The coordination and distribution of B in foraminiferal calcite

•We use X-ray spectromicroscopy to measure B in foraminifera at the sub-micron-scale.•B is solely hosted in a trigonal coordination throughout foraminiferal calcite.•Our results uphold the chemical theory that underpins the B palaeoproxies.•However, B concentration exhibits concentration banding, offset from Mg variations.•B incorporation is likely modulated by biological or crystal growth processes.

The isotopic ratio and concentration of B in foraminiferal calcite appear to reflect the pH and bicarbonate concentration of seawater. The use of B as a chemical proxy tracer has the potential to transform our understanding of the global carbon cycle, and ocean acidification processes. However, discrepancies between the theory underpinning the B proxies, and mineralogical observations of B coordination in biomineral carbonates call the basis of these proxies into question. Here, we use synchrotron X-ray spectromicroscopy to show that B is hosted solely as trigonal BO3 in the calcite test of Amphistegina lessonii, and that B concentration exhibits banding at the micron length scale. In contrast to previous results, our observation of trigonal B agrees with the predictions of the theoretical mechanism behind B palaeoproxies. These data strengthen the use of B for producing palaeo-pH records. The observation of systematic B heterogeneity, however, highlights the complexity of foraminiferal biomineralisation, implying that B incorporation is modulated by biological or crystal growth processes.