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Novel observations on biomineralization processes in foraminifera and implications for Mg/Ca ratio in the shells

¹ Institute of Earth Sciences, The Hebrew University of Jerusalem, Edmond Safra Campus, Givat Ram, Jerusalem 91904, Israel

Stable isotopes and trace elements in foraminifera shells are important for determining the paleochemistry of the ocean, and Mg/Ca ratios and ¹⁸O values in foraminifera have recently been used to determine simultaneously the temperature and salinity of past oceans. However, large variations between species and significant Mg intra-shell heterogeneity indicate a major role for biological factors in determining the Mg/Ca ratio in foraminifera. Our novel in vivo observations on the biomineralization process of perforate foraminifera show that their calcareous wall is composed of two different calcite types: a thin, high-Mg primary layer, and low-Mg layers that cover the primary layer on both sides. The data may suggest that two biomineralization pathways are employed in the formation of the different calcite types. A new conceptual framework is provided to explain part of the Mg heterogeneity in foraminiferal shells and the variable sensitivity of the Mg/Ca ratio to temperature in different species.

Key Words: foraminifera • biomineralization • Mg/Ca ratio • vital effects • paleotemperature