APPLICATION OF MODERN CAVE DRIPWATER GEOCHEMISTRY TO UNDERSTANDING SEASONAL CHANGES IN MINERAL-SOLUTION REACTIONS: IMPLICATIONS FOR RECONSTRUCTING PALEOCLIMATE FROM SPELEOTHEMS

We apply the geochemistry of modern cave drip waters and growth rate of calcite deposits in caves (speleothems) to understanding the potential for speleothems as climate proxies. In particular, the co-variations of trace elements (Sr/Ca and Ba/Ca) and Sr isotopes provide insight into the mineral-solution reactions that control the seasonal evolution of drip-water in a cave at Westcave Preserve in central Texas. Monitoring of hydrogeologic and meteorologic conditions at Westcave reveals that the cave is well-ventilated year-round, which results in similar cave-air and surface atmospheric conditions, including temperature and CO₂ concentrations. Analysis of monthly drip water samples shows that during the warmer summer months, the Sr/Ca and Ba/Ca ratios increase, and calcite grows faster. This indicates that prior calcite precipitation (PCP) is driven by high temperatures.

Geochemical modeling indicates that drip-water geochemical evolution is controlled by a combination of PCP and water-rock interaction (WRI), with PCP dominating during the summer. ⁸⁷Sr/⁸⁶Sr values for all sites range from 0.70846-0.70856, which is similar to values for soils and higher than values for the host Cretaceous limestones. The largest range of ⁸⁷Sr/⁸⁶Sr at any of the sites is 0.000035, which is similar to the analytical uncertainty. Sr isotopes will undergo negligible fractionation during mineral precipitation from solution, whereas mineral dissolution during WRI will evolve solution ⁸⁷Sr/⁸⁶Sr values. Therefore, the essentially invariant Sr isotope variability at each drip site is consistent with PCP as the primary control. Drip-site location also appears to play a role in drip-water geochemistry, as ⁸⁷Sr/⁸⁶Sr values vary between sites. This may be due to either the heterogeneity of soil above the cave, or varying flow paths of the water feeding the sites. Further studies of the host limestone and soils may constrain the origin of the local differences. Co- variations of trace elements and Sr isotope compositions may be applicable to speleothems as proxies for seasonal changes in temperature, the extent of different mineral-solution reactions, and the sources of dissolved ions.