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Neogene diatoms from the Southern Ocean; tiny fossils, big questions

Mitchison, Freya 2019. Neogene diatoms from the Southern Ocean; tiny fossils, big questions. PhD Thesis, Cardiff University.
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The Neogene (c. 23-2.6 Ma) is the most recent period of Earth history in which atmospheric CO2 was last >400 ppm; possibly associated with dramatic fluctuations in Antarctic ice volume and thus eustatic sea level. With continental configurations broadly similar to today, the Neogene therefore provides an opportunity to characterise and explore the different feedbacks and configurations of Earth’s climate that occur under climatic forcing conditions either similar to today or to projected anthropogenic warming scenarios. However, many important questions in Neogene paleoclimatology remain unanswered. For example, the role in and response of the Southern Ocean, a key modulator of the Antarctic ice sheet and a disproportionately important player in the carbon cycle, to major episodes of climate change. In this thesis, diatom-based proxies, which tend to be well-preserved in the Southern Ocean, are developed and applied to some of the big questions in Neogene paleoclimatology. Here, high resolution quantitative diatom assemblage records across the Middle Miocene Climate Transition (15-12 Ma) at a latitudinal transect of three Antarctic Zone ODP Sites (747, 744 and 1165), and a record at Site 1165 extending to the present, provide paleoceanographic insights. For example, the expansion of marine ice into Prydz Bay at 13.8 Ma, followed by eccentricity-paced Southern Ocean cooling and intensification of the Antarctic Circumpolar Current (ACC) at c. 13 Ma. Further cooling and full expression of the ACC during the Late Miocene was interspersed with transient warm intervals, possibly associated with ice sheet retreat. Unique Early Pliocene paleoceanography is indicated by diatom assemblages at high latitude circum-Antarctic sites, including Site 1165, dominated by Thalassiothrix, Thalassionema and Trichotoxon diatoms. These assemblages suggest substantially increased high latitude oceanic upwelling, which could have provided a source of heat contributing to Pliocene ice sheet retreat. Finally, the first Pliocene Southern Ocean diatom silica stable oxygen isotope record (δ18Odiatom) at Site 1165 (3.45-3.75 Ma) explores the application of this proxy to Neogene material, alongside a geochemical assessment of sample purity. A strong correlation between Al/Si and δ18Odiatom (r=-0.89, p=<0.01, n=46) suggests clay mineral-mediated secondary isotope exchange has overprinted the environmental signature at Site 1165 which, in the context of published and unpublished data, may be either age- or site-dependant. Any future study using δ18Odiatom, particularly within Neogene material, must therefore include a geochemical assessment of sample purity and carefully consider the correlation between δ18Odiatom and Al/Si before interpreting δ18Odiatom as a paleoenvironmental signal.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Subjects: Q Science > QE Geology
Funders: Cardiff University
Date of First Compliant Deposit: 22 October 2019
Last Modified: 23 Jul 2020 02:15

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