Magnesium partitioning into vaterite and its potential role as a precursor phase in foraminiferal Mg/Ca thermometer

Abstract The foraminifera shell Mg/Ca thermometer has been extensively used in reconstructing temperature evolution of the past oceans. An important, yet poorly understood observation is the 1-2 orders of magnitude lower Mg/Ca in foraminifera calcite shells than expected from inorganically precipitated calcite. Transient unstable carbonate was long hypothesized as a precursor of foraminiferal calcification, but its potential role in explaining apparent ‘vital effects’ of trace element partitioning is not well understood. In particular, a recent study has identified vaterite as a likely precursor phase during biomineralization of calcite foraminifera shells ( Jacob et al., 2017 ). Here, we explore the possibility of vaterite as a precursor phase in affecting Mg incorporation into foraminifera calcite shells by experimentally calibrating the temperature dependent Mg partition coefficients between carbonate solution and vaterite under different solution Mg concentrations. The experimentally determined sensitivity of Mg incorporation into vaterite as a function of temperature (T in °C) is expressed as: ln ⁡ ( λ vaterite Mg ) = 0.022 ± 0.002 ⋅ T − 3.65 ± 0.03 . It is demonstrated that Mg partitioning is mainly controlled by the strain energy originated from cation substitution in the discontinuous lattices of vaterite particles, without distinct kinetic-controlled effects. We have developed a two-step partition model applying the above experimental results, with the assumption that vaterite is directly precipitated from a seawater-like fluid and later converts to calcite through dissolution-reprecipitation mediated by a second fluid in a localized environment. Within the constrained range of model parameters during phase conversion, the two-step partition model could directly reproduce foraminiferal calcite Mg/Ca temperature dependencies in both magnitude and sensitivity. Our model provides a theoretical framework that could be broadly applied to understand the impact of precursor phase on trace metal partition in biominerals. Overall, our study implies that the energy-consuming biological pumping of Mg outside foraminifera calcifying fluid, as widely believed, might not be a precondition for foraminifera shell mineralization.