Temporal and spatial controls on cave water and speleothem calcite isotopic and elemental chemistry, central Texas



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The isotopic and elemental composition of speleothem calcite precipitated from dripwaters in caves have been used to elucidate changes in karst groundwater flow paths and changes in climate such as shifts in vegetation and rainfall patterns. Given that recent studies have shown seasonal isotopic and elemental changes in cave dripwater chemistry, then these changes are most likely reflected in the precipitated calcite. This affirms the need to study the environmental controls on the dripwater isotopic and elemental chemistry on seasonal time scales, since ultimately these are environmental controls on the precipitated speleothem calcite. In this study, physical, chemical, and meteorological variations at 13 drip-water and pool sites in two cave systems in the Edwards aquifer of central Texas are used to delineate the controls on the evolution of cave waters. The chemical evolution of these cave waters can be accounted for by variations in calcite precipitation, soil geochemistry, flow paths, and degassing of CO₂ driven by seasonal overturn of the cave atmosphere.Calcite precipitated on glass substrates placed under three of the thirteen dripwaters in this study was analyzed for its C and O isotopic composition in order to assess 1) seasonal variations in the isotopic composition and 2) the extent to which precipitation occurs in isotopic equilibrium with its host dripwater. Each winter season, calcite growth rates increase, which corresponds to changes in the C and O isotopic composition of the calcite: higher ơ13C and ơ18O values during winter and lower ơ13C and ơ18O values during summer. Some calcite samples were found to precipitate out of C and O isotopic equilibrium, with maximum departures of 1.3% with respect to carbon and 1.4% with respect to oxygen. The results show that ơ13C and ơ18O in the modern speleothem calcite:1) can vary sub-annually without change in vegetation or climate, and 2) reflect in the majority of cases precipitation out of isotopic equilibrium with the host dripwater that precipitates the calcites.