HR: 09:45h
AN: PP41E-08    [Abstracts]
TI: Fidelity of δ¹⁸Oseawater estimates using foraminiferal shell Mg/Ca and δ¹⁸O
AU: * deMenocal, P B
EM: peter@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Route 9W, Palisades, NY 10964, United States
AU: Arbuszewski, J A
EM: jarbo@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Route 9W, Palisades, NY 10964, United States
AU: Kaplan, A
EM: akaplan@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Route 9W, Palisades, NY 10964, United States
AU: Marley, B
EM: mbice@whoi.edu
AF: College of William & Mary, Geology and Environmental Science, Williamsburg, VA 23187, United States
AB: Paired foraminiferal shell Mg/Ca and δ¹⁸O analyses are widely used to estimate surface ocean δ¹⁸Oseawater, a proxy for surface salinity. We present a systematic assessment of the fidelity of shell- based δ¹⁸Oseawater estimates using an Atlantic meridional transect (43N-25S) of 58 coretop measurements of G. ruber (white) Mg/Ca and δ¹⁸O. Core locations were selected near the flanks of the mid-Atlantic ridge, at or above the modern lysocline depth, from trigger weight samples only, and of late Holocene age. G. ruber (white) shells for Mg/Ca analyses were cleaned using the full reductive-oxidative cleaning protocol. These results document a large, systematic offset between shell-derived and observed δ¹⁸Oseawater values. With increasing salinity, shell Mg/Ca compositions are significantly elevated above their expected values determined from isotopic calcification temperatures. This "excess Mg" residual is highly correlated with surface salinity or alkalinity (r2=0.75). This "excess Mg/Ca" effect is apparent in published Atlantic G. ruber (white) Mg/Ca and δ¹⁸O data. Temperature and salinity signals are evidently embedded in both shell δ¹⁸O and Mg/Ca and this actually enhances detection of salinity gradients. We develop multivariate regression equations that accurately estimate mean annual SST (r2=0.8) and salinity (r2=0.8) values along the transect using shell Mg/Ca, δ¹⁸O, and bottom water Δ CO₃^2- as predictors.
DE: 1065 Major and trace element geochemistry
DE: 3344 Paleoclimatology (0473, 4900)
DE: 4924 Geochemical tracers
DE: 4954 Sea surface temperature
DE: 4994 Instruments and techniques
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting