Physical and geochemical response in cave drip waters to recent drought, central Texas, USA : implications for drought reconstruction using speleothems
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A five-year study (2009-2014) of eight drip sites in Inner Space Cavern (IS), a cave on the Edwards Plateau in central Texas, was undertaken to assess the physical and geochemical response of cave drip waters to extreme drought. Drip rate, calcite growth rate, and dripwater geochemistry were monitored before, during, and after the peak of a record-breaking drought in central Texas that began and peaked in 2011, and which continued through to early 2015. Three groups of drip sites are identified based on average drip rate (slow sites, 0.2 -- 0.4 mL/min; intermediate sites, 1.2 -- 4.4 mL/min; fast sites, 6.7 -- 18 mL/min) and similarities in geochemical variation. Drip rates of slow sites have the lowest rate and magnitude of response to changes in hydrological conditions, while fast sites have the largest rate and magnitude of response. In contrast, the geochemical response to drought of the three groups does not correspond to the drip rate response. Slow and fast sites exhibit limited geochemical responses to changes in hydrologic conditions, including dripwater Mg/Ca, Sr/Ca, and Ba/Ca ratios, Sr isotope values, Ca concentrations, and oxygen isotope values. This lack of response indicates limited water-rock interaction (WRI) and/or prior calcite precipitation (PCP) affects these dripwaters. Intermediate drip sites exhibit the greatest geochemical response to changes in hydrologic conditions, including extreme drought, expressed by a decrease in Sr isotope values and an increase in Mg/Ca ratios during drier periods. Quantitative modeling indicates that both WRI and PCP can account for trace-element and Sr isotope variations at intermediate sites. The peak of the drought in 2011 coincides with high cave-air CO₂ and slow calcite growth rates, yet PCP may be an important process at two intermediate drip sites during drought. Geochemistry of intermediate drip waters at IS is likely controlled by water supplied by conduit and matrix flow and may provide the preferred speleothem record for reconstructing past droughts in central Texas using trace-elements ratios. Flow-route characteristics of drip sites at other caves that may be expected to show drought response in terms of trace elements include drip rate response to changes in moisture conditions but relatively low drip rate coefficient of variation and sub-equal matrix- and conduit-flow contributions. The monitoring of key geochemical and physical parameters at a range of sites in a given cave may allow for the identification of speleothems that are most likely to be geochemically responsive to changes in climate, making the speleothem sampling process more informed and less destructive.