APPLICATION OF SR ISOTOPES TO TRACING VADOSE FLOW PATHS IN THE EDWARDS AQUIFER AND IMPLICATIONS FOR RECONSTRUCTING PALEOCLIMATE

Previous studies in karst systems of the Caribbean and Texas provide a model for the assessment of the controls on strontium isotope variations in both modern and ancient karst waters. In a karst system that has a contrast between the Sr isotopic composition of its soil and aquifer limestone, isotopic variations in vadose waters are predicted to vary between drip sites supplied by conduit-dominated flow paths and drip sites supplied by diffuse-dominated flow paths. A ten-year study of drip water from a central Texas cave (cave IS) in the recharge zone of the Edwards aquifer affords an opportunity to assess this model. The aquifer limestone has a low Sr isotope ratio, reflecting the limestone’s Cretaceous marine origin, whereas the soil has an exchangeable Sr isotope value higher than the limestone value, reflecting the soil’s silicate component. At site ISLM, water drips slowly with little variation in drip rate. In contrast, drips at site ISST are faster and more variable. Sr isotope values of ISLM drip waters are similar to the limestone values, and vary mostly within analytical uncertainty over the 10-year monitoring period. These variations can be accounted for by diffuse flow paths that dictate long residence times in the limestone. ISST drip waters have higher and more variable Sr isotope values, consistent with this site’s more conduit flow-path characteristics. The results indicate that Sr isotope variations in speleothems from the Edwards aquifer have potential for reconstructing past changes in effective precipitation, and that this potential is greater for speleothems from drip sites with a more conduit-dominated than diffuse-dominated flow path.