Seawater carbonate chemistry, processes and elements during experiments with planktonic foraminifera Orbulina universa, 2004

We investigate the sensitivity of U/Ca, Mg/Ca, and Sr/Ca to changes in seawater [CO3[2-]] and temperature in calcite produced by the two planktonic foraminifera species, Orbulina universa and Globigerina bulloides, in laboratory culture experiments. Our results demonstrate that at constant temperature, U/Ca in O. universa decreases by 25 +/- 7% per 100 µmol [CO3[2-]] kg**-1, as seawater [CO3[2-]] increases from 110 to 470 µmol kg**-1. Results from G. bulloides suggest a similar relationship, but U/Ca is consistently offset by ~+40% at the same environmental [CO3[2-]]. In O. universa, U/Ca is insensitive to temperature between 15°C and 25°C. Applying the O. universa relationship to three U/Ca records from a related species, Globigerinoides sacculifer, we estimate that Caribbean and tropical Atlantic [CO3[2-]] was 110 +/- 70 µmol kg**-1 and 80 +/- 40 µmol kg**-1 higher, respectively, during the last glacial period relative to the Holocene. This result is consistent with estimates of the glacial-interglacial change in surface water [CO3[2-]] based on both modeling and on boron isotope pH estimates. In settings where the addition of U by diagenetic processes is not a factor, down-core records of foraminiferal U/Ca have potential to provide information about changes in the ocean's carbonate concentration.本研究通过实验室培养实验，探究了两种浮游有孔虫（planktonic foraminifera）——环球球室虫（Orbulina universa）与泡抱球虫（Globigerina bulloides）所形成方解石（calcite）中的U/Ca、Mg/Ca及Sr/Ca比值，对海水碳酸根离子浓度([CO3^2-])与温度变化的响应敏感性。实验结果显示，在温度恒定的条件下，当海水[CO3^2-]从110 μmol·kg^-1升高至470 μmol·kg^-1时，环球球室虫的U/Ca比值每升高100 μmol·kg^-1的[CO3^2-]，会降低25±7%。泡抱球虫的实验结果呈现出相似的变化规律，但在相同环境[CO3^2-]条件下，其U/Ca比值整体偏高约40%。对于环球球室虫而言，其U/Ca比值在15℃至25℃的温度区间内不受温度影响。我们将环球球室虫的U/Ca与环境[CO3^2-]的响应关系应用于近缘物种袋拟抱球虫（Globigerinoides sacculifer）的3条U/Ca记录后，估算得出：相较于全新世，末次冰期加勒比海与热带大西洋的海水[CO3^2-]分别高出110±70 μmol·kg^-1与80±40 μmol·kg^-1。这一结果与基于模型模拟与硼同位素pH估算得到的冰期-间冰期表层海水[CO3^2-]变化的估算结果相一致。在不受成岩作用（diagenetic processes）导致铀添加影响的环境中，有孔虫U/Ca的岩心剖面记录（down-core records）有望为海洋碳酸盐浓度变化提供有效信息。