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

Recent studies have revealed that Sr/Ca ratios of coccolithophores may be affected by productivity. Here we compile published Sr/Ca data from bulk carbonate, fine fraction sediment, and planktonic foraminiferal Sr/Ca records that span the past 60 Myr and attempt to place these records into a paleoceanographic framework. We account for changes in seawater Sr/Ca ratios using the curve of Lear et al. [2003] and discuss our observations with respect to changes in the partitioning coefficient of Sr through time. We discuss limitations associated with postburial processes, temporal changes in the coccolith/planktonic foraminiferal rain ratio, and the coccolith species composition of the sediment. Despite these caveats, we are able to show that in the bulk and fine fraction carbonate records there are two broad periods of enhanced partitioning of Sr relative to today, the Oligocene, and the middle to late Miocene/early Pliocene. Because these are two intervals for which we can cite evidence for a relatively productive ocean on the regional or global scale, we believe that the Sr/Ca ratios of bulk carbonate can be explained, at least in part, by the effects of oceanic nutrient levels on Sr uptake during calcification of coccoliths, which make up the vast majority of these sediments. 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.