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

Zhou, Xiaoli, Rosenthal, Yair, Haynes, Laura, Si, Weimin, Evans, David, Huang, Kuo Fang, Hönisch, Bärbel and Erez, Jonathan (2021) Planktic foraminiferal Na/Ca: a potential proxy for seawater calcium concentration. Geochimica et Cosmochimica Acta, 305, 306-322. (doi:10.1016/j.gca.2021.04.012).

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 (Ω_calcite) 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.

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Contributors

Author: David Evans

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