A new model for biomineralization and trace-element signatures of Foraminifera tests

[摘要] The Mg/Ca ratio of Foraminifera calcium carbonate tests is used as proxyfor seawater temperature and widely applied to reconstruct globalpaleo-climatic changes. However, the mechanisms involved in the carbonatebiomineralization process are poorly understood. The current paradigm holdsthat calcium ions for the test are supplied primarily by endocytosis ofseawater. Here, we combine confocal-laser scanning-microscopy observations ofa membrane-impermeable fluorescent marker in the extant benthic speciesAmmonia aomoriensis with dynamic ⁴⁴Ca-labeling and NanoSIMSisotopic imaging of its test. We infer that Ca for the test in A.aomoriensis is supplied primarily via trans-membrane transport, but that asmall component of passively transported (e.g., by endocytosis) seawater tothe site of calcification plays a key role in defining the trace-elementcomposition of the test. Our model accounts for the full range of Mg/Caand Sr/Ca observed for benthic Foraminifera tests and predicts the effectof changing seawater Mg/Ca ratio. This places foram-basedpaleoclimatology into a strong conceptual framework.

[学科分类] 地球化学与岩石