RELATING VARIATIONS IN RAINFALL TO VARIATIONS IN CAVE DRIP WATER GEOCHEMISTRY AS A PRECURSOR TO PALEOCLIMATE APPLICATION

Drought is a common occurrence in central Texas. The highest resolution paleoclimate record for the region is reconstructed from tree-ring studies extending to the mid-1600’s. Speleothems offer a potential extension to the regional climate record. To maximize the applicability of speleothem proxies in this region, we monitor cave air (temperature, relative humidity, and CO₂ concentration), drip rates, calcite growth, and drip water temperature and geochemical composition. This allows assessment of the degree of correspondence between surface conditions, cave hydrologic variability, and speleothem geochemistry.

Two of ten drip sites studied in a central Texas cave reflect sub-annual climate signals in that they exhibit (1) positive correlations between rainfall amount and drip rates as well as (2) negative correlations between drip rates and drip water Mg/Ca values. Drought in the summer of 2009 resulted in lower drip rates. This is proposed to have increased the input to these drips from diffuse flow paths, whereby the increased Mg/Ca values of the drip waters reflect an increased residence time and water-rock interaction in the overlying limestone. When rainfall increased in this region during an El Nino event in the fall of 2009, drip rates increased, as it appears that conduit flow paths became a larger input and led to decreased Mg/Ca values of the drip waters. The remaining eight drip sites do not display a clear, high resolution climate signal possibly due to (1) flowpath type, which constrains the variability of Mg/Ca values and/or (2) seasonal cave air CO₂ variations that alter drip water geochemistry due to prior calcite precipitation. This highlights the importance of targeting multiple drip sites for paleoclimate application as only a certain percentage will yield a clear, high resolution climate signal. However, a lower resolution climate signal might be gained at all sites studied as δ¹⁸O values of drip waters are similar to the weighted average regional rainfall δ¹⁸O value. Thus, δ¹⁸O values offer the potential to detect long term variations in climate while Mg/Ca values from certain sites can yield a sub-annual drought history for the region.