HR: 09:00h
AN: PP31B-05    [Abstracts]
TI: Stratification and Circulation of the Glacial Ocean: Reconstructing Watermass Geometry and Circulation with Nd Isotopes
AU: * Piotrowski, A M
EM: arcturus@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 1000 Rt. 9W, Palisades, NY 10964 United States
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 1000 Rt. 9W, Palisades, NY 10964 United States
AU: Hemming, S R
EM: sidney@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 1000 Rt. 9W, Palisades, NY 10964 United States
AB: One of the most important debates in paleoclimate research is the link between ocean circulation and climate change. On glacial-interglacial timescales, global climate is driven by Milankovich orbital cycles, though the resulting insolation variations are small and require amplifying mechanisms. Changes in the strength of global "conveyor-belt" ocean circulation is one possible amplifying mechanism, and abrupt switches between circulation modes may have triggered rapid climate changes. Understanding the ocean-climate link has been difficult because nutrient-based proxies of ocean circulation disagree with each other. During the last glacial period, benthic foraminiferal carbon isotopes suggest substantially weaker North Atlantic Deep Water (NADW) formation and export, while benthic Cd/Ca indicate a strong flow of North Atlantic -sourced water at intermediate depths. Non-circulatory effects (including fractionation during calcite dissolution, changes in global carbon budget, air-sea gas equilibration, and porewater effects) are known to overprint these proxies, likely causing this discrepancy. Neodymium isotopes can resolve this disagreement because it does not share the same sources of error as nutrient-based proxies. We report Nd isotopes measured on Fe-Mn leaches from sites located throughout the South Atlantic. These sites range in depth from 2000 - 5000 mbsl, allowing a three-dimensional perspective of South Atlantic watermass geometry. The leachates have marine Sr isotopic composition. Coretop samples have Nd isotopic compositions which match overlying bottom waters. This coretop calibration is consistent with known vertical and horizontal geometry of NADW and Antarctic Bottom Water. Samples from the last glacial maximum show a coherent glacial-interglacial pattern of circulation change which is most simply interpreted as indicating a reduction in the amount of NADW reaching the South Atlantic. The relative contribution of NADW to the glacial South Atlantic was less at all depths. The Nd isotopic compositions of Cape Basin sites (>2000 mbsl) range from -6.4 to -5.5, indicating that Southern-sourced water dominated all depths. This last glacial profile shows a reduction of vertical stratification of watermass provenance, and suggests that the strong vertical structure apparent in nutrient-based proxy studies may have been caused by depth-dependant non-circulatory effects. Finally, this study places downcore Nd isotope records from the South Atlantic in a consistent framework of glacial-interglacial circulation change.
DE: 4267 Paleoceanography
DE: 4283 Water masses
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 1635 Oceans (4203)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting