Seawater calcium isotope ratios across the Eocene-Oligocene Transition from Pacific Ocean - Marine Barite

During the Eocene-Oligocene transition (EOT, ca. 34 Ma), Earth's climate cooled significantly from a greenhouse to an icehouse climate, while the calcite (CaCO3) compensation depth (CCD) in the Pacific Ocean increased rapidly. Fluctuations in the CCD could result from various processes that create an imbalance between calcium (Ca) sources to, and sinks from, the ocean (e.g., weathering and CaCO3 deposition), with different effects on the isotopic composition of dissolved Ca in the oceans due to differences in the Ca isotopic composition of various inputs and outputs. We used Ca isotope ratios (d44/40Ca) of coeval pelagic marine barite and bulk carbonate to evaluate changes in the marine Ca cycle across the EOT. We show that the permanent deepening of the CCD was not accompanied by a pronounced change in seawater d44/40Ca, whereas time intervals in the Neogene with smaller carbonate depositional changes are characterized by seawater d44/40Ca shifts. This suggests that the response of seawater d44/40Ca to changes in weathering fluxes and to imbalances in the oceanic alkalinity budget depends on the chemical composition of seawater. A minor and transient fluctuation in the Ca isotope ratio of bulk carbonate may reflect a change in isotopic fractionation associated with CaCO3 precipitation from seawater due to a combination of factors, including changes in temperature and/or in the assemblages of calcifying organisms.

在始新世-渐新世过渡期（Eocene-Oligocene transition, EOT，约34 Ma），地球气候从温室气候显著降温至冰室气候，太平洋海域的方解石（CaCO₃）补偿深度（calcite compensation depth, CCD）快速抬升。方解石补偿深度的波动可由多种过程引发，这些过程会打破海洋钙（Ca）的源与汇之间的平衡（例如风化作用与CaCO₃沉积作用）；由于不同输入、输出过程的钙同位素组成存在差异，这些过程会对海洋溶解钙的同位素组成产生不同影响。我们利用同期远洋海相重晶石与全岩碳酸盐的钙同位素比值（δ⁴⁴/⁴⁰Ca），评估了始新世-渐新世过渡期全球海洋钙循环的变化。研究结果显示，方解石补偿深度的永久性加深并未伴随海水δ⁴⁴/⁴⁰Ca的显著变化；而新近纪（Neogene）中碳酸盐沉积变化幅度较小的时段，海水δ⁴⁴/⁴⁰Ca则呈现出偏移特征。这表明，海水δ⁴⁴/⁴⁰Ca对风化通量变化以及海洋碱度预算失衡的响应，取决于海水的化学组成。全岩碳酸盐的钙同位素比值出现微小且短暂的波动，这可能反映了多重因素共同作用下，海水CaCO₃沉淀过程中的钙同位素分馏发生改变——这些因素包括温度变化以及/或钙化生物群落组成的改变。