HR: 1340h
AN: PP43C-1535    [Abstracts]
TI: Implications of Seawater Mg/Ca Variability for Plio-Pleistocene Climate reconstruction
AU: Lea, D W
EM: lea@geol.ucsb.edu
AF: University of California, Santa Barbara, Santa Barbara, CA 93106, United States
AU: * Medina-Elizalde, M A
EM: martinmedina@umail.ucsb.edu
AF: University of California, Santa Barbara, Santa Barbara, CA 93106, United States
AU: Fantle, M S
EM: mfantle@geosc.psu.edu
AF: Penn State University, University Park, Pennsylvania, PA 16802, United States
AB: Recent reconstructions of Mg and Ca concentrations of seawater indicate that seawater Mg/Ca has changed significantly on time scales of thousands of years. This study examines the implications of adjusting available equatorial Pacific SST records based on Mg/Ca paleothermometry to account for the inferred past variations of seawater Mg/Ca. The results suggest that both the cold and the warm regions of the equatorial Pacific were much warmer during the Upper Pliocene (32-34° C), and that both regions experienced a marked cooling from ~4 Ma to ~0.8 Ma. The new interpretation of foraminiferal Mg/Ca creates a discrepancy with alkenone-based SST records from the eastern equatorial Pacific which might be due to either overestimation of changes in past seawater Mg/Ca or to factors affecting the interpretation of the alkenone-unsaturation index. The adjusted SST records are consistent with the hypothesis that higher levels of greenhouse gases maintained the warmth of the early Pliocene, and that the cooling implied by these records was at least partly driven by a drop in the atmospheric concentration of greenhouse gases. We used the adjusted warm pool record to calculate the level of greenhouse forcing, expressed as atmospheric CO2 concentration, required to explain Pliocene SSTs. Modern warm pool SST at 806B is 29.2° C, whereas peak reconstructed Pliocene SSTs were 34° C. In our calculations, we consider a range of climate sensitivity factors (l = 0.75, 1.3 and 1.5° C per Wm-2) that reflect the range from fast climate feedbacks only to those that include slow geological feedbacks such as ice sheet and surface albedo changes. For l = 0.75, 1.3 and 1.5° C per Wm-2, we estimate Pliocene atmospheric CO2 concentrations to have been 985, 580 and 525 ppm, respectively. For comparison, current estimates of Pliocene pCO2 vary between ~250 ppm and 560 ppm. The two lower values overlap with previously published estimates of Pliocene pCO2. The critical point of this analysis is that the temperature range that we calculate using a variable seawater Mg/Ca curve is not unrealistic, when considered in the context of Pliocene pCO2 estimates.
DE: 1065 Major and trace element geochemistry
DE: 4835 Marine inorganic chemistry (1050)
DE: 4922 El Nino (4522)
DE: 4954 Sea surface temperature
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting