Cenozoic pelagic Sr/Ca records: Exploring a link to paleoproductivity

Recent studies have revealed that Sr/Ca ratios of surface ocean carbonate producers are affected by productivity. In this study we explore a link between published bulk carbonate (primarily coccoliths) and planktonic foraminiferal Sr/Ca records and large-scale variations in paleoproductivity over the course of the Cenozoic. We constrain seawater Sr/Ca changes with benthic foraminiferal Sr/Ca records constructed from a number of sites from the Atlantic and Southern Ocean. In agreement with Lear et al. (2003) inferred seawater ratios vary little over the Cenozoic until the early Pliocene when ratios begin to rise steeply toward the present. Using the benthic foraminiferal-derived Sr/Ca seawater curve, we calculate the partitioning coefficient of Sr for bulk carbonate and planktonic foraminiferal Sr/Ca records. We show that over the past 50 myr, there have been two broad periods of enhanced partitioning of Sr relative to today, the Oligocene and the mid to late Miocene/Pliocene. Because these are two intervals for which we can cite evidence for a relative rise in oceanic nutrient levels, we believe that the Sr/Ca ratios of bulk carbonate and planktonic foraminiferal tests reflects the effects of oceanic nutrient levels on Sr uptake during hard part formation. Within the limits of the inferred seawater Sr/Ca record, the results of this study contribute a geologic perspective to recent laboratory and field studies that have raised the possibility that Sr incorporation into biogenic calcite is controlled by biogeochemical processes.