Seasonal Variations of Cave Dripwater Trace Element Ratios in Central Texas: Implications for Speleothem Geochemical Records

A multiyear monitoring study in a central Texas cave indicates that different processes control vadose dripwater compositions at three different groups of drip types in the same cave. Group 1 (n=4) sites exhibit negative covariations between drip rate and dripwater Mg/Ca and Sr/Ca, and positive covariations with Sr isotope value. Drip rate variations at these sites are characteristic of conduit flow paths. Group 2 (n=3) sites are characterized by strong seasonal geochemical changes, with peak Mg/Ca and Sr/Ca and low Ca concentrations in the winter. Drip rate variability is indicative of diffuse flow paths, with no correspondence between drip rate and dripwater geochemisty. Group 3 (n=2) sites exhibit limited geochemical, physical or temporal correlation.

To assess the controls on dripwater geochemistry, we apply mass balance modeling of rock-water interaction (i.e., dissolution-reprecipitation processes involving carbonate and evaporite minerals comprising host Cretaceous carbonate rocks) and calcite precipitation within the cave. Group 1 variations can be accounted for by changes in the extent of rock-water interaction governed by changes in water residence times during and following rain events. Group 2 variations require multiple processes of rock-water interaction and calcite precipitation to account for the trace element and isotopic changes. Seasonal changes in the amount of calcite growth that can account for the increased dripwater Mg/Ca and Sr/Ca in the winter, is experimentally observed in this cave. This growth is driven by seasonal ventilation of cave-air CO₂, and suggests that trace element variations in speleothems at Group-2-like sites could be indicators of seasonal banding. Group 3 sites may reflect a complex interplay between soil ion exchange processes, host rock-water interaction, and in-cave processes. The results of this study provide insight on the processes controlling the geochemical evolution of vadose karst waters and can be applied toward uncoding the paleoclimate signals recorded in speleothems.