[MIS11-10] Foraminiferal Na/Ca suggests decreased seawater Ca concentration over the past 40 Myr.
★Invited Papers
The reconstruction of seawater calcium concentration ([Ca2+]SW) in the geologic past is crucial to our understanding of long-term changes in the carbon cycle as related to climate and tectonics. Here we propose that because of the long oceanic residence time of Na+ (>>50 Myr), variations in foraminifera Na/Ca potentially offer a new proxy of seawater calcium concentration in the past. We combined evidence from culture experiments on live foraminifers, core tops calibration and down-core measurements of benthic foraminifera to show that seawater [Ca2+]SW concentration was substantially lower between ~20-40 Ma, and increased to the modern level through the past 15 Myr. Combining the Neogene [Ca2+]SW record with our global reconstructions of Mass Accumulation Rates of the main marine calcifying groups for the past 15 Myr, used to assess variations in alkalinity fluxes from continental weathering and changes in the oceanic calcite compensation depth (CCD), we explore possible causes for the observed changes in seawater chemistry.