Effects of seawater carbonate ion concentration and temperature on shell U, Mg, and Sr in cultured planktonic foraminifera

We investigate the sensitivity of U/Ca, Mg/Ca, and Sr/Ca to changes in seawater [CO32-] and temperature in calcite produced by the two planktonic foraminifera species, Orbulina universa and Globigerina bulloides, in laboratory culture experiments. Our results demonstrate that at constant temperature, U/Ca in O. universa decreases by 25 +/- 7% per 100 mumol [CO32-] kg(-1), as seawater [CO32-] increases from 110 to 470 mumol kg(-1). Results from G. bulloides suggest a similar relationship, but U/Ca is consistently offset by similar to + 40% at the same environmental [CO32-]. In O. universa, U/Ca is insensitive to temperature between 15degreesC and 25degreesC. Applying the O. universa relationship to three U/Ca records from a related species, Globigerinoides sacculifer, we estimate that Caribbean and tropical Atlantic [CO32-] was 110 +/- 70 mumol kg(-1) and 80 +/- 40 mumol kg(-1) higher, respectively, during the last glacial period relative to the Holocene. This result is consistent with estimates of the glacial-interglacial change in surface water [CO32- ] based on both modeling and on 3 boron isotope pH estimates. In settings where the addition of U by diagenetic processes is not a factor, down-core records of foraminiferal U/Ca have potential to provide information about changes in the ocean's carbonate concentration. Below ambient pH (pH < 8.2), Mg/Ca decreased by 7 +/- 5% (O. universa) to 16 +/- 6% (G. bulloides) per 0.1 unit increase in pH. Above ambient pH, the change in Mg/Ca was not significant for either species. This result suggests that Mg/Ca-based paleotemperature estimates for the Quaternary, during which surface-ocean pH has been at or above modern levels, have not been biased by variations in surface-water pH. Sr/Ca increased linearly by 1.6 +/- 0.4% per 0.1 unit increase in pH. Shell Mg/Ca increased exponentially with temperature in O. universa, where Mg/Ca = 0.85 exp (0.096*T), whereas the change in Sr/Ca with temperature was within the reproducibility of replicate measurements. Copyright (C) 2004 Elsevier Ltd.