Modelling carbon isotopes of carbonates in cave drip water

C isotopes in cave drip water are affected by both the C isotope composition of soil air and host rock carbonate. Furthermore, the C isotope composition of cave drip water strongly depends on the calcite dissolution system, i.e., open, closed and intermediate conditions. Here, we present a calcite dissolution model, which calculates the C-14 activity and delta C-13 value of the dissolved inorganic carbon of the drip water. The model is based on the chemical equations describing calcite dissolution (H2O + CaCO3 + CO2 double left right arrow Ca2+ + 2HCO(3)(-)). The most important improvement, relative to previous models, is the combination of the open and closed system conditions in order to simulate the C isotope composition during intermediate states of calcite dissolution and the application to carbon isotope measurements on cave drip waters from Grotta di Ernesto, Italy. The major changes in the C isotope composition of the drip water occur in response to variations in the open-closed system ratio. Additionally, the C-14 activity and the delta C-13 value of the drip water depend on changes in the partial pressure of soil CO2. Radiocarbon and delta C-13 values of the Grotta di Ernesto drip water are well reproduced by the model. (C) 2011 Elsevier Ltd. All rights reserved.