HR: 0830h
AN: PP11A-0209    [PDF]
TI: Unraveling primary and diagenetic controls over U/Ca in planktonic foraminifera
AU: * Russell, A D
EM: russell@geology.ucdavis.edu
AF: Department of Geology, University of California, One Shields Avenue, Davis, CA 95616 United States
AU: Schmidt, M
EM: schmidt@geology.ucdavis.edu
AF: Department of Geology, University of California, One Shields Avenue, Davis, CA 95616 United States
AB: The quest to understand the marine controls over atmospheric carbon dioxide has led paleoceanographers to seek reliable reconstructions of past seawater carbonate chemistry. Laboratory culture experiments have shown that the uranium/calcium ratio of planktonic foraminifera ({\it O. universa} and {\it G. bulloides}) is sensitive to changes in seawater carbonate ion concentration, decreasing by 28% per 100 $\mu$mol kg$^{-1}$ increase in [CO$_{3}$$^{2-}$]. However, the application of U/Ca as a proxy for carbonate ion has been limited by its apparent association with a diagenetic manganese-rich phase in many cores. Here we present a U/Ca profile from ODP999A in the Caribbean over the last 140 kyr, showing that U/Ca in {\it G. ruber} covaries with pCO$_{2}$ from the Vostok ice core, consistent with a primary signal controlled by variations in surface-water [CO$_{3}$$^{2-}$]. The U/Ca record from ODP999A is similar to shorter records from two other Caribbean cores ({\it G. ruber} VM28-122 and {\it G. sacculifer} in CP6001-4), in spite of striking differences in their Mn/Ca records, suggesting that Mn-rich phases are not the dominant control over U/Ca in these cores. When U/Ca from ODP999A is corrected for covariation with Mn/Ca, its correspondence with the Vostok CO$_{2}$ record improves. We estimated surface-water pCO$_{2}$ using the ODP999A U/Ca record and SST derived from {\it G. ruber} Mg/Ca. Paleosalinities were obtained from Mg/Ca-SST and {\it G. ruber} $\delta$$^{18}$O records from the same core. Finally, we assumed a constant alkalinity/salinity ratio throughout the past 140 kyr. The calculated pCO$_{2}$ shows maxima during the Holocene and MIS Stage 5, with glacial (MIS Stage 2) pCO$_{2}$ 20% lower than Holocene levels. The correspondence between the Mn-corrected U/Ca record from ODP999A and the Vostok pCO$_{2}$ record is consistent with a strong influence of surface-water [CO$_{3}$$^{2-}$] over foraminiferal U/Ca in this core.
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1065 Trace elements (3670)
DE: 4835 Inorganic marine chemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting