HR: 15:30h
AN: OS13A-08    [Abstracts]
TI: Glacial to Interglacial Changes in the Provenance and Flux of Sediment to the Cape Basin, South Atlantic: Implications for Indian-Atlantic Ocean Exchange
AU: * Franzese, A M
EM: franzese@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 91 Route 9W, Palisades, NY 10964 United States
AU: Hemming, S R
EM: sidney@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 91 Route 9W, Palisades, NY 10964 United States
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 91 Route 9W, Palisades, NY 10964 United States
AU: Anderson, R F
EM: boba@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 91 Route 9W, Palisades, NY 10964 United States
AB: The southeast Atlantic is a region where thermocline and intermediate waters are exchanged with the Indian Ocean by the ``Agulhas Leakage''. This leakage is thought to be an important source of heat and salt to the Atlantic Ocean, possibly acting as a positive feedback for the formation of North Atlantic Deep Water (NADW). Changes in the strength of NADW formation and the intensity of global thermohaline circulation (THC) have been called upon as major amplifiers for climate change during the Pleistocene glacial cycles, with shallower and probably weaker NADW formation during the cold periods of the glacial cycles, particularly during the Last Glacial Maximum (LGM), approximately 18,000 years ago. In this study, we use the radiogenic isotope compositions of strontium and neodymium (⁸⁷Sr/⁸⁶Sr and ε_Nd) of terrigenous sediments from the oceans surrounding South Africa as tracers of sediment provenance, and we use the initial excess ²³⁰Th of the bulk sediments as a constant flux proxy. Until now provenance and flux measures have been applied to separate problems, and this study is unique in using the combination of these two approaches. The combined measurements of sediment provenance and flux has great potential for tracing sediment transport patterns, and how they have changed through time. ⁸⁷Sr/⁸⁶Sr, ε_Nd and ²³⁰Th _xs⁰- normalized terrigenous flux measurements from Holocene and LGM sediment samples support the Agulhas Current as a major source of sediment to the southern Cape Basin. The results are consistent with 3 end-member mixing between sediment carried by (1) the Agulhas Current, (2) the South Atlantic or Antarctic Circumpolar Current (SAC or ACC) and (3) a local source from western South Africa. From a comparison of the two time-slices it is shown that a smaller proportion of sediment deposited in the Cape Basin and in the Natal Valley off the southeast coast of Africa is derived from the Agulhas Current during the LGM. In the Holocene the compositions can be approximately explained by a two-component mix between the Agulhas and local end-members while in the LGM they can be approximately explained by a two-component mix between the local and South Atlantic components. The flux results suggest a more sediment-laden ACC and/or SAC during the LGM, which could entirely explain the Cape Basin record. More vigorous deep currents south of Africa may have also increased the input of local and South Atlantic sediments to the Cape Basin and Natal Valley. The Agulhas Current was never entirely isolated from the Atlantic Basin; however, the possibility remains that there was a reduced Agulhas leakage, and hence less interocean exchange. The Cape Basin is not the best setting for making the evaluation of changes in the Agulhas Current and its Retroflection with sediment provenance/flux studies. Further investigation of sediment cores closer to the Agulhas Current and Retroflection will help to constrain whether or not the Agulhas Current was significantly different during the LGM.
DE: 1040 Radiogenic isotope geochemistry
DE: 1051 Sedimentary geochemistry
DE: 1635 Oceans (1616, 3305, 4215, 4513)
DE: 3022 Marine sediments: processes and transport
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005