Dissertation/Thesis Abstract

Paleoceanography of the Agulhas Current and retroflection determined by radiogenic isotopes in deep -sea sediments
by Franzese, Allison M., Ph.D., Columbia University, 2008, 289; 3299357
Abstract (Summary)

One of the most widely studied aspects of paleoceanography is the thermohaline circulation (THC). Many studies have focused on the deep arm of the THC, trying to assess changes in the strength of North Atlantic Deep Water (NADW) formation and its characteristic properties over glacial-interglacial and millennial timescales. Far fewer studies have focused on the surface return flow of the THC. The Agulhas Leakage represents the warm return route of the THC, bringing surface and intermediate waters from the tropical Indian Ocean into the Atlantic. The Agulhas Leakage occurs as turbulent eddies form when the Agulhas Current, the western boundary current of the Indian Ocean, retroflects after passing the southern tip of Africa. This thesis uses the stable products of long-lived radioactive isotopes to reconstruct the Agulhas Current and its Retroflection during the Last Glacial Maximum (LGM, ∼20,000 years ago).

The radiogenic isotopes of strontium and neodymium (87Sr/ 86Sr and ϵNd) in terrigenous sediments are used as tracers of provenance (source regions), and initial excess thorium-230 ( 230Thxs°) as a constant-flux proxy, to quantify particle flux and sediment redistribution. Results imply that strong surface currents are important for long distance sediment transport in the South Atlantic, while bottom currents play a secondary role in sediment redistribution. A map of the 87Sr/86Sr of modern sediments is used to show that the sediments in regions underlying the Agulhas Current and the Agulhas Return Current have higher 87Sr/86Sr than sediments in surrounding regions. The high 87Sr/ 86Sr character can therefore be used to trace the path of the Agulhas Current, Retroflection and Return Current through time.

In Chapter 2, combining the 87Sr/86Sr and ϵ Nd measurements with concentrations of Sr, Nd, and their parent elements Rb and Sm, I define three major sedimentary components that contribute to the mixing south of Africa, and calculate the relative importance of each during both the modern and the LGM. Combining this with the constant-flux proxy 230Thxs°, I demonstrate that the Agulhas Current and Leakage were both probably weaker during the LGM than they are today. In Chapter 3, I use a map of terrigenous 87Sr/ 86Sr to show that the LGM Agulhas Retroflection was close to its modern location. In Chapter 4, I expand the geochemical dataset to include a large suite of major and trace element concentrations, and several analyses on grain size fractions. These results, along with sortable silt mean size further implicate provenance changes as the most important factor controlling the composition of sediments in the Agulhas Current and Retroflection region. I present a composite record of down-core measurements of terrigenous 87Sr/86Sr from these cores in Chapter 5. Chapter 6 presents some steps toward measuring the terrigenous 87Sr/ 86Sr of suspended particles to begin calibrating the proxy.

This thesis work provides us with new insights about the Agulhas Current system both today and at the LGM, from the unique perspective of the terrigenous sediments. It also serves as an example of what we can learn from the combination of several terrigenous sediment proxies.

Indexing (document details)
Advisor: Hemming, Sidney M.
School: Columbia University
School Location: United States -- New York
Source: DAI-B 69/01, Dissertation Abstracts International
Subjects: Physical oceanography, Geochemistry
Keywords: Agulhas Current, Deep-sea sediments, Radiogenic isotopes, Retroflection
Publication Number: 3299357
ISBN: 978-0-549-43056-8
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