Planktic foraminiferal Na/Ca: A potential proxy for seawater calcium concentration

The reconstruction of seawater calcium concentration ([Ca2+]SW) in the geologic past is crucial to our understanding biogeochemical processes and elemental cycling as linked long-term climate change. Published [Ca2+]SW estimates for Cenozoic differ from each other both direction magnitude change, are associated with large uncertainties. Here we demonstrate potential Na/Ca planktic foraminifera a new proxy reconstructing variations Na/Ca. Because long oceanic residence time Na+ (≫50 Myr), foraminiferal should principle reflect changes [Ca2+]SW. Our culture experiments on live species validate approach, showing that responds when [Na+]SW kept constant, consistent previous shallow water benthic foraminifer inorganic calcite. study suggests affected, small extent, by calcite saturation state (Ωcalcite) but not resolvably affected temperature or salinity. A core tops transect Trilobatus sacculifer shows similar decreasing trends Sr/Ca depth can be dissolution tests. strong covariance Sr supports hypothesis dominant fraction Na resides lattice positions within test. Down records Atlantic Pacific Oceans consistently show 13–28% lower during mid-Miocene than late Pleistocene. After considering effects temperature, salinity diagenesis, conclude observed down decrease primarily reflects concentration. Using calibrations generated top data reconstructed [Ca2+]SW, suggesting was 46 ± 22% higher at present, decreased toward present pattern resembling Neogene evolution represented δ18O record. 16 Myr narrows range suggested proxies, potentially offers way generate continuous sub-million years resolution.