Modelling delta C-13 and delta O-18 in the solution layer on stalagmite surfaces

摘要

We derive equations describing the evolution of the carbon and oxygen isotope composition of the bicarbonate in a calcite precipitating solution on the surface of a stalagmite using a classical Rayleigh approach. The combined effects of calcite precipitation, degassing of CO2 and the buffering effect of the water reservoir are taken into account. Whereas VC shows a progressive increase to a final constant value, 8180 shows an initial isotopic enrichment, which exponentially decays due to the buffering effect of the water reservoir. The calculated evolution is significantly different for both carbon and oxygen isotopes than derived in a recent paper [Dreybrodt W. (2008) Evolution of the isotopic composition of carbon and oxygen in a calcite precipitating H2O-CO2-CaCO3 solution and the related isotopic composition of calcite in stalagmites. Geochim. Cosmochim. Acta 72, 4712-4724.]. Furthermore, we discuss the isotopic evolution of the bicarbonate in the solution for long residence times on the stalagmite surface, i.e., for t --> infinity. The equilibrium isotope ratio of the bicarbonate is then determined by isotopic exchange between the cave atmosphere and the bicarbonate in the solution and can be calculated by equilibrium isotope fractionation. For strongly ventilated caves exchange with the cave atmosphere will result in higher delta C-13 and delta O-18 values than those observed in a pure Rayleigh distillation scenario, for sparsely ventilated caves it will result in lower delta C-13 and delta O-18 values.