Abstract:
Records of seawater chemistry help constrain temporal variations in geochemical processes that impact the global carbon cycle and climate through Earth's history. Here we reconstruct Cenozoic seawater Sr/Ca (Sr/Ca-sw) using fossil Conus and turritellid gastropod Sr/Ca. Combined with an oxygen isotope paleotemperature record from the same samples, the gastropod record suggests that Sr/Ca-sw was slightly higher in the Eocene (similar to 11.4 +/- 3 mmol/mol) than today (similar to 8.54 mmol/mol) and remained relatively stable from the mid- to late Cenozoic. We compare our gastropod Cenozoic Sr/Ca-sw record with a published turritellid gastropod Sr/Ca-sw record and other published biogenic (benthic foraminifera, fossil fish teeth) and inorganic precipitate (calcite veins) Sr/Ca-sw records. Once the uncertainties with our gastropod-derived Sr/Ca-sw are taken into account the Sr/Ca-sw record agrees reasonably well with biogenic Sr/Ca-sw records. Assuming a seawater [Ca] history derived from marine evaporite inclusions, all biogenic-based Sr/Ca-sw reconstructions imply decreasing seawater [Sr] through the Cenozoic, whereas the calcite vein Sr/Ca-sw reconstruction implies increasing [Sr] through the Cenozoic. We apply a simple geochemical model to examine the implications of divergence among these seawater [Sr] reconstructions and suggest that the interpretation and uncertainties associated with the gastropod and calcite vein proxies need to be revisited. Used in conjunction with records of carbonate depositional fluxes, our favored seawater Sr/Ca scenarios point to a significant increase in the proportion of aragonite versus calcite deposition in shelf sediments from the Middle Miocene, coincident with the proliferation of coral reefs. We propose that this occurred at least 10 million years after the seawater Mg/Ca threshold was passed, and was instead aided by declining levels of atmospheric carbon dioxide.
