The link between seawater and foraminiferal calcite chemistry

Geochemical signatures of the calcium carbonate shells of foraminifera form the basis of many proxies used in climate reconstructions. Yet, key physiological controls governing mineral precipitation remain unknown, limiting a mechanistic understanding of proxy sensitivity and interspecific variability. Here, we combine cryo–focused ion beam milling with scanning electron microscopy to reconstruct the in vivo three-dimensional ultrastructure of the foraminiferal calcification site. The images reveal a semi-open system, in which the calcifying fluid is largely isolated from ambient seawater by a thin cellular barrier, while discrete ~30 nm openings allow limited fluid exchange. This configuration provides a link between seawater and the site of calcification, offering a mechanistic explanation for the correlation between shell trace-element ratios and ambient seawater composition, complementing the ion uptake through transmembrane transport. This, in turn, improves understanding of element incorporation and enhances the reliability of foraminiferal calcite as a paleoproxy. Foraminiferal calcification occurs within a semi-open system, explaining the link between shell trace-element ratios and seawater composition, according to cryo–focused ion beam milling with scanning electron microscopy.