4. Boron Incorporation into Marine CaCO₃

The isotopic composition (δ¹¹B) and abundance (B/Ca) of boron in the marine CaCO₃ minerals calcite and aragonite are used as paleoceanographic tracers for past oceanic pH and carbon chemistry. These environmental proxies depend upon the ability of CaCO₃ minerals to incorporate trace concentrations of B within their structure, and record the state of the pH-dependent equilibrium between B(OH)₃ and \( {{{\text{B}}\left( {\text{OH}} \right)_{4}}^{ - }} \), and the relative abundance of B and C in seawater. To achieve this CaCO₃ 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 \( {{{\text{B}}\left( {\text{OH}} \right)_{4}}^{ - }} \) into the anion site of CaCO₃ minerals. These observations established the required link between aqueous B chemistry and CaCO₃ ^– hosted B, and provided the foundation for the development and application of the δ¹¹B 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 CaCO₃. Studies of aragonite appear to support a relatively straightforward substitution of \( {{{\text{B}}\left( {\text{OH}} \right)_{4}}^{ - }} \) into the mineral anion site. Conversely, a growing number of studies of calcite suggest either that both aqueous B(OH)₃ and \( {{{\text{B}}\left( {\text{OH}} \right)_{4}}^{ - }} \) are taken up into the mineral, or that B is subject to a significant isotopic fractionation during incorporation. While a growing body of theoretical and experimental work are moving toward an understanding of B uptake in CaCO₃, we currently lack a systematic description of this key process, particularly in calcite. As long as the mechanisms of B incorporation remain unknown, the relationships between δ¹¹B and B/Ca and ocean chemistry must be treated as empirical, adding uncertainty to the paleoceanographic records derived from them. This chapter will explore our current understanding of B incorporation into marine CaCO₃ minerals, in context of their structure and growth mechanisms. We will consider the broad question of ‘how does B get from seawater into calcite and aragonite?’