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.

地质过去的海水钙浓度（[Ca ²⁺ ] _SW）的重建对于我们对与长期气候变化有关的生物地球化学过程和元素循环的理解至关重要。已发表的新生代[Ca ²⁺ ] _SW估算值在变化的方向和幅度上互不相同，并且存在较大的不确定性。在这里，我们证明了有孔有孔虫中Na / Ca的潜力，可以作为重建海水Na / Ca新生代变化的新代理。由于Na ⁺（>> 50 Myr）在海洋中的停留时间长，有孔虫Na / Ca的变化原则上应反映[Ca ²⁺ ] _SW的变化。我们在活的浮游物种上的培养实验验证了该方法的有效性，表明有孔虫的Na / Ca在[Na ⁺ ] _SW保持恒定的情况下对[Ca ²⁺ ] _SW的变化有响应，这与之前在浅水底栖有孔虫和无机物上的实验一致方解石。文化研究表明，方解石的Na / Ca在较小程度上受方解石饱和状态（_方解石Ω ）的影响，但不受温度或盐度的影响。浮游有孔虫Trilobatus sacculifer的核心顶断面结果表明，随着水深的增加，Na / Ca和Sr / Ca的下降趋势类似，这可能与测试的溶解有关。与Sr的强协方差支持以下假设：Na的主要部分位于方解石测试中的晶格位置。

来自大西洋和太平洋的下层板状有孔虫记录始终显示，中新世中期的Na / Ca比更新世晚期的低13-28％。考虑温度，盐度和成岩作用的影响后，我们得出结论，观察到的Na / Ca下降核心的减少主要反映了海水钙浓度的变化。利用我们的培养实验和核心数据得出的标定值，我们重建了[Ca ²⁺ ] _SW，表明中新世中期的[Ca ²⁺ ] _SW比目前高46±22％，并以一定的模式向现在降低类似的底栖δ代表的新近纪气候演化¹⁸ Ø记录。[Ca ²⁺过去16 Myr的] _SW缩小了其他[Ca ²⁺ ] _SW代理所建议的范围，并可能提供一种新的方法来生成亚百万年分辨率下海水钙浓度的连续记录。