Pleistocene – recent reconstruction of Antarctic climate in the Amundsen Sea

Abstract

The West Antarctic Ice Sheet (WAIS) is highly sensitive to atmospheric and oceanographic change, and, at least since the beginning of satellite era (1960’s), has been rapidly losing ice from large outlet glaciers such as Thwaites and Pine Island Glaciers in the Amundsen Sea Embayment. Due to its sensitivity to climate change, this region of Antarctica is an area of intense and urgent interest for geological, oceanographic, and glaciological research. Improving our understanding of the impact changing oceanographic and atmospheric conditions has on the cryosphere for both the past and the present in this region is key for predicting the WAIS response to a warming climate and, hence, future sea level projections. International Ocean Discovery Program (IODP) Expedition 379 sailed to the Amundsen Sea in order to obtain new sediment records for reconstruction of past WAIS history. In Chapter 2, I present two new, independent chronostratigraphies for the Latest Pliocene to Pleistocene section of IODP Site U1533, which was drilled on the continental rise of the Amundsen Sea Embayment. New age models for drift sediments recovered at this site were constructed using i) X-ray fluorescence (XRF) scan-derived barium cycles as a palaeoproductivity proxy and correlation to global deep-sea oxygen isotope stratigraphy, and ii) geomagnetic relative palaeointensity (RPI) correlation to published reference records. Both age models correlate well to global reference records, but have an average ±12 kyr offset, due to uncertainties in their methods (e.g., lead/lags between local productivity and global δ18O records; sediment magnetisation lock-in effects). A final hybrid age model is developed for Site U1533 and is applied, in conjunction with published Antarctic RPI records, in the construction of a regional RPI stack (ANT-1600) that is strongly coherent with a published global stack on time scales 20–200 kyrs. In Chapter 3, I present a glacial-interglacial history of Site U1533 as recorded by u-channel and rock magnetic analyses for the same Plio-/Pleistocene sequence investigated in Chapter 2. In these records, I identify a higher concentration of magnetically coarser material present during glacial periods. The main magnetic mineralogy present is magnetite, with maghemite contributing to the magnetic assemblage in varying quantities (typically higher during glacial periods). During glacial periods, magnetic particles are interpreted as sourced from the Amundsen Sea interior. At ~2.5 Ma, there is a large change in magnetic coercivity at Site U1533 that is interpreted to be the result of a change in bottom water conditions as atmospheric and oceanographic conditions after Northern Hemisphere glaciation intensifies. In Chapter 4, carbon and oxygen stable isotope analyses of eight benthic foraminiferal species are presented for sites on the Amundsen Sea shelf and compared to bottom water isotopic values for the same sites. Two species (Trifarina angulosa and Bulimina aculeata) show promise in reliably recording bottom water oxygen and carbon isotope values, and support a regional change in δ18O and, to a lesser extent, δ13CDIC, with proximity to the ice sheet and variations in local productivity. This study helps to refine our understanding of Antarctic foraminifera and their ability to record stable isotopes. Future work could include using these species to reconstruct downcore changes in order to detect changes in bottom water proximal to important outlet glaciers such as Thwaites Glacier in the Amundsen Sea Embayment.

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Identifiers

ORCID for Chuang Xuan: orcid.org/0000-0003-4043-3073

ORCID for Paul Wilson: orcid.org/0000-0001-6425-8906

ORCID for Steven Bohaty: orcid.org/0000-0002-1193-7398

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