Partitioning of multiple elements as a function of seawater [Ca2+]: results from foraminiferal culture experiments.

Climate reconstructions are important for validating climate models and hence constrain climate change predictions. Such reconstructions are based on indirect tools (proxies) in which a fossil or chemical remnant corresponds to an environmental parameter. When employing foraminifera as proxies, elemental concentrations in their shells have been shown to correlate with environmental parameters including temperature (that determines the calcite’s Mg/Ca), seawater [HCO₃^-] (Sr/Ca and B/Ca) and [CO₃^2-] (S/Ca). When reconstructing theses parameters on longer geological timescales (i.e. millions of years), the seawater’s major ion concentrations may affect these proxy relationships. Especially the concentration of calcium will affect all of these calcitic elemental ratios and therefore, we varied [Ca²⁺] in controlled growth experiments to test its effect on incorporation of Mg, Sr, B and S in the benthic foraminifer Amphistegina lessonii. For the divalent cations we find a decrease in the partition coefficient (D) for Mg (D_Mg) with a changing [Ca²⁺] (and hence seawater Mg/Ca) while D_Sr does not vary significantly with [Ca²⁺]. For SO₄^2- and B(OH)₄^- against S/Ca_sw and B/Ca_sw respectively, we observe a significant decrease in the partition coefficients with decreasing [Ca²⁺]. These results gives new information for SO₄^2- and B(OH)₄^- incorporation and represents a step forward towards comprehending the impact of [Ca²⁺]_sw alterations on foraminiferal calcite chemistry and hence their application as paleoproxies in deep time.