Inorganic carbon transformations between phases and its impact on its isotopic signature under open conditions

Abstract

In order to evaluate the possibility of tracing dissolved inorganic carbon (DIC) derived from calcite dissolution under open system conditions, controlled laboratory experiments using stable carbon isotopes were conducted in a specially designed reaction chamber with constant pH monitoring. The system design also allowed us to measure the forward and backward (degassing) gas transfer reactions rates. We confirmed that the degassing reaction rate can be two to four times slower than the gas transfer reaction rate depending on the differences in pCO2 between the gas phase and the liquid. The isotopic analyses suggest that the carbon isotopic signature of DIC under open system conditions is controlled by the CO2 carbon isotopic signature, the isotopic fractionation between CO2, the relative occurrence of the different DIC species, and changes in pH. We found that the isotopic signature of DIC is not affected by the isotopic signature of the calcite or the mechanism controlling the dissolution (protonation or hydration). Practically this study suggests that it is not possible to trace the DIC derived from calcite dissolution under open system conditions. It supports the approach of evaluating the pedogenic carbonate content by stable carbon isotopes methods. In open system conditions: a) pH can explain up to 8‰ variation in the DIC δ13C signature and b) temperature is the second most important factor affecting the δC13-DIC signature, and only at pH above neutrality.