Abstract The Mg/Ca of planktic foraminifera is widely used to determine past surface ocean temperatures but temperature is not the only factor that controls test Mg/Ca. Here we quantify the combined influence of seawater temperature, carbon chemistry, and cation chemistry on Orbulina universa Mg/Ca, based on experimental cultures where these factors were varied independently and simultaneously. We fit a new empirical multi-parameter model that quantifies the effects of each of these variables on the Mg/Ca of O. universa: Mg/Ca O. universa = Mg/Ca sw A · DIC sw B · exp C · C a sw + D · T + E We extend our approach to published Mg/Ca from cultured and sediment trap collected Globigerinoides ruber, and show that a similar equation can be used to describe test Mg/Ca, using [CO32−]sw or pH in place of DIC. The cause of this difference is unknown, but the sensitivity of these two ecophysiologically similar species to different carbon chemistry parameters highlights the uncertainty inherent in applying modern Mg/Ca calibrations to uncalibrated and/or extinct species. In both O. universa and G. ruber, Mg/Casw, [Ca]sw and carbon chemistry (DIC, [CO32−]sw or pH) each modulate the sensitivity of Mg/Ca to temperature. These variables are not constant in the ocean over time-scales relevant to palaeotemperature reconstructions, and must be accounted for when deriving temperature from foraminiferal Mg/Ca. Over time-scales longer than the residence times of Mg and Ca, secular changes in Mg/Casw and [Ca]sw will exert a primary influence on foraminiferal Mg/Ca and its sensitivity to temperature. On shorter timescales, co-variance between temperature and seawater carbon chemistry will influence Mg/Ca-derived temperature estimates. This is particularly relevant to transient hyperthermal events, where the Mg/Ca palaeothermometer is used to evaluate the relationship between atmospheric CO2 and ocean temperature. We explore the potential impact of these effects in an illustrative re-interpretation of Mg/Ca records across the Paleocene-Eocene Thermal Maximum.

中文翻译：

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限制对浮游有孔虫 Mg/Ca 的多重控制

摘要 浮游有孔虫的 Mg/Ca 被广泛用于确定过去的海洋表面温度，但温度并不是控制测试 Mg/Ca 的唯一因素。在这里，我们量化了海水温度、碳化学和阳离子化学对 Orbulina universa Mg/Ca 的综合影响，基于这些因素独立且同时变化的实验培养。我们拟合了一个新的经验多参数模型，该模型量化了这些变量中的每一个对 O. universa 的 Mg/Ca 的影响：Mg/Ca O. universa = Mg/Ca sw A·DIC sw B·exp C·C a sw + D · T + E 我们将我们的方法扩展到来自培养和沉积物捕集器收集的 Globigerinoides ruber 的已发布 Mg/Ca，并表明类似的方程可用于描述测试 Mg/Ca，使用 [CO32-]sw 或 pH DIC的地方。这种差异的原因尚不清楚，但是，这两种生态生理学相似的物种对不同碳化学参数的敏感性突出了将现代 Mg/Ca 校准应用于未校准和/或灭绝物种所固有的不确定性。在 O. universa 和 G. ruber 中，Mg/Casw、[Ca]sw 和碳化学（DIC、[CO32-]sw 或 pH）各自调节 Mg/Ca 对温度的敏感性。在与古温度重建相关的时间尺度上，这些变量在海洋中并不是恒定的，并且在从有孔虫 Mg/Ca 推导温度时必须考虑在内。在比 Mg 和 Ca 的停留时间更长的时间尺度上，Mg/Casw 和 [Ca]sw 的长期变化将对有孔虫 Mg/Ca 及其对温度的敏感性产生主要影响。在较短的时间范围内，温度和海水碳化学之间的协方差将影响镁/钙衍生的温度估计。这与瞬态高温事件特别相关，其中 Mg/Ca 古温度计用于评估大气 CO2 与海洋温度之间的关系。我们在对古新世-始新世热最大值的 Mg/Ca 记录的说明性重新解释中探索了这些影响的潜在影响。