The isotopic composition (d 11 B) and abundance (B/Ca) of boron in the marine CaCO 3 minerals calcite and aragonite are used as paleoceanographic tracers for past oceanic pH and carbon chemistry. These environmental proxies depend upon the ability of CaCO 3 minerals to incorporate trace concentrations of B within their structure, and record the state of the pH-dependent equilibrium between B(OH) 3 and B OH ð Þ 4 À , and the relative abundance of B and C in seawater. To achieve this CaCO 3 minerals must either incorporate a single species of aqueous B, or take up a predictable mixture of both species. Initial investigations found evidence to suggest the sole incorporation of aqueous B OH ð Þ 4 À into the anion site of CaCO 3 minerals. These observations established the required link between aqueous B chemistry and CaCO 3 hosted B, and provided the foundation for the development and application of the d 11 B and B/Ca proxies. However, advances in our understanding of aqueous B chemistry, improvements in the accuracy of B isotopic measurements of carbonates, and new data from controlled precipitation experiments have since revealed more complex, structure-dependent mechanisms of B incorporation into CaCO 3 . Studies of aragonite appear to support a relatively straightforward substitution of B OH ð Þ 4 À into the mineral anion site.Conversely, a growing number of studies of calcite suggest either that

Boron Incorporation into Marine CaCO3

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