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. / text

The playas of the Salt Basin in Trans-Pecos Texas are natural laboratories for the study of the hydrodynamic, hydrochemical, and sedimentologic properties of the unsaturated zone under the conditions of evaporation from a shallow water table. Water beneath the salt-flat surface moves upward from the saturated zone, through a thick capillary fringe, to the unsaturated zone where it is removed by evaporation. Daily temperature fluctuations change soil suction values and seasonal variations in temperature alter the thickness of the capillary fringe. There is little change in the chemical composition of the pore water as it moves from below the water table to the capillary fringe because the filled pore spaces of the capillary fringe prevent evaporation from taking place. However, an enrichment in the heavy isotopes of hydrogen and oxygen in the groundwater, as compared to area precipitation, suggests that evaporation may have occurred earlier along the flow path. As water moves from the top of the capillary fringe into the unsaturated zone, evaporation in the partially-filled pore spaces increases the total dissolved solids content. According to the increase in chlorinity, the brine has lost over 60% of its original volume by the time it has moved to within 20 cm of the surface. Evaporation in the unsaturated zone further enriches the brine in deuterium and oxygen-18. Gypsum precipitation in the unsaturated zone depletes the shallow pore water in calcium and sulfate, relative to chloride, and forms white patches, enterolithic bands, and discontinuous lenses of pure gypsum. The sediments are made almost entirely of gypsum and dolomite. The high magnesium-to-calcium molar ratio in the brines and the poorly ordered nature of the dolomite mud in the sediment column indicate that the salt-flat dolomite formed by the alteration of a calcium carbonate precursor. A lack of lateral continuity in sediment structures and a change in sediment character from massive below the water table to laminated above indicate that the shallow salt-flat sediments were formed by vadose-zone processes rather than by sedimentation in an ancient lake. / text