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 (B_T), 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, [B_T], [DIC] as well as [Ca²⁺], when a given parameter was solely manipulated while keeping the others constant. Except for [B_T], 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 [B_T]/[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 [B_T]/[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.