Boron incorporation into marine CaCO3

Abstract

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?’.