Abstract: The element and isotope compositions of calcite formed by calcification of foraminifera shells are widely used in reconstructing paleo-marine evolution. However, it has been found that the chemical composition of foraminiferal calcite is
significantly different from that of inorganic calcite precipitated directly in seawater, indicating that the chemical composition of foraminiferal shell is affected by the “vital effect”. In order to explore the controlling factors behind this “vital effect” and
get more reliable paleo-oceanic indicators, a series of studies have been carried out from the aspects of calcification process and element partition mechanism. This study first summarized the two main ways of foraminifera calcification: seawater vacuolization model and Ca2+ transmembrane model. In the enclosed or semi-enclosed calcified space, the composition of trace elements in the fluid produces a Rayleigh fractionation effect with the precipitation of calcium carbonate, which becomes a classical model to explain the mechanism of the “ vital effect” of trace element partition in foraminifera. However, this model is still difficult to explain quantitatively the mechanism of low magnesium in foraminifera calcite shells, the composition of calcium isotopes, and the sensitivity of Mg/Ca ratios to temperature. Bio-carbonates, including foraminiferal calcite, may be common exist metastable precursor carbonate. The partition effect of trace elements during the conversion of metastable precursor carbonates to calcite may be an important reason to the formation of low magnesium in foraminifera. The recent discovery of vaterite in living foraminifera supports this hypothesis, but the partition of trace elements and isotope effects during the conversion of carbonate precursors to calcite are rarely studied. This paper focuses on the partition model of trace elements when the precursor of vaterite in foraminifera transformed to calcite. This model can explain the phenomenon of low magnesium in foraminifera quantitatively. At the same time, combined with the calcium isotope data synthesized in the laboratory and the calcium isotope fractionation mechanism in the calcification process of coccolith, this paper attempts to further conjecture the mechanism of “vital effect” of the chemical composition of foraminifera calcite shells from the perspective of calcium isotope. Metastable precursors may contribute significantly to the “vital effect” of trace elements partition and isotope fractionation during foraminifera calcification, and this model needs to be further verified from the perspective of other trace elements and isotopes. 

Key words: foraminifera calcification, vital effect, vaterite precursor, Ca isotope