The decrease of uranium to calcium ratios (U/Ca) in biogenic CaCO₃ with increasing pH and/or [CO₃^2–] is being explored as a proxy to reconstruct past seawater carbonate chemistry. This inverse relationship was interpreted to reflect the changes in the abundances of dissolved U(VI) species that were incorporated into CaCO₃ with pH and/or [CO₃^2–]. Aqueous U(VI) speciation calculations in all published work on U/Ca in CaCO₃ only considered UO₂-CO₃ and UO₂-OH complexes. The recent discovery that Ca/Mg-UO₂-CO₃ complexes are the dominant U(VI) species in seawater rather than UO₂-CO₃ complexes calls this interpretation into question. Considering the Ca/Mg-UO₂-CO₃ complexes, I revisited published data of foraminifera culturing experiments to examine the effects of U(VI) speciation on U/Ca in these calcite precipitates. My results reveal that concentrations of Ca/Mg-UO₂-CO₃ complexes and UO₂(CO₃)₃^4– remain statistically invariant within uncertainty when the pH and [CO₃^2–] increase, whereas the minor U(VI) species UO₂(CO₃)₂^2– decreases by 10-fold. The decrease of [UO₂(CO₃)₂^2–] with increasing pH and [CO₃^2–] best predicts the decrease of U/Ca in foraminifera, suggesting that UO₂(CO₃)₂^2– is most likely the U(VI) species incorporated into calcite. The incorporation of the minor U(VI) species UO₂(CO₃)₂^2– into calcite provides an alternative interpretation of significantly lower U concentration in calcite relative to aragonite, if the more abundant UO₂(CO₃)₃^4– is incorporated into aragonite. Theoretical aqueous U(VI) speciation further demonstrates that [UO₂(CO₃)₂^2–] is very sensitive to changes in pH, [CO₃^2–], and [Ca²⁺], suggesting that U/Ca is a good proxy for seawater pH and [CO₃^2–] within ∼400 kyr when both U and Ca concentrations remain constant.

中文翻译：

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有孔虫方解石中U / Ca上的铀水溶液形态：次要物种UU ₂（CO ₃）₂ ^2–的重要性

人们正在探索随着pH和/或[CO ₃ ^2- ]的增加，生物型CaCO ₃中铀钙比（U / Ca）的降低，以替代过去的海水碳酸盐化学。这种反比关系被解释为反映了溶解的U（VI）物种的丰度变化，这些物种在pH和/或[CO ₃ ^2– ]下并入CaCO _3中。在CaCO _3中有关U / Ca的所有已发表工作中，U（VI）水溶液的形态计算仅考虑UO ₂ -CO ₃和UO ₂ -OH配合物。Ca / Mg-UO ₂ -CO ₃的最新发现络合物是海水中主要的U（VI）物种，而不是UO ₂ -CO ₃络合物使这种解释成为疑问。考虑到Ca / Mg-UO ₂ -CO ₃配合物，我重新查看了有孔虫培养实验的公开数据，以研究U（VI）形态对这些方解石沉淀物中U / Ca的影响。我的结果表明，当pH和[CO ₃ ^2- ]增加时，Ca / Mg-UO ₂ -CO ₃复合物和UO ₂（CO ₃）₃ ^4–的浓度在不确定性内保持统计不变，而次要的U（VI）物种UO ₂（CO ₃）₂ ^2–减少10倍。随着pH值的升高，[UO ₂（CO ₃）₂ ^2– ]的降低和[CO ₃ ^2– ]的最佳预测是有孔虫中U / Ca的降低，表明UO ₂（CO ₃）₂ ^2–最可能是有孔虫。U（VI）物种合并到方解石中。次要U（Ⅵ）物种UO掺入₂（CO ₃）₂ ^2-成方解石提供的显著底部U浓度相对于文石方解石，可替换的解释，如果更丰富的UO ₂（CO ₃）₃ ^4–被掺入文石中。U（VI）水溶液的理论形态学进一步证明了[UO ₂（CO ₃）₂ ^2– ]对pH，[CO ₃ ^2– ]和[Ca ²⁺ ]的变化非常敏感，表明U / Ca是一种当U和Ca浓度保持恒定时，海水pH和[CO ₃ ^2- ]的良好替代物在约400 kyr内。