The isotopic geochemistry of speleothems—I. The calculation of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as palaeoclimatic indicators

The effects of different modes of formation on the isotopic composition of speleothems has been examined to delineate the conditions under which they may be used to give palaeo-climatic data. The modes of deposition considered were: equilibrium loss of CO2, kinetic loss of CO2, and evaporation of water. If isotopic equilibrium is maintained between HCO3− and CO2(aq) the calcite precipitated will be in isotopic equilibrium with the water and variations in 18O16O will depend on climate alone, variations in 13C12C will be independent of climate. This will occur if the loss of CO2 from solution is slow. If loss of CO2 is rapid a kinetic fractionation will occur between HCO3− and CO2(aq) and the calcite precipitated will show a simultaneous enrichment in 13C and 18O. If evaporation of water occurs the calcite precipitated will be enriched in 18O and will be close to 13C and 14C equilibrium with the cave atmosphere. Speleothems deposited under conditions of kinetic loss of CO2 or evaporation of water cannot be used to give palaeoclimate data.

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