Cave Speleothems as Potential Tracers of Long-term Variations in Groundwater Flow Routes in a Karst Aquifer, Central Texas, USA

    Carbonate cements deposited from groundwater in caves (speleothems) over recent geologic time can be used to understand mechanisms and time scales of variations in groundwater chemistry. 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 flow regimes. Carbonate aquifers can be characterized as a mix of different permeability networks along a continuum from low-permeability diffuse flow pathways to high permeability conduit flow pathways. Strontium isotope variations in speleothems offer insight into the sources of dissolved ions in groundwater, water-rock interaction processes, and corresponding type of fluid flow. Application of this technique to two carbonate aquifer systems (the Pleistocene limestone aquifer of Barbados and the Cretaceous Edwards aquifer of central Texas) suggests that groundwater migration shifted between higher and lower permeability pathways on century to millennial timescales during the Pleistocene and Holocene. Speleothems are continuous and precisely dateable over a range of timescales for the Pleistocene and Holocene and provide a record of how groundwater chemistry evolves through time. Speleothem Sr isotope values in each aquifer system range from values for exchangeable Sr in local soils to values similar to host marine carbonate aquifer rocks. We propose that temporal variations in the Sr isotope values of groundwater recorded by speleothems in these aquifers reflect changes in the relative Sr flux from soils versus limestones. Changes in vadose flow routes as a function of rainfall recharge is a mechanism by which groundwater may receive temporally varying fluxes from these two endmembers. In this model, groundwater migration is predominantly along slow seepage routes during periods of low rainfall, which results in relatively long residence times in the host limestones. During high-rainfall periods, the capacity of seepage routes is exceeded and groundwater migration is dominated by conduit flow along paths of enhanced permeability.