
    Nitsche, Frank O.
    Columbia Univ, Lamont Doherty Earth Observ, Palisades, NY 10964 USA..
    Post-LGM deglaciation in Pine Island Bay, West Antarctica2012In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 38, p. 11-26Article in journal (Refereed)
    Abstract [en]

    To date, understanding of ice sheet retreat within Pine Island Bay (PIB) following the Last Glacial Maximum (LGM) was based on seven radiocarbon dates and only fragmentary seafloor geomorphic evidence. During the austral summer 2009-2010, restricted sea ice cover allowed for the collection of 27 sediment cores from the outer PIB trough region. Combining these cores with data from prior cruises, over 133 cores have been used to conduct a detailed sedimentological facies analysis. These results, augmented by 23 new radiocarbon dates, are used to reconstruct the post-LGM deglacial history of PIB. Our results record a clear retreat stratigraphy in PIB composed of, from top to base; terrigenous sandy silt (distal glacimarine), pebbly sandy mud (ice-proximal glacimarine), and till. Initial retreat from the outer-continental shelf began shortly after the LGM and before 16.4 k cal yr BP, as a likely response to rising sea level. Bedforms in outer PIB document episodic retreat in the form of back-stepping grounding zone wedges and are associated with proximal glacimarine sediments. A sub-ice shelf facies is observed in central PIB and spans similar to 12.3-10.6 k cal yr BR It is possible that widespread impingement of warm water onto the continental shelf caused an abrupt and widespread change from sub-ice shelf sedimentation to distal glacimarine sedimentation dominated by widespread dispersal of terrigenous silt between 7.8 and 7.0 k cal yr BP. The final phase of retreat ended before similar to 1.3 k cal yr BP, when the grounding line migrated to a location near the current ice margin. (C) 2012 Elsevier Ltd. All rights reserved.

  • 119. Kirshner, Alexandra E.
    et al.
    Anderson, John B.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    O'Regan, Matthew
    Majewski, Wojciech
    Nitsche, Frank O.
    Post-LGM deglaciation in Pine Island Bay, West Antarctica2012In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 38, p. 11-26Article in journal (Refereed)
    Abstract [en]

    To date, understanding of ice sheet retreat within Pine Island Bay (PIB) following the Last Glacial Maximum (LGM) was based on seven radiocarbon dates and only fragmentary seafloor geomorphic evidence. During the austral summer 2009-2010, restricted sea ice cover allowed for the collection of 27 sediment cores from the outer PIB trough region. Combining these cores with data from prior cruises, over 133 cores have been used to conduct a detailed sedimentological facies analysis. These results, augmented by 23 new radiocarbon dates, are used to reconstruct the post-LGM deglacial history of PIB. Our results record a clear retreat stratigraphy in PIB composed of, from top to base; terrigenous sandy silt (distal glacimarine), pebbly sandy mud (ice-proximal glacimarine), and till. Initial retreat from the outer-continental shelf began shortly after the LGM and before 16.4 k cal yr BP, as a likely response to rising sea level. Bedforms in outer PIB document episodic retreat in the form of back-stepping grounding zone wedges and are associated with proximal glacimarine sediments. A sub-ice shelf facies is observed in central PIB and spans similar to 12.3-10.6 k cal yr BR It is possible that widespread impingement of warm water onto the continental shelf caused an abrupt and widespread change from sub-ice shelf sedimentation to distal glacimarine sedimentation dominated by widespread dispersal of terrigenous silt between 7.8 and 7.0 k cal yr BP. The final phase of retreat ended before similar to 1.3 k cal yr BP, when the grounding line migrated to a location near the current ice margin.

  • 120.
    Kleman, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Applegate, Patrick J.
    Durations and propagation patterns of ice sheet instability events2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 92, p. 32-39Article in journal (Refereed)
    Abstract [en]

    Continued atmospheric and ocean warming places parts of the West Antarctic Ice Sheet at risk for collapse through accelerated ice flow and grounding line retreat over reversed bed slopes. However, understanding of the speed and duration of ice sheet instability events remains incomplete, limiting our ability to include these events in sea level rise projections. Here, we use a first-order, empirical approach, exploring past instability events in the Fennoscandian (FIS) and Laurentide (LIS) ice sheets to establish a relationship between catchment size and the duration of instability events. We also examine how instabilities propagate through ice sheet catchments, and how this propagation is controlled by topography and existing flow organisation at the onset of an event. We find that the fastest documented paleo-collapses involved streaming or surging in corridors that are wide compared to their length, and in which fast flow did not resume after the event. Distributed ice stream networks, in which narrow ice streams were intertwined with slow-flow interstream ridges, are not represented among the fastest documented events. For the FIS and LIS, there is geological evidence for instability events covering areas of similar to 100,000 km(2), with durations between 100 and 300 yr. Comparison of the spatial patterns and topographic contexts of Lateglacial collapse events in former Northern Hemisphere ice sheets and the current WAIS suggest that only a minor part of the WAIS area may be at risk for unimpeded collapse, and that negative feedbacks will likely slow or halt ice drawdown in remaining areas. The Pine Island Glacier (PIG) and Thwaites Glacier (TG) catchments in West Antarctica are likely to respond in very different ways to possible further grounding line retreat. The PIG may experience a minor collapse over its main trunk, but the bed topography favours a less dramatic retreat thereafter. The TG is probably not as close to a threshold as PIG, but once efficient drainage has progressed inwards to reach the Bentley Subglacial Basin (BSB) and Bentley Subglacial Trench (BST), a full collapse of the area may occur. The likely time perspective for a BSB BST collapse is the time required for 100-200 km of grounding line retreat in the TG system plus 100-300 years for an actual collapse event.

  • 121.
    Kleman, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Jansson, Krister
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    de Angelis, Hernán
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Stroeven, Arjen
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Hättestrand, Clas
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Alm, Göran
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Glasser, Neil
    Aberystwyth University.
    North American Ice Sheet build-up during the last glacial cycle, 115-21 kyr2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 17-18, p. 2036-2051Article in journal (Refereed)
    Abstract [en]

    The last glacial maximum (LGM) outline and subsequent retreat pattern (21e7 kyr) of North Americanice sheets are reasonably well established. However, the evolution of the ice sheets during their build-upphase towards the LGM between 115 and 21 kyr has remained elusive, making it difficult to verifynumerical ice sheet models for this important time interval. In this paper we outline the pre-LGM icesheet evolution of the Laurentide and Cordilleran ice sheets by using glacial geological and geomorphologicalrecords to make a first-order reconstruction of ice sheet extent and flow pattern. We mappedthe entire area covered by the Laurentide and Cordilleran ice sheets in Landsat MSS images andapproximately 40% of this area in higher resolution Landsat ETMþ images. Mapping in aerial photographsadded further detail primarily in Quebec-Labrador, the Cordilleran region, and on Baffin Island.Our analysis includes the recognition of approximately 500 relative-age relationships from crosscuttinglineations. Together with previously published striae and till fabric data, these are used as the basis forrelative-age assignments of regional flow patterns. For the reconstruction of the most probable ice sheetevolution sequence we employ a stepwise inversion scheme with a clearly defined strategy for delineatingcoherent landforms swarms (reflecting flow direction and configuration), and linking these topreviously published constraints on relative and absolute chronology. Our results reveal that icedispersalcentres in Keewatin and Quebec were dynamically independent for most of pre-LGM time andthat a massive Quebec dispersal centre, rivalling the LGM in extent, existed at times when the SW sectorof the ice sheet had not yet developed. The oldest flow system in eastern Quebec-Labrador (Atlanticswarm had an ice divide closer to the Labrador coast than later configurations). A northern Keewatin-Central Arctic Ice Sheet existed prior to the LGM, but is poorly chronologically constrained. There is alsoevidence for older and more easterly Cordilleran Ice Sheet divide locations than those that prevailedduring the Late Wisconsinan. In terms of ice sheet build-up dynamics, it appears that “residual” ice capsafter warming phases may have played an important role. In particular, the location and size of remnantice masses at the end of major interstadials, i.e. OIS 5c and 5a, must have been critical for subsequentbuild-up patterns, because such remnant “uplands” may have fostered much more rapid ice sheetgrowth than what would have occurred on a fully deglaciated terrain. The ice-sheet configuration duringstadials would also be governed largely by the additional topography that such “residual” ice constitutesbecause of inherent mass balance-topography feedbacks.

  • 122. Kliem, P.
    et al.
    Buylaert, J. P.
    Hahn, A.
    Mayr, C.
    Murray, A. S.
    Ohlendorf, C.
    Veres, D.
    Wastegård, Stefan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Zolitschka, B.
    Magnitude, geomorphologic response and climate links of lake level oscillations at Laguna Potrok Aike, Patagonian steppe (Argentina)2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 71, p. 131-146Article in journal (Refereed)
    Abstract [en]

    Laguna Potrok Aike is a large maar lake located in the semiarid steppe of southern Patagonia known for its Lateglacial and Holocene lake level fluctuations. Based on sedimentary, seismic and geomorphological evidences, the lake level curve is updated and extended into the Last Glacial period and the geomorphological development of the lake basin and its catchment area is interpreted. Abrasion and lake level oscillations since at least similar to 50 ka caused concentric erosion of the surrounding soft rocks of the Miocene Santa Cruz Formation and expanded the basin diameter by approximately 1 km. A high lake level and overflow conditions of the lake were dated by luminescence methods and tephra correlation to the early Lateglacial as well as to similar to 45 ka. The lowest lake level of record occurred during the mid-Holocene. A further lake level drop was probably prevented by groundwater supply. This low lake level eroded a distinct terrace into lacustrine sediments. Collapse of these terraces probably caused mass movement deposits in the profundal zone of the lake. After the mid-Holocene lake level low stand a general and successive transgression occurred until the Little Ice Age maximum; i.e. ca 40 m above the local groundwater table. Frequent lake level oscillations caused deflation of emerged terraces only along the eastern shoreline due to prevailing westerly winds. Preservation of eolian deposits might be linked to relatively moist climate conditions during the past 2.5 ka. Precisely dated lake level reconstructions in the rain-shadow of the Andes document high Last Glacial and low Holocene lake levels that could suggest increased precipitation during the Last Glacial period. As permafrost in semiarid Patagonia is documented and dated to the Last Glacial period we argue that the frozen ground might have increased surficial runoff from the catchment and thus influenced the water balance of the lake. This is important for investigating the glacial to Holocene latitudinal shift and/or strengthening of the Southern Hemispheric Westerlies by using lake level reconstructions as a means to assess the regional water balance. Our interpretation explains the contradiction with investigations based on pollen data indicating drier climatic conditions for the Last Glacial period.

  • 123. Kliem, Pierre
    et al.
    Enters, Dirk
    Hahn, Annette
    Ohlendorf, Christian
    Lise-Pronovost, Agathe
    St-Onge, Guillaume
    Wastegård, Stefan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Zolitschka, Bernd
    Lithology, radiocarbon chronology and sedimentological interpretation of the lacustrine record from Laguna Potrok Aike, southern Patagonia2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 71, p. 54-69Article in journal (Refereed)
    Abstract [en]

    The 106 m long composite profile from site 2 of ICDP expedition 5022 (PASADO) at Laguna Potrok Aike documents a distinct change in sedimentation patterns from pelagic sediments at the top to dominating mass movement deposits at its base. The main lithological units correspond to the Holocene, to the Lateglacial and to the last glacial period and can be interpreted as the result of distinct environmental variations. Overflow conditions might have been achieved during the last glacial period, while signs of desiccation are absent in the studied sediment record. Altogether, 58 radiocarbon dates were used to establish a consistent age-depth model by applying the mixed-effect regression procedure which results in a basal age of 51.2 cal. ka BP. Radiocarbon dates show a considerable increase in scatter with depth which is related to the high amount of reworking. Validation of the obtained chronology was achieved with geomagnetic relative paleointensity data and tephra correlation.

  • 124. Kohfeld, K. E.
    et al.
    Graham, Robert
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    de Boer, Agatha
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sime, L. C.
    Wolff, E. W.
    Le Quere, C.
    Bopp, L.
    Southern Hemisphere westerly wind changes during the Last Glacial Maximum: paleo-data synthesis2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 68, p. 76-95Article in journal (Refereed)
    Abstract [en]

    Changes in the strength and position of Southern Hemisphere westerly winds during the Last Glacial cycle have been invoked to explain both millennial and glacial interglacial climate fluctuations. However, neither paleo models nor paleodata agree on the magnitude, or even the sign, of the change in wind strength and latitude during the most studied glacial period, the Last Glacial Maximum (LGM), compared to the recent past. This paper synthesizes paleo-environmental data that have been used to infer changes in LGM winds. Data compilations are provided for changes in terrestrial moisture, dust deposition, sea surface temperatures and ocean fronts, and ocean productivity, and existing data on Southern Hemisphere ocean circulation changes during the LGM are summarized. We find that any hypothesis of LGM wind and climate change needs to provide a plausible explanation for increased moisture on the west coast of continents, cooler temperatures and higher productivity in the Subantarctic Zone, and reductions in Agulhas leakage around southern Africa. Our comparison suggests that an overall strengthening, an equatorward displacement, or no change at all in winds could all be interpreted as consistent with observations. If a single cause related to the southern westerlies is sought for all the evidence presented, then an equatorward displacement or strengthening of the winds would be consistent with the largest proportion of the observations. However, other processes, such as weakening or poleward shifts in winds, a weakened hydrological cycle, extended sea-ice cover, and changed buoyancy fluxes, cannot be ruled out as potential explanations of observed changes in moisture, surface temperature, and productivity. We contend that resolving the position and strength of westerly winds during the LGM remains elusive based on data reconstructions alone. However, we believe that these data reconstructions of environmental conditions can be used in conjunction with model simulations to identify which processes best represent westerly wind conditions during the LGM.

  • 125.
    Kremer, A.
    et al.
    Alfred Wegener Inst, Helmholtz Ctr Polar & Marine Res, Bremerhaven, Germany..
    Stein, R.
    Alfred Wegener Inst, Helmholtz Ctr Polar & Marine Res, Bremerhaven, Germany..
    Fahl, K.
    Alfred Wegener Inst, Helmholtz Ctr Polar & Marine Res, Bremerhaven, Germany..
    Ji, Z.
    State Ocean Adm, Inst Oceanog 2, Guangzhou, Guangdong, Peoples R China..
    Yang, Z.
    State Ocean Adm, Inst Oceanog 2, Guangzhou, Guangdong, Peoples R China..
    Wiers, S.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Natural Resources and Sustainable Development.
    Matthiessen, J.
    Alfred Wegener Inst, Helmholtz Ctr Polar & Marine Res, Bremerhaven, Germany..
    Forwick, M.
    UiT Arctic Univ Norway, Tromso, Norway..
    Lowemark, L.
    Stockholm Univ, Stockholm, Sweden..
    O'Regan, M.
    Stockholm Univ, Stockholm, Sweden..
    Chen, J.
    State Ocean Adm, Inst Oceanog 2, Guangzhou, Guangdong, Peoples R China..
    Snowball, Ian
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Natural Resources and Sustainable Development.
    Changes in sea ice cover and ice sheet extent at the Yermak Plateau during the last 160 ka - Reconstructions from biomarker records2018In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 182, p. 93-108Article in journal (Refereed)
    Abstract [en]

    The Yermak Plateau is located north of Svalbard at the entrance to the Arctic Ocean, i.e. in an area highly sensitive to climate change. A multi proxy approach was carried out on Core PS92/039-2 to study glacial interglacial environmental changes at the northern Barents Sea margin during the last 160 ka. The main emphasis was on the reconstruction of sea ice cover, based on the sea ice proxy IP25 and the related phytoplankton - sea ice index PIP25. Sea ice was present most of the time but showed significant temporal variability decisively affected by movements of the Svalbard Barents Sea Ice Sheet. For the first time, we prove the occurrence of seasonal sea ice at the eastern Yermak Plateau during glacial intervals, probably steered by a major northward advance of the ice sheet and the formation of a coastal polynya in front of it. Maximum accumulation of terrigenous organic carbon, IP25 and the phytoplankton biomarkers (brassicasterol, dinosterol, HBI III) can be correlated to distinct deglaciation events. More severe, but variable sea ice cover prevailed at the Yermak Plateau during interglacials. The general proximity to the sea ice margin is further indicated by biomarker (GDGT) - based sea surface temperatures below 2.5 degrees C.

  • 126. Kremer, A.
    et al.
    Stein, R.
    Fahl, K.
    Ji, Z.
    Yang, Z.
    Wiers, S.
    Matthiessen, J.
    Forwick, M.
    Löwemark, Ludvig
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    O'Regan, Matthew
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chen, J.
    Snowball, I.
    Changes in sea ice cover and ice sheet extent at the Yermak Plateau during the last 160 ka - Reconstructions from biomarker records2018In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 182, p. 93-108Article in journal (Refereed)
    Abstract [en]

    The Yermak Plateau is located north of Svalbard at the entrance to the Arctic Ocean, i.e. in an area highly sensitive to climate change. A multi proxy approach was carried out on Core PS92/039-2 to study glacial interglacial environmental changes at the northern Barents Sea margin during the last 160 ka. The main emphasis was on the reconstruction of sea ice cover, based on the sea ice proxy IP25 and the related phytoplankton - sea ice index PIP25. Sea ice was present most of the time but showed significant temporal variability decisively affected by movements of the Svalbard Barents Sea Ice Sheet. For the first time, we prove the occurrence of seasonal sea ice at the eastern Yermak Plateau during glacial intervals, probably steered by a major northward advance of the ice sheet and the formation of a coastal polynya in front of it. Maximum accumulation of terrigenous organic carbon, IP25 and the phytoplankton biomarkers (brassicasterol, dinosterol, HBI III) can be correlated to distinct deglaciation events. More severe, but variable sea ice cover prevailed at the Yermak Plateau during interglacials. The general proximity to the sea ice margin is further indicated by biomarker (GDGT) - based sea surface temperatures below 2.5 degrees C.

  • 127.
    Kullman, Leif
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Early postglacial appearance of tree species in northern Scandinavia: review and perspective2008In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 27, no 27-28, p. 2467-2472Article in journal (Refereed)
    Abstract [en]

    This paper reviews megafossil evidence for the first postglacial records of different tree species in northern Scandinavia. Betula pubescens coll. appeared at the Arctic coast of northern Norway by 16. 900 yr BR In addition, Betula Pubescens (14, 000 yr BP), Pinus sylvestris (11, 700 yr BP) and Picea abies (11, 000 yr BP) existed on early ice- free mountain peaks (nunataks) at different locations in the Scandes during the Lateglacial. Larix sibirica, currently not native to Fennoscandia, and several thermophilous broadleaved tree species were recorded in the earliest part of the Holocene. The conventional interpretation of pollen and macrofossil records from peat and sediment stratigraphies do not consider the Occurrence of the species mentioned above that early at these northern and high altitude sites. This very rapid arrival after the local deglaciation implies that the traditional model of far distant glacial refugial areas for tree species has to be challenged. The Current results are more compatible with a situation involving scattered "cryptic" refugia quite close to margin of the ice sheet at its full-glacial extension. This fits a more general pattern currently emerging on different continents. In general, "cryptic" refugia should be considered in connection with modelling extinction risks related to modern and possible future "climatic crises".

  • 128. Kwiatkowski, Cornelia
    et al.
    Prange, Matthias
    Varma, Vidya
    Stockholm University, Faculty of Science, Department of Meteorology . University of Bremen, Germany.
    Steinke, Stephan
    Hebbeln, Dierk
    Mohtadi, Mahyar
    Holocene variations of thermocline conditions in the eastern tropical Indian Ocean2015In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 114, p. 33-42Article in journal (Refereed)
    Abstract [en]

    Climate phenomena like the monsoon system, El Nino Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are interconnected via various feedback mechanisms and control the climate of the Indian Ocean and its surrounding continents on various timescales. The eastern tropical Indian Ocean is a key area for the interplay of these phenomena and for reconstructing their past changes and forcing mechanisms. Here we present records of upper ocean thermal gradient, thermocline temperatures (TT) and relative abundances of planktic foraminifera in core SO 189-39KL taken off western Sumatra (0 degrees 47.400' S, 99 degrees 54.510' E) for the last 8 ka that we use as proxies for changes in upper ocean structure. The records suggest a deeper thermocline between 8 ka and ca 3 ka compared to the late Holocene. We find a shoaling of the thermocline after 3 ka, most likely indicating an increased occurrence of upwelling during the late Holocene compared to the mid-Holocene which might represent changes in the IOD-like mean state of the Indian Ocean with a more negative IOD-like mean state during the mid-Holocene and a more positive IOD-like mean state during the past 3 ka. This interpretation is supported by a transient Holocene climate model simulation in which an IOD-like mode is identified that involves an insolation-forced long-term trend of increasing anomalous surface easterlies over the equatorial eastern Indian Ocean.

  • 129.
    Kylander, Malin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bindler, Richard
    Cortizas, Antonio Martinez
    Gallagher, Kerry
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Rauch, Sebastien
    A novel geochemical approach to paleorecords of dust deposition and effective humidity: 8500 years of peat accumulation at Store Mosse (the Great Bog), Sweden2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 69, p. 69-82Article in journal (Refereed)
    Abstract [en]

    Both bog surface wetness and atmospheric dust deposition are intricately linked to changes in the hydrological cycle and pairing these types of records at the same site provides complementary information. Here a peat core from Store Mosse in southern Sweden covering the last 8500 years was used to make a high-resolution paleoclimate reconstruction based on a combination of bog development, colourimetric humification and inorganic geochemistry data. The coupling of Principal Component Analysis with changepoint modelling allowed for precise linking of changes in bog surface wetness and dust deposition records. A long-term trend towards warm (and possibly wet) conditions starts ca 8150 cal yr BP and culminates with the most pronounced conditions from 6900 to 6600 cal yr BP. The most significant arid period at Store Mosse occurred between 6500 and 5600 cal yr BP during which dust deposition was significantly higher. Wetter conditions dominate from 5500 to 4980 cal yr BP as the transition from the Hypsithermal and into the Neoglacial is made. After a shift to drier conditions, humification enters a more stable period that lasts from 4525 until 3200 cal yr BP. It is during this time that the first possible anthropogenic dust signals occur at ca 4200 cal yr BP. From 3200 cal yr BP to present humification generally shows a long-term decline moving towards wetter conditions. The main exceptions are during the transition from the Neoglacial to Roman Warm Period which is registered as a significantly wetter period and two dry periods recorded 2365 to 2155 cal yr BP and 1275-1105 cal yr BP. In general, the observed changes agree well with regional records of effective humidity and temperature. The high temporal resolution of the Store Mosse record reveals that palaeoclimatic change over the last 8500 years in southern Sweden has had a complex and variable structure.

  • 130. Kylander, Malin E.
    et al.
    Bindler, Richard
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Cortizas, Antonio Martinez
    Gallagher, Kerry
    Morth, Carl-Magnus
    Rauch, Sebastien
    A novel geochemical approach to paleorecords of dust deposition and effective humidity: 8500 years of peat accumulation at Store Mosse (the "Great Bog"), Sweden2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 69, p. 69-82Article in journal (Refereed)
    Abstract [en]

    Both bog surface wetness and atmospheric dust deposition are intricately linked to changes in the hydrological cycle and pairing these types of records at the same site provides complementary information. Here a peat core from Store Mosse in southern Sweden covering the last 8500 years was used to make a high-resolution paleoclimate reconstruction based on a combination of bog development, colourimetric humification and inorganic geochemistry data. The coupling of Principal Component Analysis with changepoint modelling allowed for precise linking of changes in bog surface wetness and dust deposition records. A long-term trend towards warm (and possibly wet) conditions starts ca 8150 cal yr BP and culminates with the most pronounced conditions from 6900 to 6600 cal yr BP. The most significant arid period at Store Mosse occurred between 6500 and 5600 cal yr BP during which dust deposition was significantly higher. Wetter conditions dominate from 5500 to 4980 cal yr BP as the transition from the Hypsithermal and into the Neoglacial is made. After a shift to drier conditions, humification enters a more stable period that lasts from 4525 until 3200 cal yr BP. It is during this time that the first possible anthropogenic dust signals occur at ca 4200 cal yr BP. From 3200 cal yr BP to present humification generally shows a long-term decline moving towards wetter conditions. The main exceptions are during the transition from the Neoglacial to Roman Warm Period which is registered as a significantly wetter period and two dry periods recorded 2365 to 2155 cal yr BP and 1275-1105 cal yr BP. In general, the observed changes agree well with regional records of effective humidity and temperature. The high temporal resolution of the Store Mosse record reveals that palaeoclimatic change over the last 8500 years in southern Sweden has had a complex and variable structure. (c) 2013 Elsevier Ltd. All rights reserved.

  • 131. Large, David
    et al.
    Spiro, Baruch
    Ferrat, Marion
    Shopland, Maria
    Kylander, Malin
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Gallagher, Kerry
    Li, X
    Chengde, S
    Possnert, G
    Gan, Z
    Darling, WG
    Weiss, Dominik
    The influence of climate, hydrology and permafrost on Holocene peat accumulation at 3500 m on the Eastern Qinghai-Tibetan Plateau2009In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 28, no 27-28, p. 3303-3314Article in journal (Refereed)
    Abstract [en]

    Peatland of the eastern Qinghai–Tibetan Plateau lies at the convergence of the East Asian and Indian monsoon systems in eastern Asia. To understand the evolution of this peatland and its potential to provide new insights into the Holocene evolution of the East Asian monsoon a 6 m peat core was collected from the undisturbed central part of a peat deposit near Hongyuan. The age-depth profile was determined using 16 14C-AMS age dates, the peat analysed for a range of environmental variables including carbon, nitrogen and hydrogen concentration, bulk density, δ13C and the associated spring water analysed for hydrogen and oxygen isotopes. The age-depth profile of the recovered peat sequence covers the period from 9.6 to 0.3 kyr BP and is linear indicating that the conditions governing productivity and decay varied little over the Holocene. Using changes in carbon density, organic carbon content and its δ13C, cold dry periods of permafrost characterised by low density and impeded surface drainage were identified. The low δ18O and δD values of the spring water emanating around the peat deposit, down to −13.8 and −102‰ (VSMOW), respectively, with an inverse relationship between electrical conductivity and isotopic composition indicate precipitation under colder and drier conditions relative to the present day. In view of the current annual mean air temperature of 1 °C this suggests conditions in the past have been conducive to permafrost. Inferred periods of permafrost correspond to independently recognised cold periods in other Holocene records from across China at 8.6, 8.2–7.8, 5.6–4.2, 3.1 and 1.8–1.5 kyr BP. The transition to a cold dry climate appears to be more rapid than the subsequent recovery and cold dry periods at Hongyuan are of longer duration than equivalent cold dry periods over central and eastern China. Light–dark banding peat on a scale of 15–30 years from 9.6 to 5.5 kyr BP may indicate a strong influence of decadal oscillations possibly the Pacific Decadal Oscillation and a potential link between near simultaneous climatic changes in the northwest Pacific, ENSO, movement of the Intertropical Convergence Zone and the East Asian Monsoon.

  • 132. Large, DJ
    et al.
    Spiro, B
    Ferrat, M
    Shopland, M
    Kylander, M
    Gallagher, K
    Li, X
    Shen, C
    Possnert, Göran
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Ion Physics. Uppsala University, Disciplinary Domain of Science and Technology, För teknisk-naturvetenskapliga fakulteten gemensamma enheter, Tandem Laboratory.
    Zhang, G
    Darling, WG
    Weiss, D
    The influence of climate, hydrology and permafrost on Holocene peat accumulation at 3500 m on the eastern Qinghai-Tibetan plateau2009In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 28, no 27-28, p. 3303-3314Article in journal (Refereed)
    Abstract [en]

    Peatland of the eastern Qinghai–Tibetan Plateau lies at the convergence of the East Asian and Indian monsoon systems in eastern Asia. To understand the evolution of this peatland and its potential to provide new insights into the Holocene evolution of the East Asian monsoon a 6 m peat core was collected from the undisturbed central part of a peat deposit near Hongyuan. The age-depth profile was determined using 16 14C-AMS age dates, the peat analysed for a range of environmental variables including carbon, nitrogen and hydrogen concentration, bulk density, δ13C and the associated spring water analysed for hydrogen and oxygen isotopes. The age-depth profile of the recovered peat sequence covers the period from 9.6 to 0.3 kyr BP and is linear indicating that the conditions governing productivity and decay varied little over the Holocene. Using changes in carbon density, organic carbon content and its δ13C, cold dry periods of permafrost characterised by low density and impeded surface drainage were identified. The low δ18O and δD values of the spring water emanating around the peat deposit, down to −13.8 and −102‰ (VSMOW), respectively, with an inverse relationship between electrical conductivity and isotopic composition indicate precipitation under colder and drier conditions relative to the present day. In view of the current annual mean air temperature of 1 °C this suggests conditions in the past have been conducive to permafrost. Inferred periods of permafrost correspond to independently recognised cold periods in other Holocene records from across China at 8.6, 8.2–7.8, 5.6–4.2, 3.1 and 1.8–1.5 kyr BP. The transition to a cold dry climate appears to be more rapid than the subsequent recovery and cold dry periods at Hongyuan are of longer duration than equivalent cold dry periods over central and eastern China. Light–dark banding peat on a scale of 15–30 years from 9.6 to 5.5 kyr BP may indicate a strong influence of decadal oscillations possibly the Pacific Decadal Oscillation and a potential link between near simultaneous climatic changes in the northwest Pacific, ENSO, movement of the Intertropical Convergence Zone and the East Asian Monsoon.

  • 133. Larocque, I.
    et al.
    Hall, R. I.
    Holocene temperature estimates and chironomid community composition in the Abisko Valley, northern Sweden2004In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 23, no 23, p. 2453-2465Article in journal (Refereed)
    Abstract [en]

    Multi-proxy paleoenvironmental reconstructions are useful to determine the various factors affecting the biological communities of a lake, but to assess if changes in community composition of one indicator organism accurately reconstructs climatic changes through time, it may be more useful to compare temperature reconstructions using the same indicator in several lakes. Here, we compare reconstructions of mean July air temperature using chironomid-based transfer functions from Holocene records at three nearby lakes in the Abisko Valley of northern Sweden to assess if chironomids can be used as indicators of regional temperature changes. The three study lakes experience the same regional climatic conditions, but are located along gradients of elevation (348-999m a.s.l), temperature (8.1-12°C) and terrestrial vegetation (coniferous to alpine). Chironomid-temperature reconstructions from the three sites indicate a general pattern of temperature decrease (1.5-2.4°C) during the Holocene, consistent with decreases observed from analyses of other proxies in this area, and from other alpine regions in Europe and North America. Similarities between these reconstructions suggest that chironomids can adequately record general patterns of temperature changes through the Holocene, although effects of site-specific factors such as variations in lake water pH can cause deviations in inferred temperature among sites during some periods.

  • 134.
    Larocque, I.
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Halla, R.I.
    Holocene temperature estimates and chironomid community composition in the Abisko Valley, northern Sweden2004In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 23, no 23-24, p. 2453-2465Article in journal (Refereed)
    Abstract [en]

    Multi-proxy paleoenvironmental reconstructions are useful to determine the various factors affecting the biological communities of a lake, but to assess if changes in community composition of one indicator organism accurately reconstructs climatic changes through time, it may be more useful to compare temperature reconstructions using the same indicator in several lakes. Here, we compare reconstructions of mean July air temperature using chironomid-based transfer functions from Holocene records at three nearby lakes in the Abisko Valley of northern Sweden to assess if chironomids can be used as indicators of regional temperature changes. The three study lakes experience the same regional climatic conditions, but are located along gradients of elevation (348–999 m a.s.l), temperature (8.1–12°C) and terrestrial vegetation (coniferous to alpine). Chironomid-temperature reconstructions from the three sites indicate a general pattern of temperature decrease (1.5–2.4°C) during the Holocene, consistent with decreases observed from analyses of other proxies in this area, and from other alpine regions in Europe and North America. Similarities between these reconstructions suggest that chironomids can adequately record general patterns of temperature changes through the Holocene, although effects of site-specific factors such as variations in lake water pH can cause deviations in inferred temperature among sites during some periods.

  • 135.
    Larocque, I.
    et al.
    Umeå universitet, Institutionen för ekologi, miljö och geovetenskap.
    Halla, R.I.
    Holocene temperature estimates and chironomid community composition in the Abisko Valley, northern Sweden2004In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 23, no 23-24, p. 2453-2465Article in journal (Refereed)
    Abstract [en]

    Multi-proxy paleoenvironmental reconstructions are useful to determine the various factors affecting the biological communities of a lake, but to assess if changes in community composition of one indicator organism accurately reconstructs climatic changes through time, it may be more useful to compare temperature reconstructions using the same indicator in several lakes. Here, we compare reconstructions of mean July air temperature using chironomid-based transfer functions from Holocene records at three nearby lakes in the Abisko Valley of northern Sweden to assess if chironomids can be used as indicators of regional temperature changes. The three study lakes experience the same regional climatic conditions, but are located along gradients of elevation (348–999 m a.s.l), temperature (8.1–12°C) and terrestrial vegetation (coniferous to alpine). Chironomid-temperature reconstructions from the three sites indicate a general pattern of temperature decrease (1.5–2.4°C) during the Holocene, consistent with decreases observed from analyses of other proxies in this area, and from other alpine regions in Europe and North America. Similarities between these reconstructions suggest that chironomids can adequately record general patterns of temperature changes through the Holocene, although effects of site-specific factors such as variations in lake water pH can cause deviations in inferred temperature among sites during some periods.

  • 136. Larsen, Nicolaj K.
    et al.
    Funder, Svend
    Linge, Henriette
    Möller, Per
    Schomacker, Anders
    Fabel, Derek
    Xu, Sheng
    Kjær, Kurt H.
    A Younger Dryas re-advance of local glaciers in north GreenlandIn: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457XArticle in journal (Refereed)
    Abstract [en]

    Abstract The Younger Dryas (YD) is a well-constrained cold event from 12,900 to 11,700 years ago but it remains unclear how the cooling and subsequent abrupt warming recorded in ice cores was translated into ice margin fluctuations in Greenland. Here we present 10Be surface exposure ages from three moraines in front of local glaciers on a 50 km stretch along the north coast of Greenland, facing the Arctic Ocean. Ten ages range from 11.6 ± 0.5 to 27.2 ± 0.9 ka with a mean age of 12.5 ± 0.7 ka after exclusion of two outliers. We consider this to be a minimum age for the abandonment of the moraines. The ages of the moraines are furthermore constrained using Optically Stimulated Luminescence (OSL) dating of epishelf sediments, which were deposited prior to the ice advance that formed the moraines, yielding a maximum age of 12.4 ± 0.6 ka, and bracketing the formation and subsequent abandonment of the moraines to within the interval 11.8–13.0 ka ago. This is the first time a synchronous YD glacier advance and subsequent retreat has been recorded for several independent glaciers in Greenland. In most other areas, there is no evidence for re-advance and glaciers were retreating during YD. We explain the different behaviour of the glaciers in northernmost Greenland as a function of their remoteness from the Atlantic Meridional Overturning Circulation (AMOC), which in other areas has been held responsible for modifying the YD drop in temperatures.

  • 137. Larsen, Nicolaj K.
    et al.
    Kjaer, Kurt H.
    Funder, Svend
    Moller, Per
    van der Meer, Jaap J. M.
    Schomacker, Anders
    Linge, Henriette
    Darby, Dennis A.
    Late Quaternary glaciation history of northernmost Greenland - Evidence of shelf-based ice2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 25-26, SI, p. 3399-3414Article in journal (Refereed)
    Abstract [en]

    We present the mapping of glacial landforms and sediments from northernmost Greenland bordering 100 km of the Arctic Ocean coast. One of the most important discoveries is that glacial landforms, sediments, including till fabric measurements, striae and stoss-lee boulders suggest eastward ice-flow along the coastal plain. Volcanic erratic boulders document ice-transport from 80 to 100 km west of the study area. We argue that these findings are best explained by local outlet glaciers from the Greenland Ice Sheet and local ice caps that merged to form a shelf-based ice in the Arctic Ocean and possibly confirming an extensive ice shelf in the Lincoln Sea between Greenland and Ellesmere Island. It is speculated that the shelf-based ice was largely affected by the presence of thick multiyear sea ice in the Arctic Ocean that prevented it from breaking up and forced the outlet glaciers to flow eastwards. During the initial retreat the coastal area was dammed by the shelf-based ice and kame and glaciolacustrine sediments were deposited up to 50 m above the marine limit before the final deglaciation and marine transgression. The timing of the shelf-based ice is constrained on land by dating glaciolacustrine sediments with OSL and marine molluscs with radiocarbon and by re-evaluating IRD events in cores from the Fram Strait. Results show that the shelf-based ice started to build-up as early as 30 cal ka BP and reached a maximum during the Last Glacial Maximum (LGM). The shelf-based ice began to retreat ca 16 ka to 10.3 cal ka BP before the final break-up, which took place ca 10.1 cal ka BP probably as a combined result of increased inflow of warm Atlantic water through the Fram Strait, a shallower halocline and higher summer temperatures, corresponding to orbital maximum solar insolation at this time. The existence of extensive shelf-based ice north of Greenland provides an important contribution to the understanding of the LGM glaciation history of the Arctic Ocean. (C) 2010 Elsevier Ltd. All rights reserved.

  • 138.
    Larter, Robert D.
    et al.
    British Antarctic Survey, Cambridge CB3 0ET, England..
    Anderson, John B.
    Rice Univ, Dept Earth Sci, Houston, TX 77005 USA..
    Graham, Alastair G. C.
    British Antarctic Survey, Cambridge CB3 0ET, England.;Univ Exeter, Coll Life & Environm Sci, Exeter EX4 4RJ, Devon, England..
    Gohl, Karsten
    Helmholtz Ctr Polar & Marine Res, Alfred Wegener Inst, D-27568 Bremerhaven, Germany..
    Hillenbrand, Claus-Dieter
    British Antarctic Survey, Cambridge CB3 0ET, England..
    Jakobsson, Martin
    Stockholm Univ, Dept Geol Sci, S-10691 Stockholm, Sweden..
    Johnson, Joanne S.
    British Antarctic Survey, Cambridge CB3 0ET, England..
    Kuhn, Gerhard
    Helmholtz Ctr Polar & Marine Res, Alfred Wegener Inst, D-27568 Bremerhaven, Germany..
    Nitsche, Frank O.
    Columbia Univ, Lamont Doherty Earth Observ, Palisades, NY USA..
    Smith, James A.
    British Antarctic Survey, Cambridge CB3 0ET, England..
    Witus, Alexandra E.
    Rice Univ, Dept Earth Sci, Houston, TX 77005 USA..
    Bentley, Michael J.
    Univ Durham, Dept Geog, Durham DH1 3LE, England..
    Dowdeswell, Julian A.
    Univ Cambridge, Scott Polar Res Inst, Cambridge CB2 1ER, England..
    Ehrmann, Werner
    Univ Leipzig, Inst Geol & Geophys, D-04103 Leipzig, Germany..
    Klages, Johann P.
    Helmholtz Ctr Polar & Marine Res, Alfred Wegener Inst, D-27568 Bremerhaven, Germany..
    Lindow, Julia
    Univ Bremen, Dept Geosci, D-28359 Bremen, Germany..
    Cofaigh, Colm O.
    Univ Durham, Dept Geog, Durham DH1 3LE, England..
    Spiegel, Cornelia
    Univ Bremen, Dept Geosci, D-28359 Bremen, Germany..
    Reconstruction of changes in the Amundsen Sea and Bellingshausen Sea sector of the West Antarctic Ice Sheet since the Last Glacial Maximum2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 100, p. 55-86Article in journal (Other academic)
    Abstract [en]

    Marine and terrestrial geological and marine geophysical data that constrain deglaciation since the Last Glacial Maximum (LGM) of the sector of the West Antarctic Ice Sheet (WAIS) draining into the Amundsen Sea and Bellingshausen Sea have been collated and used as the basis for a set of time-slice reconstructions. The drainage basins in these sectors constitute a little more than one-quarter of the area of the WAIS, but account for about one-third of its surface accumulation. Their mass balance is becoming increasingly negative, and therefore they account for an even larger fraction of current WAIS discharge. If all of the ice in these sectors of the WAIS were discharged to the ocean, global sea level would rise by ca 2 m. There is compelling evidence that grounding lines of palaeo-ice streams were at, or close to, the continental shelf edge along the Amundsen Sea and Bellingshausen Sea margins during the last glacial period. However, the few cosmogenic surface exposure ages and ice core data available from the interior of West Antarctica indicate that ice surface elevations there have changed little since the LGM. In the few areas from which cosmogenic surface exposure ages have been determined near the margin of the ice sheet, they generally suggest that there has been a gradual decrease in ice surface elevation since pre-Holocene times. Radiocarbon dates from glacimarine and the earliest seasonally open marine sediments in continental shelf cores that have been interpreted as providing approximate ages for post-LGM grounding-line retreat indicate different trajectories of palaeo-ice stream recession in the Amundsen Sea and Bellingshausen Sea embayments. The areas were probably subject to similar oceanic, atmospheric and eustatic forcing, in which case the differences are probably largely a consequence of how topographic and geological factors have affected ice flow, and of topographic influences on snow accumulation and warm water inflow across the continental shelf. Pauses in ice retreat are recorded where there are "bottle necks" in cross-shelf troughs in both embayments. The highest retreat rates presently constrained by radiocarbon dates from sediment cores are found where the grounding line retreated across deep basins on the inner shelf in the Amundsen Sea, which is consistent with the marine ice sheet instability hypothesis. Deglacial ages from the Amundsen Sea Embayment (ASE) and Eltanin Bay (southern Bellingshausen Sea) indicate that the ice sheet had already retreated close to its modern limits by early Holocene time, which suggests that the rapid ice thinning, flow acceleration, and grounding line retreat observed in this sector over recent decades are unusual in the context of the past 10,000 years. (C) 2014 The Authors. Published by Elsevier Ltd. All rights reserved.

  • 139. Larter, Robert D.
    et al.
    Anderson, John B.
    Graham, Alastair G.C.
    Gohl, Karsten
    Hillenbrand, Claus-Dieter
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Johnson, Joanne S.
    Kuhn, Gerhard
    Nitsche, Frank O.
    Smith, James A.
    Witus, Alexandra E.
    Bentley, Michael J.
    Dowdeswell, Julian A.
    Ehrmann, Werner
    Klages, Johann P.
    Lindow, Julia
    Ó Cofaigh, Colm
    Spiegel, Cornelia
    Reconstruction of changes in the Amundsen Sea and BellingshausenSea sector of the West Antarctic Ice Sheet since the Last GlacialMaximum2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 100, p. 56-86Article in journal (Refereed)
    Abstract [en]

    Marine and terrestrial geological and marine geophysical data that constrain deglaciation since the LastGlacial Maximum (LGM) of the sector of theWest Antarctic Ice Sheet (WAIS) draining into the AmundsenSea and Bellingshausen Sea have been collated and used as the basis for a set of time-slice reconstructions.The drainage basins in these sectors constitute a little more than one-quarter of the area ofthe WAIS, but account for about one-third of its surface accumulation. Their mass balance is becomingincreasingly negative, and therefore they account for an even larger fraction of currentWAIS discharge. Ifall of the ice in these sectors of the WAIS were discharged to the ocean, global sea level would rise byca 2 m.There is compelling evidence that grounding lines of palaeo-ice streams were at, or close to, thecontinental shelf edge along the Amundsen Sea and Bellingshausen Sea margins during the last glacialperiod. However, the few cosmogenic surface exposure ages and ice core data available from the interiorofWest Antarctica indicate that ice surface elevations there have changed little since the LGM. In the fewareas from which cosmogenic surface exposure ages have been determined near the margin of the icesheet, they generally suggest that there has been a gradual decrease in ice surface elevation since pre-Holocene times. Radiocarbon dates from glacimarine and the earliest seasonally open marine sedimentsin continental shelf cores that have been interpreted as providing approximate ages for post-LGMgrounding-line retreat indicate different trajectories of palaeo-ice stream recession in the Amundsen Seaand Bellingshausen Sea embayments. The areas were probably subject to similar oceanic, atmosphericand eustatic forcing, in which case the differences are probably largely a consequence of how topographicand geological factors have affected ice flow, and of topographic influences on snow accumulation andwarm water inflow across the continental shelf.Pauses in ice retreat are recorded where there are “bottle necks” in cross-shelf troughs in both embayments.The highest retreat rates presently constrained by radiocarbon dates from sediment cores arefound where the grounding line retreated across deep basins on the inner shelf in the Amundsen Sea,which is consistent with the marine ice sheet instability hypothesis. Deglacial ages from the Amundsen 

    Sea Embayment (ASE) and Eltanin Bay (southern Bellingshausen Sea) indicate that the ice sheet hadalready retreated close to its modern limits by early Holocene time, which suggests that the rapid icethinning, flow acceleration, and grounding line retreat observed in this sector over recent decades areunusual in the context of the past 10,000 years.

  • 140. Leng, Melanie J.
    et al.
    Wagner, Bernd
    Boehm, Anne
    Panagiotopoulos, Konstantinos
    Vane, Christopher H.
    Snelling, Andrea
    Haidon, Cheryl
    Woodley, Ewan
    Vogel, Hendrik
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Zanchetta, Gianni
    Baneschi, Ilaria
    Understanding past climatic and hydrological variability in the Mediterranean from Lake Prespa sediment isotope and geochemical record over the last glacial cycle2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 66, p. 123-136Article in journal (Refereed)
    Abstract [en]

    Here we present stable isotope and geochemical data from Lake Prespa (Macedonia/Albania border) over the Last Glacial cycle (Marine Isotope Stages 5-1) and discuss past lake hydrology and climate (TIC, oxygen and carbon isotopes), as well as responses to climate of terrestrial and aquatic vegetation (TOC, Rock Eval pyrolysis, carbon isotopes, pollen). The Lake Prespa sediments broadly fall into 5 zones based on their sedimentology, geochemistry, palynology and the existing chronology. The Glacial sediments suggest low supply of carbon to the lake, but high summer productivity; intermittent siderite layers suggest that although the lake was likely to have mixed regularly leading to enhanced oxidation of organic matter, there must have been within sediment reducing conditions and methanogenesis. MIS 5 and 1 sediments suggest much more productivity, higher rates of organic material preservation possibly due to more limited mixing with longer periods of oxygen-depleted bottom waters. We also calculated lakewater delta O-18 from siderite (authigenic/Glacial) and calcite (endogenic/Holocene) and show much lower lakewater delta O-18 values in the Glacial when compared to the Holocene, suggesting the lake was less evaporative in the Glacial, probably as a consequence of cooler summers and longer winter ice cover. In the Holocene the oxygen isotope data suggests general humidity, with just 2 marked arid phases, features observed in other Eastern and Central Mediterranean lakes. (C) 2012 Elsevier Ltd. All rights reserved.

  • 141.
    Lewis, J.P.
    et al.
    Loughborough University, Loughborough, England.
    Ryves, D.B.
    Loughborough University, Loughborough, England.
    Rasmussen, P.
    National Museum of Denmark, Lyngby, Denmark / Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark.
    Olsen, J.
    Aarhus University, Aarhus, Denmark.
    Knudsen, K.-L.
    Aarhus University, Aarhus, Denmark.
    Andersen, S.H.
    Moesgård Museum, Højbjerg, Danmark.
    Weckström, K.
    Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark.
    Clarke, A.L.
    APEM Aquatic Scientists Ltd, Stockport, UK.
    Andrén, Elinor
    Södertörn University, School of Natural Sciences, Technology and Environmental Studies, Environmental Science.
    Juggins, S.
    Newcastle University, Newcaslte, England.
    The shellfish enigma across the Mesolithic-Neolithic transition in southern Scandinavia2016In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 151, p. 315-320, article id http://dx.doi.org/10.1016/j.quascirev.2016.09.004Article in journal (Refereed)
    Abstract [en]

    The well-known and widespread replacement of oysters (abundant during the Mesolithic period) by cockles and mussels in many Danish Stone Age shell middens ca. 5900 cal yrs BP coincides with the transition to agriculture in southern Scandinavia. This human resource shift is commonly believed to reflect changing resource availability, driven by environmental and/or climatic change at the Mesolithic-Neolithic transition rather than cultural choice. While several hypotheses have been proposed to explain the “Mesolithic-Neolithic oyster decline”, an explanation based on a sudden freshening of the inner Danish waters has received most attention. Here, for the first time, we test and refute this long-standing hypothesis that declining salinity explains the marked reduction in oysters identified within numerous shell middens across coastal Denmark at the Mesolithic-Neolithic transition using quantitative and qualitative salinity inference from several, independent proxies (diatoms, molluscs and foraminifera) from multiple Danish fjord sites. Alternatively, we attribute the oyster decline to other environmental causes (particularly changing sedimentation), ultimately driven by external climatic forcing. Critical application of such high-quality environmental archives can reinvigorate archaeological debates and can aid in understanding and managing environmental change in increasingly impacted coastal regions.

  • 142. Li, Yingkui
    et al.
    Liu, Gengnian
    Chen, Yixin
    Li, Yanan
    Harbor, Jon
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Stroeven, Arjen P.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Caffee, Marc
    Zhang, Mei
    Li, Chuanchuan
    Cui, Zhijiu
    Timing and extent of Quaternary glaciations in the Tianger Range, eastern Tian Shan, China, investigated using Be-10 surface exposure dating2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 98, p. 7-23Article in journal (Refereed)
    Abstract [en]

    Reconstructing glacial chronologies with consistent methods is critical for efforts to examine the timing and pattern of past climate change. Cosmogenic Be-10 surface exposure dating has been widely used to constrain the timing of glacial events on the Tibetan Plateau and in Central Asia. However, few such studies have been conducted in the Chinese Tian Shan and available Be-10 ages from this region have only provided evidence for glacial events during the global Last Glacial Maximum (gLGM) and Lateglacial. Here, we present 45 Be-10 surface exposure ages from glacial landforms in the Ala and Daxi valleys, two formerly glaciated valleys draining the Tianger Range, eastern Tian Shan. Combined with previously published Be-10 surface exposure ages from the Daxi Valley in the source area of the Urumqi River, the new ages record five major glacial events during Marine Oxygen Isotope Stages (MIS) 6 or older, 4, 3, 2, and 1 (during the Little Ice Age, LIA). Landforms from glacial events since MIS 2 are found on the northern slope of the Tianger Range (Daxi Valley), whereas evidence for the older glacial events is only preserved on its southern slope (Ala Valley). This disparity may be caused by different preservation- and micro-climatic conditions on the northern and southern slopes of this mountain range, due to differences in gradient and aspect. The LIA glacial advances are apparently the only Holocene glacial event recorded in this area. Earlier Holocene glacial events were probably so restricted in extent that they were destroyed by subsequent LIA advances.

  • 143. Lifton, Nathaniel
    et al.
    Beel, Casey
    Hättestrand, Clas
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kassab, Christine
    Rogozhina, Irina
    Heermance, Richard
    Oskin, Michael
    Burbank, Douglas
    Blomdin, Robin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Purdue University, USA.
    Gribenski, Natacha
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Caffee, Marc
    Goehring, Brent M.
    Heyman, Jakob
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Ivanov, Mikhail
    Li, Yanan
    Li, Yingkui
    Petrakov, Dmitry
    Usubaliev, Ryskul
    Codilean, Alexandru T.
    Chen, Yixin
    Harbor, Jon
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Purdue University, USA.
    Stroeven, Arjen P.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Constraints on the late Quaternary glacial history of the Inylchek and Sary-Dzaz valleys from in situ cosmogenic Be-10 and Al-26, eastern Kyrgyz Tian Shan2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 101, p. 77-90Article in journal (Refereed)
    Abstract [en]

    Paleoclimatic constraints from regions at the confluence of major climate systems are particularly important in understanding past climate change. Using geomorphic mapping based on remote sensing and field investigations, combined with in situ cosmogenic Be-10 and Al-26 dating of boulders associated with glacial landforms, we investigate the chronology of past glaciation in the Inylchek and Sary-Dzaz valleys in the eastern Kyrgyz Tian Shan, a tectonically active area with some of the highest peaks in the world outside of the Himalayas. Cosmogenic Be-10 and (26) Al exposure ages of boulders on moraines record up to five glacial advances including: Lateglacial age lateral moraine remnants and meltwater channels in the upper Inylchek Valley; Last Glacial Maximum (LGM, Marine Oxygen Isotope Stage [MIS] 2) moraines in the Sary-Dzaz Valley and in a terminal moraine complex at the west end of the Inylchek Valley, overriding older moraines; an MIS 4 or 5 moraine remnant above the Inylchek terminal moraine complex; and an older high moraine remnant down-valley from the confluence of the Inylchek and Sary-Dzaz valleys. The evidence for glacial extent in this study is consistent with a limited ice expansion hypothesis for Tian Shan glaciation. Published results from the western and central Kyrgyz Tian Shan do not show evidence for significant LGM glacier expansion, which in combination with the results presented here, indicate a spatial variation in glacier records along the Tian Shan. This may reflect either paleoclimatic gradients or the impact of local physiographic conditions on responses to regional climate change, or both.

  • 144. Ljung, Karl
    et al.
    Holmgren, Sofia
    Kylander, Malin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sjolte, Jesper
    Van der Putten, Nathalie
    Kageyama, Masa
    Porter, Charles T.
    Björck, Svante
    The last termination in the central South Atlantic2015In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 123, p. 193-214Article in journal (Refereed)
    Abstract [en]

    Lake sediments and peat deposits from two basins on Nightingale Island (37 degrees S), in the Tristan da Cunha archipelago, South Atlantic, have been analyzed. The studies were focused on the time period 16.2-10.0 cal ka BP, determined by 36 C-14 dates from the two sequences. A wide variety of proxies were used, including pollen and diatom analyzes, biogenic silica content, C and N analyzes, stable isotopes (C-13 and N-15), elemental concentrations and magnetic susceptibility measurements, to detect environmental changes that can be related to shifts of the circulation belts of the Southern Ocean. In addition, climate model simulations were carried out. We find that the sediments are underlain by a >2 cal ka BP long hiatus, possibly representing a dried-out lake bed. The climate simulations corroborate that the area might have been exposed to arid conditions as a consequence of the Heinrich I event in the north and a southward displacement of the ITCZ. The development on the island after 16.2 cal ka BP is determined by the position of the Subtropical Front (STF) and the Southern Hemisphere Westerlies (SHW). The period 16.2-14.75 cal ka BP was characterized by varying influence from SHW and with STF situated south of Tristan da Cunha, ending with a humidity peak and cooler conditions. The stable conditions 14.7-14.1 cal ka BP with cool and fairly arid conditions imply that STF and SHW were both north of the islands during the first part of the Antarctic Cold Reversal. The most unstable period, 14.1-12.7 cal ka BP, indicates incessant latitudinal shifts of the zonal circulation, perhaps related to climate variability in the Northern Hemisphere and bipolar seesaw mechanisms as the strength of the Atlantic Meridional Overturning Circulation (AMOC) varied. At 12.7 cal ka BP the Holocene warming began with a gradually drier and warmer climate as a result of a dampened AMOC during the Younger Dryas cooling in the north with ITCZ, STF and SHW being displaced southwards. Peak warming seems to have occurred in the earliest part of the Holocene, but this period was also characterized by humidity shifts, possibly an effect of retraction and expansion phases of SHW during AMOC variations in the north.

  • 145. Loader, N. J.
    et al.
    Young, G. H. F.
    Grudd, H.
    McCarroll, D.
    Stable carbon isotopes from Torneträsk, northern Sweden provide a millennial length reconstruction of summer sunshine and its relationship to Arctic circulation2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 62, no Supplement C, p. 97-113Article in journal (Refereed)
    Abstract [en]

    Abstract This paper presents results from the first 1100 years of a long stable carbon isotope chronology currently in development from Scots Pine (Pinus sylvestris L.) trees growing in the Torneträsk region of northern Sweden. The isotope record currently comprises a total of 74 trees with a mean annual replication of >12, thereby enabling it to be compared directly with other tree-ring based palaeoclimate reconstructions from this region. In developing the reconstruction, several key topics in isotope dendroclimatology (chronology construction, replication, CO2 adjustment and age trends) were addressed. The resulting carbon isotope series is calibrated against instrumental data from the closest meteorological station at Abisko (AD1913-2008) to provide a record of June-August sunshine for northern Fennoscandia. This parameter is closely linked to the direct control of assimilation rate; Photosynthetically Active Radiation (PAR) and the indirect measures; mean July-August temperature and percent cloud cover. The coupled response of summer sunshine and temperature in this region permits a multi-parameter comparison with a local reconstruction of past temperature variability based upon tree growth proxies to explore the stability of this coupling through time. Several periods are identified where the temperature (X-ray density) and sunshine (stable carbon isotope ratio) records diverge. The most significant and sustained of these occur between c AD1200-1380 and c AD1550-1780, providing evidence for a cool, sunny, two-phase "Little Ice Age". Whilst summer sunshine reconstructed for the 20th century is significantly different from the mean of the last 1100 years (P < 0.01), conditions during the early mediaeval period are similar to those experienced in northern Fennoscandia during the 20th century (P > 0.01), so it is the 17th–18th, and to a lesser extent, the 13th centuries rather than the early mediaeval period that appear anomalous when viewed within the context of the last 1100 years. The observed departures between temperature and sunshine are interpreted as indicating a change in large-scale circulation associated with a southward migration of the Polar Front. Such a change, affecting the Northern Annular Mode (Arctic Oscillation) would result in more stable anticyclonic conditions (cool, bright, summers) over northern Fennoscandia, thus providing a testable mechanism for the development of a multi-phase, time-transgressive "Little Ice Age"€ across Europe.

  • 146. Loader, N. J.
    et al.
    Young, G. H. F.
    Grudd, Håkan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    McCarroll, D.
    Stable carbon isotopes from Tornetrask, northern Sweden provide a millennial length reconstruction of summer sunshine and its relationship to Arctic circulation2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 62, p. 97-113Article in journal (Refereed)
    Abstract [en]

    This paper presents results from the first 1100 years of a long stable carbon isotope chronology currently in development from Scots Pine (Pinus sylvestris L.) trees growing in the Tornetrask region of northern Sweden. The isotope record currently comprises a total of 74 trees with a mean annual replication of >12, thereby enabling it to be compared directly with other tree-ring based palaeoclimate reconstructions from this region. In developing the reconstruction, several key topics in isotope dendroclimatology (chronology construction, replication, CO2 adjustment and age trends) were addressed. The resulting carbon isotope series is calibrated against instrumental data from the closest meteorological station at Abisko (AD1913-2008) to provide a record of June August sunshine for northern Fennoscandia. This parameter is closely linked to the direct control of assimilation rate; Photosynthetically Active Radiation (PAR) and the indirect measures; mean July August temperature and percent cloud cover. The coupled response of summer sunshine and temperature in this region permits a multiparameter comparison with a local reconstruction of past temperature variability based upon tree growth proxies to explore the stability of this coupling through time. Several periods are identified where the temperature (X-ray density) and sunshine (stable carbon isotope ratio) records diverge. The most significant and sustained of these occur between c AD1200-1380 and c AD1550-1780, providing evidence for a cool, sunny, two-phase Little Ice Age. Whilst summer sunshine reconstructed for the 20th century is significantly different from the mean of the last 1100 years (P < 0.01), conditions during the early medival period are similar to those experienced in northern Fennoscandia during the 20th century (P > 0.01), so it is the 17th-18th, and to a lesser extent, the 13th centuries rather than the early medival period that appear anomalous when viewed within the context of the last 1100 years. The observed departures between temperature and sunshine are interpreted as indicating a change in large-scale circulation associated with a southward migration of the Polar Front. Such a change, affecting the Northern Annular Mode (Arctic Oscillation) would result in more stable anticyclonic conditions (cool, bright, summers) over northern Fennoscandia, thus providing a testable mechanism for the development of a multi-phase, time-transgressive Little Ice Age across Europe.

  • 147. Lougheed, Bryan C.
    et al.
    Nilsson, Andreas
    Björck, Svante
    Snowball, Ian
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Muscheler, Raimund
    A deglacial palaeomagnetic master curve for Fennoscandia Providing a dating template and supporting millennial-scale geomagnetic field patterns for the past 14 ka2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 106, p. 155-166Article in journal (Refereed)
    Abstract [en]

    Reconstructions of palaeomagnetic secular variation (PSV) in sediment cores can be compared to well-dated regional PSV master curves to infer deposition age. The existing PSV master curve for Fennoscandia, "Fennostack" (Snowball et al., 2007), is limited to the past 10 ka. In this study, we construct a deglacial (for the interval 14-11 ka) PSV master curve for Fennoscandia by including data from a number of existing studies in the region, updating geochronologies where necessary. We also produce new deglacial PSV data from Baltic Sea long-core sediments. By selecting three suitable sites, one in southern Sweden and two in northwest Russia, we produce, for the first time, a deglacial PSV master curve for Fennoscandia, which will provide a useful alternative dating tool for deglacial time intervals, especially considering that deglacial sediments are often unsuitable for C-14 dating. Additionally, we use the deglacial PSV master curve to assess current hypotheses regarding geomagnetic field changes. Time varying geomagnetic field models constrained by Holocene PSV data from around the globe have predicted the presence of latitudinal and longitudinal patterns in the position of the north geomagnetic pole (NGP). Specifically, a 1350 year cycle in NGP latitude has been noted, along with two preferred dominant mode longitudinal bands for NGP; in Europe and North America (Korte et al., 2011; Nilsson et al., 2011). Most PSV studies of sediment are, however, limited to the Holocene epoch. By combining our deglacial PSV master curve with 'Fennostack', we are able to assess general patterns in inclination for the past 14 ka, and compare these to a general prediction of regional inclination for the last 14 ka, based on an extrapolation of the latitudinal and longitudinal NGP periodicity noted by Nilsson et al. (2011). The model prediction suggests that the Fennoscandian PSV for the past 14 ka should reveal three recurring intervals of generally steeper inclination due to a dominant NGP longitudinal band in Europe. We find that the Fennoscandian PSV does indeed show these intervals of generally steeper inclination for the time periods expected, supporting the hypothesised periodic NGP variation, which may represent an inherent feature of the geodynamo. (C) 2014 Elsevier Ltd. All rights reserved.

  • 148. Löwemark, L.
    et al.
    Marz, C.
    O'Regan, M.
    Gyllencreutz, R.
    Arctic Ocean Mn-stratigraphy: genesis, synthesis and inter-basin correlation2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 92Article in journal (Refereed)
    Abstract [en]

    Across the Arctic Ocean, late Quaternary deep marine sediments are characterized by the occurrence of brownish layers intercalated with yellowish to olive gray sediments. These layers show enhanced levels of bioturbation, peaks in Mn content, and typically contain elevated abundances of planktonic and benthic micro-and nannofossils. It was early surmised that these layers were deposited under interglacial conditions and that their cyclical downcore occurrence could be correlated to the global benthic oxygen isotope curve. However, the synchronicity of Mn layers with interglacial conditions and the underlying mechanisms responsible for their formation remain controversial. Here we compile and synthesize findings of the last decades with several recent studies that shed light on issues such as the sources of Mn to the Arctic Ocean, the processes and pathways for Mn to the deep sea, the chemical processes active in the sediment, and the spatial and temporal distribution of Mn-rich layers in Arctic deep marine sediments. Budget calculations show that about 90% of Mn input to the Arctic Ocean originates from Arctic rivers or coastal erosion, two sources effectively shut down during mid-to late Quaternary glacial intervals by continental ice sheets blocking or redirecting the rivers and vast subaerial exposure of the shelf areas. Thus, the strong late Quaternary interglacial-glacial cyclicity in Mn content is clearly an input-related signal, and only secondarily influenced by chemical processes in the water column and in the sediment. On the shelves, the Mn undergoes repeated geochemical recycling caused by the high organic carbon content in the sediments before it is ultimately exported to the deep basins where scavenging processes in the water column effectively bring the Mn to the sea floor in the form of Mn (oxyhydr)oxides. The close synchronicity with enhanced bioturbation and elevated micro and nannofossil abundances shows that the Mn peaks are preserved at a stratigraphic level closely corresponding to the interglacial intervals. However, under certain biogeochemical conditions, Mn (oxyhydr)oxides may diagenetically become both dissolved and re-precipitated deep in the sediments, as shown by pore water analyses and X-ray radiograph studies. Dissolution is particularly conspicuous in late Quaternary sediments from the Lomonosov Ridge, where in rapidly deposited coarse grained intervals (diamictons) with elevated total organic carbon (TOC) contents, Mn appears almost completely removed from within the glacial sediments, and also the surrounding interglacial sediments. Correspondingly, bundles of closely spaced, mm-thick, Mn-rich horizontal bands are observed in sediment otherwise devoid of indicators for interglacial conditions, suggesting that these bands were purely formed by diagenetic processes redistributing the Mn from deeper sediment layers. This type of diagenetic Mn redistribution within the sediment can be recognized in XRF-core scanner data combined with sedimentological information from X-ray radiographs, while pore water data are highly promising if clear diagenetic features in the sediment are missing. With this increasing ability to recognize intervals where a diagenetic overprint exists in the Mn record, the recently improved understanding of the Mn cycle in the Arctic Ocean provides a conceptual paleoenvironmental framework in which carefully applied Mn stratigraphy can provide a powerful correlation tool, when combined with other paleoceanographic proxies and sedimentological data. (C) 2013 Elsevier Ltd. All rights reserved.

  • 149. Löwemark, L.
    et al.
    März, C.
    O'Regan, Matthew
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Arctic Ocean Mn-stratigraphy: genesis, synthesis and inter-basin correlation2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 92, p. 97-111Article in journal (Refereed)
    Abstract [en]

    Across the Arctic Ocean, late Quaternary deep marine sediments are characterized by the occurrence of brownish layers intercalated with yellowish to olive gray sediments. These layers show enhanced levels of bioturbation, peaks in Mn content, and typically contain elevated abundances of planktonic and benthic micro-and nannofossils. It was early surmised that these layers were deposited under interglacial conditions and that their cyclical downcore occurrence could be correlated to the global benthic oxygen isotope curve. However, the synchronicity of Mn layers with interglacial conditions and the underlying mechanisms responsible for their formation remain controversial. Here we compile and synthesize findings of the last decades with several recent studies that shed light on issues such as the sources of Mn to the Arctic Ocean, the processes and pathways for Mn to the deep sea, the chemical processes active in the sediment, and the spatial and temporal distribution of Mn-rich layers in Arctic deep marine sediments. Budget calculations show that about 90% of Mn input to the Arctic Ocean originates from Arctic rivers or coastal erosion, two sources effectively shut down during mid-to late Quaternary glacial intervals by continental ice sheets blocking or redirecting the rivers and vast subaerial exposure of the shelf areas. Thus, the strong late Quaternary interglacial-glacial cyclicity in Mn content is clearly an input-related signal, and only secondarily influenced by chemical processes in the water column and in the sediment. On the shelves, the Mn undergoes repeated geochemical recycling caused by the high organic carbon content in the sediments before it is ultimately exported to the deep basins where scavenging processes in the water column effectively bring the Mn to the sea floor in the form of Mn (oxyhydr)oxides. The close synchronicity with enhanced bioturbation and elevated micro and nannofossil abundances shows that the Mn peaks are preserved at a stratigraphic level closely corresponding to the interglacial intervals. However, under certain biogeochemical conditions, Mn (oxyhydr)oxides may diagenetically become both dissolved and re-precipitated deep in the sediments, as shown by pore water analyses and X-ray radiograph studies. Dissolution is particularly conspicuous in late Quaternary sediments from the Lomonosov Ridge, where in rapidly deposited coarse grained intervals (diamictons) with elevated total organic carbon (TOC) contents, Mn appears almost completely removed from within the glacial sediments, and also the surrounding interglacial sediments. Correspondingly, bundles of closely spaced, mm-thick, Mn-rich horizontal bands are observed in sediment otherwise devoid of indicators for interglacial conditions, suggesting that these bands were purely formed by diagenetic processes redistributing the Mn from deeper sediment layers. This type of diagenetic Mn redistribution within the sediment can be recognized in XRF-core scanner data combined with sedimentological information from X-ray radiographs, while pore water data are highly promising if clear diagenetic features in the sediment are missing. With this increasing ability to recognize intervals where a diagenetic overprint exists in the Mn record, the recently improved understanding of the Mn cycle in the Arctic Ocean provides a conceptual paleoenvironmental framework in which carefully applied Mn stratigraphy can provide a powerful correlation tool, when combined with other paleoceanographic proxies and sedimentological data.

  • 150. Löwemark, Ludvig
    et al.
    Chao, Weng-Si
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hanebuth, Till J. J.
    Chiu, Pin-Yao
    Yang, Tien-Nan
    Su, Chih-Chieh
    Chuang, Chih-Kai
    Leon Dominguez, Dora Carolina
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Variations in glacial and interglacial marine conditions over the last two glacial cycles off northern Greenland2016In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 147, no SI, p. 164-177Article in journal (Refereed)
    Abstract [en]

    Five sediment cores from the Lomonosov Ridge and the Morris Jesup Rise north of Greenland show the history of sea-ice coverage and primary productivity over the last two glacial cycles. Variations in Manganese content, benthic and planktonic foraminifera, bioturbation, and trace fossil diversity are interpreted to reflect differences in sea-ice cover and sediment depositional conditions between the identified interglacials. Marine Isotope Stage (MIS) 1 and MIS 2 are represented by thin (<< 5 cm) sediment units while the preceding interglacial MIS 5 and glacial MIS 6 are characterized by thick (10 -20 cm) deposits. Foraminiferal abundances and bioturbation suggest that MIS 1 was generally characterized by severe sea-ice conditions north of Greenland while MISS appears to have been considerably warmer with more open water, higher primary productivity, and higher sedimentation rates. Strengthened flow of Atlantic water along the northern continental shelf of Greenland rather than development of local polynyas is here suggested as a likely cause for the relatively warmer marine conditions during MIS 5 compared to MIS 1. The cores also suggest distinct differences between the glacial intervals MIS 2 and MIS 6. While MIS 6 is distinguished by a relatively thick sediment unit poor in foraminifera and with low Mn values, MIS 2 is practically missing. We speculate that this could be the effect from a paleocrystic sea-ice cover north of Greenland during MIS 2 that prevented sediment delivery from sea ice and icebergs. In contrast, the thick sequence deposited during MIS 6 indicates a longer glacial period with dynamic intervals characterized by huge drifting icebergs delivering ice rafted debris (IRD). A drastic shift from thinner sedimentary cycles where interglacial sediment parameters indicate more severe sea-ice conditions gave way to larger amplitude cycles with more open water indicators was observed around the boundary between MIS 7/8. This shift is in agreement with a sedimentary regime shift previously identified in the Eurasian Basin and may be an indicator for the growth of larger ice sheets on the Eurasian landmass during the penultimate glacial period.

12345 101 - 150 of 236
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf