HR: 15:10h
AN: PP43E-07    [Abstracts]
TI: Deep Pacific Carbonate Chemistry During the LGM and Deglaciation: Reconstructing ΔCO₃^2- using foraminiferal Mg/Ca ratios.
AU: * Fehrenbacher, J S
EM: jsf1@uchicago.edu.com
AF: University of Chicago, Department of Geophysical Sciences 5734 S. Ellis Ave., Chicago, IL 60561, United States
AU: Martin, P
EM: pmartin@uchicago.edu
AF: University of Chicago, Department of Geophysical Sciences 5734 S. Ellis Ave., Chicago, IL 60561, United States
AU: Eshel, G
EM: geshel@simons-rock.edu
AF: Bard College, Simon's Rock College of Bard 84 Alford Road, Great Barrington, MA 01230, United States
AB: Deep Pacific carbonate preservation reconstructions are at odds suggesting both enhanced preservation (Farell and Prell, 1989) and increased dissolution (Broecker and Clark, 2003). We present a reconstruction of the western equatorial Pacific deep-water carbonate ion profile for the Last Glacial Maximum (LGM) and a deglacial preservation event based on changes in the Mg/Ca ratio of planktonic foraminifera with increased water depth. Mg/Ca measurements were generated for three species commonly used in paleothermometry, G. ruber, G. sacculifer, and N. dutertrei, obtained from a suite of six cores spanning 1.6 to 4.4 km on the Ontong Java Plateau. Mg/Ca ratios obtained from the shallow core are used to reconstruct sea surface and thermocline temperatures. Changes in the Mg/Ca ratio with increased water depth are used to assess temporal changes in the deep-water carbonate saturation state and generate a paleo-carbonate ion profile. Our data confirm the strong dissolution effect on N. dutertrei Mg/Ca. The N. dutertrei Mg-derived LGM ΔCO₃^2- profile suggests that preservation was enhanced in the shallowest three cores in comparison to modern. In the modern deep Pacific, there is little change in the carbonate ion concentration with increasing water depth; decreases in the calcite saturation state are primarily controlled by the pressure effect. Overall the carbonate ion concentration gradient in the deep Pacific is steeper during the LGM than today. Our reconstruction suggests that there were changes in both ocean carbonate chemistry and circulation. While the shape of our reconstructed profile is consistent with the gradient derived from shell weight data (Broecker and Clark, 2003), the carbonate ion concentrations our data imply are higher and, at least in the shallowest three cores, more consistent with reconstructions based on % carbonate (e.g. Farell and Prell, 1989). During the deglaciation at ~14 ky bp a preservation event occurred that is marked by increased Mg/Ca in N. dutertrei and the presence of pteropods in the shallowest three cores. During this interval, the data imply a whole ocean increase in ΔCO₃^2- with enhanced preservation in all cores shallower than 4.0 km. The shape of the reconstructed gradient is similar to the modern gradient which suggests that by 14 ky bp the circulation in the Pacific was similar to modern circulation.
DE: 4200 OCEANOGRAPHY: GENERAL
DE: 4825 Geochemistry
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 4924 Geochemical tracers
DE: 4926 Glacial
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting