Carbonate cements deposited from groundwater in caves (speleothems) over recent geologic time can be used to understand mechanisms and timescales of variations in groundwater geochemistry and groundwater flow routes. Geochemical and isotopic variations in speleothems have been used as records of continental paleoclimatic and hydrologic variables. Although the mechanisms and timescales that link climatic and hydrologic processes are not well constrained, this knowledge provides a framework for assessing the controls of factors such as climatic variations on aquifer and karst development, long-term patterns of recharge and changes in groundwater flow regimes. Speleothems are precisely datable over a range of timescales for the Pleistocene and Holocene, and may provide continuous temporal sequences of growth and corresponding records of aquifer and karst development, groundwater geochemistry, and paleoclimatic parameters over such timescales.

The Edwards aquifer of central Texas is developed in karstified Cretaceous limestone and is the most significant groundwater resource in the region. Numerous studies have detailed the development of the aquifer, fluid hydrodynamics and groundwater geochemistry. Speleothem and temporal series of cave dripwater samples were collected across the region from multiple caves. This regional sampling coverage facilitates discerning local variations from individual sites versus regional variations. A detailed chronology for four stalagmites from three central Texas caves, up to 130 kilometers apart, provides a 72,000-year record of temporal changes in hydrology and climate. The stalagmites have similar growth histories with periods of relatively rapid and slow growth. These growth rate shifts correspond in part with global glacial – interglacial climatic shifts. Growth rate accuracy is assessed using two independent uranium-series dating methods.

Geochemical and isotopic variations in central Texas vadose groundwaters and speleothems (i.e., Sr, U, C and O isotopes, Mg/Ca and Sr/Ca ratios) offer insight into the sources of dissolved constituents in groundwaters, water-rock interaction pathways, residence time, and fluxes of geochemical constituents from distinct sources (e.g., soils versus aquifer rocks). Geochemical variability in soils exerts a important control on regional and local water compositions. Changes in vadose flow routes as a function of rainfall and corresponding recharge is a mechanism to account for the range of temporal and spatial variability observed in cave dripwater geochemistry and recorded in speleothems. The timing of geochemical fluctuations, as well as the inferred processes that they reflect, are generally consistent with 1) growth rate variations in the same speleothems, 2) independent paleontological evidence for the region, and 3) aspects of global climate records. Results of integrated geochemical and isotopic techniques suggests that, in spite of complexities of regional and local variability, controlling processes on dripwater and speleothem geochemistry are regionally extensive. An understanding of processes controlling geochemical variations in the modern aquifer system provides a framework within which to interpret the paleohydrologic and paleoclimatic information recorded in speleothems.