HR: 0830h
AN: PP11A-0205    [PDF]
TI: Glacial-Interglacial Change in Seawater Carbonate Chemistry Inferred From Foraminiferal Mg/Ca
AU: * Coombs, M
EM: coombs@umail.ucsb.edu
AF: University of California, Department of Geological Sciences, Santa Barbara, CA 93106 United States
AU: Fehrenbacher, J S
EM: jsf1@uchicago.edu
AF: The University of Chicago, Department of Geophysical Sciences 5734 S. Ellis Avenue, HGS 421, Chicago, IL 60637 United States
AU: Lea, D W
EM: lea@geol.ucsb.edu
AF: University of California, Department of Geological Sciences, Santa Barbara, CA 93106 United States
AU: Martin, P A
EM: pmartin@uchicago.edu
AF: The University of Chicago, Department of Geophysical Sciences 5734 S. Ellis Avenue, HGS 421, Chicago, IL 60637 United States
AB: Carbonate ion concentration, which is associated with both the concentration of dissolved CO$_{2}$ in and pH of seawater, can be used to infer changes in the carbon cycle and ocean circulation over time. Recent studies have presented reconstructions of glacial-interglacial deep-water carbonate chemistry changes using shell thickness (Broecker and Clark; 2001, 2003) and foraminiferal assemblages (Anderson and Archer, 2002); however, these independent proxies yield conflicting views of glacial seawater chemistry. We explore an alternative method of quantifying variation in carbonate ion concentration of seawater over time using changes in planktonic foraminiferal shell chemistry. Many studies have shown the utility of planktonic foraminiferal Mg/Ca analysis as a paleotemperature indicator; detailed core-top studies demonstrate that Mg/Ca is also sensitive to shell dissolution. Dekens {\it et al}. (2002) have quantified the relation between Mg/Ca, water depth, and dissolution for several species of planktonic foraminifera. Here, we use new Mg/Ca data from a set of cores from the Ontong Java Plateau, western equatorial Pacific Ocean (1.6 - 4.0 km), a set of cores from Ceara Rise, western equatorial Atlantic Ocean (2.8 - 4.0 km), and core-top calibrations defined by Dekens {\it et al}. (2002) to quantify temporal changes in carbonate ion concentration of seawater. By applying Mg/Ca calibrations to the shallowest core from each region, we infer sea surface temperature. Differences in Mg/Ca between the shallowest and deeper cores in each region should reflect differences in preservation, indicating differences in deep-water carbonate chemistry. Preliminary Mg/Ca results from the Pacific reveal changes in both sea-surface temperature and dissolution between the Last Glacial Maximum (LGM) and the Holocene. Mg/Ca temperatures reconstructed from {\it G. ruber} reveal $\sim$3 $\deg$C lower temperature during the LGM, consistent with previous results from the region (Lea {\it et al.}, 2000). Smaller planktonic Mg/Ca differences between the shallow (1.6 km) and deep (3.4 km) Pacific cores during the LGM imply better preservation and therefore higher deep-water carbonate ion concentration ($\sim$20 $\mu$mol/kg higher). These preliminary {\it G. ruber} results are consistent with the results of Broecker and Clark (2001); however, reconstructions of the carbonate ion gradient using a second species, {\it G. sacculifer}, imply opposite changes. These differences may reflect the different depth habitats of the two species.
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1635 Oceans (4203)
DE: 4267 Paleoceanography
DE: 4271 Physical and chemical properties of seawater
DE: 4835 Inorganic marine chemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting