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Title: Insights into glacial terminations from a South Atlantic perspective
Author: Roberts, Jenny
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2016
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The last two glacial terminations represent the most recent, and best documented, periods of Earth warming in the geological record. During these terminations atmospheric CO\textsubscript{2 }rose by approximately 100 ppm and global mean temperatures increased by 4-6\textsuperscript{o}C. Whilst the driver for these deglaciations ultimately derives from changes in the insolation forcing at the edge of the atmosphere, feedbacks within the Earth\textquoteright s climate system act to amplify these small external forcings tipping the Earth from a cold glacial climate state to a warm interglacial climate state. A key question in Quaternary climate science is understanding which feedbacks are important in regulating global climate on glacial-interglacial timescales. On this topic, the Southern Ocean has long been considered to be an important player in regulating atmospheric CO\textsubscript{2 } on glacial-interglacial timescales. This thesis investigates some of the hypothesised drivers of changes in atmospheric CO\textsubscript{2 } on glacial-interglacial timescales by generating high-resolution multi-proxy records from the Southern Ocean spanning the last two glacial terminations. In particular, I focus on changes in the structure, circulation and biological productivity within the sub-Antarctic zone. A change in the deep ocean density structure has been hypothesised to have resulted in the release of CO\textsubscript{2 } from the deep ocean. Centennial records from the sub-Antarctic are used to reconstruct deep and intermediate water density for the first time. I demonstrate that timing of the major breakdown in the density gradient of the ocean significantly lagged the breakdown in the chemical gradient, suggesting that changes in the deep ocean density structure were not the major driver of the deglacial rise in atmospheric CO\textsubscript{2 }. Changes in the density structure of the Southern Ocean likely had significant implications for global circulation. In particular, the flow of low salinity water through the Drake Passage is thought to be important in setting the strength and geometry of Atlantic Overturning Circulation. Drake Passage through-flow speed was reconstructed from two sites in the central and northern margins of the Antarctic Circumpolar Current downstream of Drake Passage. These records suggest a very different structure of Antarctic Circumpolar flow through Drake Passage during glacial periods, and evidence significant changes in ocean temperature as a result of pronounced reductions in Drake Passage through-flow. The strength of the biological pump has long been identified as an important player in regulating atmospheric CO\textsubscript{2 }. In particular, a strong glacial increase in sub-Antarctic productivity has been observed at open ocean sites in the South Atlantic and Indian Ocean. However, the glacial-interglacial changes in productivity in sub-Antarctic shelf settings are less well-documented. The new high-resolution records presented here from the sub-Antarctic southwest Atlantic suggest a significant change in the CaCO\textsubscript{3}:C\textsubscript{org} ratio which likely has implications for the surface ocean\textquoteright s ability to uptake CO\textsubscript{2 }.
Supervisor: Hodell, David A. ; Peck, Vicky L. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Antarctic Intermediate Water ; Southwest Atlantic ; Paleoceanography