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

Abstract

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