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Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system
Stockholm University, Faculty of Science, Department of Geological Sciences.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The long warming transition from the Last Ice Age into the present Interglacial period, the last deglaciation, holds the key to our understanding of future abrupt climate change. In the last decades, a great effort has been put into deciphering the linkage between freshwater fluxes from melting ice sheets and rapid shifts in global ocean-atmospheric circulation that characterized this puzzling climate period. In particular, the regional expressions of climate change in response to freshwater forcing are still largely unresolved.

This projects aims at evaluating the environmental, hydro-climatic and oceanographic response in the Eastern North Atlantic domain to freshwater fluxes from the Scandinavian Ice Sheet during the last deglaciation (~19,000-11,000 years ago). The results presented in this thesis involve an overview of the regional representations of climate change across rapid climatic transitions and provide the groundwork to better understand spatial and temporal propagations of past atmospheric and ocean perturbations.

Specifically, this thesis comprises i) a comparison of pollenstratigraphic records from densely 14C dated lake sediment sequences, which provides insight into the regional sensitivity of North European vegetation to freshwater forcing in the Nordic Seas around the onset of the Younger Dryas stadial (~12,900 years ago); ii) a reconstruction of North European hydro-climate, which, together with transient climate simulations, shed light on the mechanisms and regionality of climate shortly prior to the transition into the Younger Dryas stadial; iii) studies of a ~1250-year long glacial varve chronology, which provides an accurate timing for the sudden drainage of proglacial freshwater stored in the former ice-dammed Baltic Ice Lake into the North Atlantic Ocean; iv) a 5000-year long terrestrial-marine reconstruction of Eastern North Atlantic hydro-climate and oceanographic changes that clarifies the hitherto elusive relationship between freshwater forcing and the transient behaviour of the North Atlantic overturning circulation system. The results presented in this thesis provide new important temporal constraints on the events that punctuated the last deglaciation in Northern Europe, and give a clearer understanding of the ocean – atmosphere – ice-sheet feedbacks that were at work in the North Atlantic. This increases our understanding of how the Earth climate system functions in more extreme situations.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences , 2016.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, ISSN 1101-1599 ; 362
Keyword [en]
climate, North Atlantic, last deglaciation, isotope geochemistry, chronology, climate modeling
National Category
Earth and Related Environmental Sciences
Research subject
Marine Geology
Identifiers
URN: urn:nbn:se:su:diva-128147ISBN: 978-91-7649-368-7 (print)OAI: oai:DiVA.org:su-128147DiVA: diva2:913346
Public defence
2016-05-13, De Geersalen, Geovetenskapens hus, Svante Arrenhius väg 14, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: In press. Paper 4: Manuscript.

Available from: 2016-04-20 Created: 2016-03-21 Last updated: 2017-02-23Bibliographically approved
List of papers
1. Time-transgressive environmental shifts across Northern Europe at the onset of the Younger Dryas
Open this publication in new window or tab >>Time-transgressive environmental shifts across Northern Europe at the onset of the Younger Dryas
2015 (English)In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 109, 49-56 p.Article in journal (Refereed) Published
Abstract [en]

Until lately, it has commonly been assumed that the last major reorganization of the North Atlantic ocean-atmosphere system, the Younger Dryas climatic reversal, spread synchronously on continental to hemispheric scales. This assumption arose because reliable chronologies, which would allow capturing the complexity surrounding local responses to abrupt climate change, were lacking. To better understand the temporal structure at the inception of the Younger Dryas across the North Atlantic, we revised, updated and compared the chronological framework of four Northern European sediment sequences (Lake Krakenes, Lake Madtjarn, Lake Gammelmose, Sluggan Bog) by applying classical Bayesian modelling. We found distinct and spatially consistent age differences between the inferred ages of the Allerod interstadial - Younger Dryas stadial pollen zone boundaries among the four sites. Our results suggest an earlier vegetation response at sites along latitude 56-54 degrees N as compared to sites located at 60 -58 degrees N. We explain this time lag by a gradual regional cooling that started as early as c. 12,900 -13,100 cal. BP. This phenomenon was probably linked to cooling around the Nordic Seas as a result of enhanced iceberg calving from the Fennoscandian Ice Sheet during the final stage of the Aneroid inter-stadial. By contrast, vegetation shifts at sites located further north occurred significantly later and in concert with the establishment of full stadial climate conditions (c. 12,600-12,750 cal. BP). Our study emphasizes the need to develop solid regional C-14 chronologies and to employ the same age modelling approach to determine the temporal and spatial response to a climatic shift.

Keyword
Northern Europe, Younger Dryas, Radiocarbon, Bayesian age-depth models
National Category
Earth and Related Environmental Sciences
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-115294 (URN)10.1016/j.quascirev.2014.11.015 (DOI)000348957200005 ()
Note

AuthorCount:2;

Available from: 2015-03-31 Created: 2015-03-18 Last updated: 2017-12-04Bibliographically approved
2. Fennoscandian freshwater control on Greenland hydroclimate shifts at the onset of the Younger Dryas
Open this publication in new window or tab >>Fennoscandian freshwater control on Greenland hydroclimate shifts at the onset of the Younger Dryas
Show others...
2015 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, 8939Article in journal (Refereed) Published
Abstract [en]

Sources and timing of freshwater forcing relative to hydroclimate shifts recorded in Greenland ice cores at the onset of Younger Dryas, similar to 12,800 years ago, remain speculative. Here we show that progressive Fennoscandian Ice Sheet (FIS) melting 13,100-12,880 years ago generates a hydroclimate dipole with drier-colder conditions in Northern Europe and wetter-warmer conditions in Greenland. FIS melting culminates 12,880 years ago synchronously with the start of Greenland Stadial 1 and a large-scale hydroclimate transition lasting similar to 180 years. Transient climate model simulations forced with FIS freshwater reproduce the initial hydroclimate dipole through sea-ice feedbacks in the Nordic Seas. The transition is attributed to the export of excess sea ice to the subpolar North Atlantic and a subsequent southward shift of the westerly winds. We suggest that North Atlantic hydroclimate sensitivity to FIS freshwater can explain the pace and sign of shifts recorded in Greenland at the climate transition into the Younger Dryas.

National Category
Earth and Related Environmental Sciences
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-124781 (URN)10.1038/ncomms9939 (DOI)000366380100010 ()26573386 (PubMedID)
Available from: 2016-01-05 Created: 2016-01-04 Last updated: 2017-12-01Bibliographically approved
3. Precise timing of the first drainage of the Baltic Ice Lake from Swedish glacial-varve chronologies
Open this publication in new window or tab >>Precise timing of the first drainage of the Baltic Ice Lake from Swedish glacial-varve chronologies
Show others...
(English)In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885Article in journal (Refereed) In press
Abstract

Glacial varves can give significant insights into recession and melting rates of decaying ice sheets. Moreover, varve chronologies can provide an independent means of comparison to other annually resolved climatic archives, which ultimately help to assess the timing and response of an ice sheet to changes across rapid climate transitions. Here we report a composite 1257-year long varve chronology from south-eastern Sweden spanning the regional late Allerød-late Younger Dryas pollen zone. The chronology was correlated to the Greenland Ice Core Chronology 2005 using the time-synchronous Vedde Ash volcanic marker, which can be found in both successions. For the first time, this enables secure placement of the Lateglacial Swedish varve chronology in absolute time. Geochemical analysis from new varve successions indicate a marked change in sedimentation regime accompanied by an interruption of ice-rafted debris deposition synchronous with the onset of Greenland Stadial 1 (GS-1; 12 846 years before 1950 AD). With the support of a simple ice flow/calving model, we suggest that slowdown of sediment transfer can be explained by ice-sheet margin stabilisation/advance in response to a significant drop of the Baltic Ice Lake level. A reassessment of chronological evidence from central-western and southern Sweden further supports the hypothesis of synchronicity between the first (penultimate) catastrophic drainage of the Baltic Ice Lake and the start of GS-1 in Greenland ice cores. Our results may therefore provide the first chronologically robust evidence linking continental meltwater forcing to rapid atmosphere-ocean circulation changes in the North Atlantic.

National Category
Earth and Related Environmental Sciences
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-126156 (URN)
Available from: 2016-01-25 Created: 2016-01-25 Last updated: 2017-11-30
4. North Atlantic overturning and climate response to meltwater forcing during the last deglaciation
Open this publication in new window or tab >>North Atlantic overturning and climate response to meltwater forcing during the last deglaciation
(English)Manuscript (preprint) (Other academic)
National Category
Climate Research
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-128203 (URN)
Available from: 2016-03-21 Created: 2016-03-21 Last updated: 2016-03-21Bibliographically approved

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