The coordination and distribution of B in foraminiferal calcite

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.We would like to acknowledge David Nicol, Iris Buisman and Martin Walker for invaluable technical assistance, and James Bryson for his help with synchrotron data collection. Wewould like to thank Jean DeMouthe (California Academy of Sciences) and Mike Rumsey (Natural History Museum, London) for provision of B-containing minerals for use as reference materials. This work was funded by ERC (grant 2010-ADG-267931 to HE), NERC, Jesus College (Cambridge)and the US Department of Energy (via ALS).This is the final published version. It first appeared at http://www.sciencedirect.com/science/article/pii/S0012821X15000849