Sr/Ca in foraminiferal calcite as a proxy for calcifying fluid composition

Abstract Foraminifera are unicellular organisms that inhabit the oceans. They play an important role in the global carbon cycle and record valuable paleoclimate information through the uptake of trace elements such as strontium (Sr) into their calcitic (CaCO3) shells. Understanding how foraminifera control their internal fluid composition to make CaCO3 is important for predicting their response to ocean acidification and for reliably interpreting the chemical and isotopic compositions of their shells. Here, we model foraminiferal calcification and strontium partitioning in the benthic foraminifera Cibicides wuellerstorfi and Cibicidoides mundulus based on insights from inorganic calcite experiments. The model reconciles inter-ocean and taxonomic differences in benthic foraminifer Sr/Ca partitioning relationships and enables us to reconstruct the composition of the calcifying fluid. We find that Sr partitioning and mineral growth rates of foraminiferal calcite are not significantly affected by changes in external seawater pH (within 7.8–8.1) and [DIC] (within 2100–2300 µmol/kg) due to a regulated calcite saturation state at the site of shell formation. Such homeostasis of the calcifying fluid could explain why foraminifera have been resilient to changes in ocean carbonate chemistry for more than 500 million years. Nevertheless, our model indicates that past foraminiferal DSr values were lower than its modern value due to overall lower ocean pH and higher seawater temperature during the early and middle Cenozoic.