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Eastern equatorial Pacific bulk sediment properties and paleoceanography since the late Neogene
Stockholm University, Faculty of Science, Department of Geological Sciences. (Marine geology)ORCID iD: 0000-0002-9939-5836
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Reconstructing eastern equatorial Pacific (EEP) oceanography since the late Neogene (about 8 million years ago, Ma) is a central topic in current paleoceanographic investigations. The reason for this is two-fold. First, the EEP exerts a strong control on global climate because steep gradients in sea surface temperature (SST) and biological productivity linked to equatorial wind-driven upwelling affect global climate and carbon cycling, including the exchange of upwelled CO2 to the atmosphere. Second, during the last 8 Ma global climate underwent major changes before arriving at its current state, evolving through the last period of widespread global warmth (the early Pliocene) to the colder ‘mean’ global climate state of the pre-industrial world. Deciphering the dynamics of the EEP system since the late Neogene is thus important for understanding how this ocean area works under changing climatic conditions.

 

Despite the large numbers of studies devoted to the EEP, its paleoceanographic evolution since 8 Ma is still debated and contrasting scenarios have often been proposed. This is in part because paleoceanographic reconstructions are challenging in the EEP due to the high environmental, and thus sedimentary, heterogeneity, as well as the extreme seafloor depth, which compromises the preservation of useful foraminiferal archives in many regions. Moreover, some existing legacy data sets are confounded by some basic issues with the way in which the data were collected. Yet, reconstructing properties of surface mixed-layer remains a crucial requirement for deciphering EEP paleoceanography. Fossil foraminifera tests are typically not available for tracing EEP surface ocean properties because of strong sea floor diagenetic alteration. However, calcareous nannofossil carbonate, also produced in the surface mixed layer and accessible in the form of the bulk sedimentary carbonate or sediment ‘fine fraction’, is available. The challenge is to understand what these bulk geochemical signals mean.

 

Bulk sediment comprises a mixture of different, mostly biogenic particles. The information carried by its properties, including physical and geochemical signals, comprises a mixed signal reflecting different ecological, metabolic and depositional processes associated with the formation and sedimentation of the various calcite particles. The purpose of this thesis is to understand what bulk sediment records represent in terms meaningful for deciphering the paleoceanographic history of the EEP. The results add to, and significantly improve the paleoceanographic “tool box” available for developing proxy records in this complex oceanic region. This thesis comprises a kappa, two published papers and a manuscript, in review as this is written (April 2019). Paper I examines the factors linking sediment composition and physical properties in the EEP through analysis of the relationship between sediment carbonate content and sediment density. Paper II and Paper III focuses on improving the understanding of bulk carbonate carbon and oxygen stable isotope signals by disentangling the contribution of different carbonate components. In these studies, a multiproxy approach was adopted to reconstruct ocean evolution of the EEP since the late Miocene. A major conclusion is that bulk carbonate stable isotopes reflect the isotopic composition of calcareous nannofossils, and therefore surface water conditions, despite complication by several factors. The ideas and findings of the latter two papers have been further tested in an unpublished inter-basin comparison presented in the Chapter 6 of this kappa. The findings of this doctoral thesis demonstrate that bulk sediment is more than just a correlation tool and can provide a reliable indicator of surface ocean conditions that can be used to decipher EEP oceanographic history since 8 Ma.

 

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University , 2019. , p. 83
Keywords [en]
Paleoceanography, late Neogene, eastern equatorial Pacific, deep sea sediments, bulk sediment, stable isotopes, surface mixed layer, sediment physical properties
National Category
Geology
Research subject
Marine Geology
Identifiers
URN: urn:nbn:se:su:diva-168162ISBN: 978-91-7797-678-3 (print)ISBN: 978-91-7797-679-0 (electronic)OAI: oai:DiVA.org:su-168162DiVA, id: diva2:1306587
Public defence
2019-06-12, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.

Available from: 2019-05-20 Created: 2019-04-24 Last updated: 2019-05-23Bibliographically approved
List of papers
1. The relationship between wet bulk density and carbonate content in sediments from the Eastern Equatorial Pacific
Open this publication in new window or tab >>The relationship between wet bulk density and carbonate content in sediments from the Eastern Equatorial Pacific
2013 (English)In: Marine Geology, ISSN 0025-3227, E-ISSN 1872-6151, Vol. 344, p. 41-52Article in journal (Refereed) Published
Abstract [en]

Sediment cores collected from the Eastern Equatorial Pacific Ocean display a clear positive second-order relationship between wet bulk density (WED) and carbonate content. This has long interested the paleoceanography community because detailed Gamma Ray Attenuation Porosity Evaluator (GRAPE) measurements, which approximate WBD, might be used to determine records of carbonate content at very high temporal resolution. Although general causes for the relationship are known, they have not been presented and discussed systematically on the basis of first principles. In this study, we measure the mass and carbonate content of 50 sediment samples with known WBD from Site U1338, before and after rinsing with de-ionized water; we also determine the mass related proportion of coarse (>63 mu m) material. Samples exhibit clear relationships between WBD, carbonate content, mass loss upon rinsing, and grain size. We develop a series of mathematical expressions to describe these relationships, and solve them numerically. As noted by previous workers, the second-order relationship between WBD and carbonate content results from the mixing of biogenic carbonate and biogenic silica, which have different grain densities and different porosities. However, at high carbonate content, a wide range in WBD occurs because samples with greater amounts of coarse carbonate have higher porosity. Moreover compaction impacts carbonate particles more than biogenic silica particles. As such, a single two-component equation cannot be used to determine carbonate content accurately across depth intervals where both the porosity and type of carbonate vary. Instead, the WBD-carbonate relationship is described by an infinite series of curves, each which represents mixing of multiple sediment components with different densities and porosities. Dissolved ions also precipitate from pore space during sample drying, which adds mass to the sediment. Without rinsing samples, simple empirical relationships between WBD and carbonate content are further skewed by salt dilution.

Keywords
Eastern Equatorial Pacific, density, carbonate content, grain size, sediment compaction, seawater salt correction
National Category
Oceanography, Hydrology and Water Resources
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-96159 (URN)10.1016/j.margeo.2013.07.007 (DOI)000325600500005 ()
Note

AuthorCount:3;

Available from: 2013-11-12 Created: 2013-11-12 Last updated: 2019-04-25Bibliographically approved
2. Carbon and oxygen isotopes of bulk carbonate in sediment deposited beneath the eastern equatorial Pacific over the last 8 million years
Open this publication in new window or tab >>Carbon and oxygen isotopes of bulk carbonate in sediment deposited beneath the eastern equatorial Pacific over the last 8 million years
2015 (English)In: Paleoceanography, ISSN 0883-8305, E-ISSN 1944-9186, Vol. 30, no 10, p. 1261-1286Article in journal (Refereed) Published
Abstract [en]

To improve the understanding and utility of bulk carbonate stable carbon and oxygen isotope measurements, we examine sediment from cores in the eastern equatorial Pacific that span the last 8Ma. We measured C-13 and O-18 in 791 samples from Integrated Ocean Drilling Program Site U1338 and Deep Sea Drilling Project Site 573, both located close to the Pacific equator. In 100 samples, we measured C-13 and O-18 on isolated <63 mu m and <38 mu m fractions, which concentrates calcareous nannofossil carbonate and progressively excludes foraminiferal carbonate. Bulk carbonate C-13 and O-18 records are similar to published records from other sites drilled near the equator and seem to reflect mixed layer conditions, albeit with some important caveats involving the precipitation of calcite by coccolithophores. The comparatively lower C-13 and O-18 of the <63 mu m and <38 mu m fractions in sediments younger than 4.4Ma is attributed to an increase in deep-dwelling planktic foraminifera material in bulk carbonate, shifting the bulk isotopic signals toward higher values. Bulk carbonate C-13 is similar over 2500km along the Pacific equator, suggesting covarying concentrations and C-13 of dissolved inorganic carbon within surface waters since 8Ma. Greater bulk sediment C-13 and O-18, higher sedimentation rates, and low content of coarse material suggest intensified wind-driven upwelling and enhanced primary productivity along the Pacific equator between 8.0 and 4.4Ma, although a full understanding of bulk carbonate records will require extensive future work.

Keywords
eastern equatorial Pacific, late Miocene-early Pliocene high productivity, carbon and oxygen isotopes, bulk carbonate, sediment size fractions
National Category
Earth and Related Environmental Sciences
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-125028 (URN)10.1002/2015PA002825 (DOI)000366061400003 ()
Available from: 2016-01-07 Created: 2016-01-07 Last updated: 2019-04-25Bibliographically approved
3. Comparison of stable isotope records across the Eastern Equatorial Pacific, from the Biogenic Bloom to Present-day
Open this publication in new window or tab >>Comparison of stable isotope records across the Eastern Equatorial Pacific, from the Biogenic Bloom to Present-day
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The oceanographic evolution of the eastern equatorial Pacific Ocean (EEP) since the late Neogene is still under debate. One school of thought proposes weaker than modern equatorial upwelling and sustained warm sea surface temperatures (SSTs) during the late Miocene to early Pliocene, while another suggests stronger than modern upwelling and cool SSTs over this period. These opposing theories appear to be proxy dependent and new perspectives are needed. Bulk carbonate stable isotopes signals have shown potential to carry information on EEP mixed layer water properties including temperature but the relative contribution of different biogenic carbonate components, and thus the origin of these signals, remains uncertain. Here we measured δ13C and δ18O of bulk carbonate, and several finer fractions that concentrate coccoliths, from ODP Site 851 sediments over the last 7 Ma. These data are compared to a series of new and published planktic and benthic foraminifera and foraminifera fragment δ13C and δ18O records to help disentangle coccolith ecological and ocean signals, and refine the use of bulk carbonate d13C and d18O as palaeoceanographic proxies. Our results imply that, once coccolith vital effects are accounted for, bulk δ13C and δ18O records mixed layer signals shallower than the depth of the planktonic foraminifera Globigerinoides sacculifer. Higher bulk carbonate δ13C, δ18O, sedimentation rate and opal content, combined with lower CaCO3 and >63 µm content during the late Miocene and until 4.6 Ma imply enhanced upwelling and cool SSTs along the EEP at this time, supporting the biogenic bloom hypothesis.

National Category
Geology
Research subject
Marine Geology
Identifiers
urn:nbn:se:su:diva-168155 (URN)
Available from: 2019-04-23 Created: 2019-04-23 Last updated: 2019-04-25Bibliographically approved

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