Kinetic Effects on B/Ca in Synthetic Calcite: Implications for B(OH)4- and B(OH)3 Incorporation.
Abstract:In this experimental study, we investigated the influence of solution chemistry on the boron abundance in synthetic calcite using a pH-stat system. We systematically varied solution pH as well as the concentration of total dissolved boron (BT), inorganic carbon (DIC) and calcium ion. We found robust positive correlations between boron abundance in calcite (measured as the boron to calcium ratio, B/Ca) and solution pH, [BT], [DIC] as well as [Ca2+], when a given parameter was solely manipulated while keeping the others constant. Except for [BT], raising these parameters also caused simultaneous increase in calcite saturation and precipitation rate. We found that much of the B/Ca variability as a results of the chemical manipulations tested here can be essentially explained by just precipitation rate and the [BT]/[DIC] ratio in the solution, which was particularly the case for relatively rapidly precipitated calcite samples. On the contrary, for relatively slowly precipitated samples, the [B(OH)4-]/[DIC] and [BT]/[DIC] ratios are equally effective in explaining the B/Ca variability (along with precipitation rate). This observation suggests the possibility of a hitherto unrecognized and apparently kinetically-controlled mechanism that promotes B(OH)3 incorporation for rapidly forming calcite.
In recent years both the abundance and isotopic composition of boron in marine biogenic CaCO3 (B/Ca and δ11B, respectively) has been increasingly utilized to constrain past ocean carbonate chemistry. But these boron-based proxies crucially rely on the first order assumption that B(OH)4- is predominantly incorporated. Thus, the possibility of kinetically-controlled B(OH)3 incorporation presented here raises a concern for the reliability of the B/Ca and δ11B proxy. If it is similarly applicable to foraminifers, shell B/Ca and δ11B may be prone to significant uncertainties due to long-term changes in seawater chemistry. Our results further suggest that future calibrations of these proxies clearly need to take calcification rates into consideration.