Investigating the effects of growth rate and temperature on the B/Ca ratio and δ11B during inorganic calcite formation

To deconvolve the effect of growth rate and temperature on the boron partitioning into calcite and its isotope fractionation, seeded calcite precipitation experiments were performed at a constant temperature and various growth rates and at a constant growth rate and various temperatures. We show that boron partitioning increases with increasing growth rate and decreases with increasing temperature. The B isotope fractionation between calcite and B(OH)4− increases with increasing growth rate favoring the lighter B isotope for incorporation into calcite whereas no effect of temperature was observed within the temperature range investigated (12°C to 32°C). At the lowest temperature and growth rate δ11B of the calcite almost equals that of B(OH)4− in solution. Applying the surface entrapment model (SEMO) of Watson and Liang (1995) to our data, we demonstrate that the observed effects of temperature and growth rate on B concentration can be explained by processes in the near surface layer of the calcite crystal.