Controls on oxygen isotope variability in precipitation and cave drip waters, central Texas, USA

SummaryThis study investigates the factors that control the oxygen isotopic composition of precipitation and cave drip waters in central Texas. Precipitation samples collected in Austin, Texas at monthly to bimonthly intervals between 1999 and 2007 have δ18O values between −12.6‰ and −1.1‰ VSMOW and a weighted average value of −4.1‰. This weighted average value likely reflects the isotopic composition of precipitation generated predominantly by moisture masses that originate from the Gulf of Mexico. Samples with the lowest δ18O values (−12.6 ± 0.1‰) are associated with precipitation from moisture masses related to Pacific tropical cyclones. Precipitation amount appears to be an important control on the oxygen isotope composition of rainfall during summertime periods with the warmest weather, but does not account for the variability of the precipitation isotopic data for colder months. Drip-water samples collected at monthly to quarterly intervals from multiple drip sites in three regionally distributed central Texas caves between 1998 and 2007 have δ18O values between −5.5‰ and −0.6‰ with individual drip sites generally varying by no more than 1‰ of their average values. This much narrower range of δ18O values relative to those for rainfall is consistent with mixing of meteoric water in the unsaturated zone. Average drip-water δ18O values of all caves are lower than the weighted average of Austin precipitation, which may indicate that a threshold of precipitation amount must be exceeded to provide recharge to drip sites. Unlike Austin precipitation, drip waters at most sites lack seasonal variations and therefore must have a residence time in the unsaturated zone of approximately annual scale or greater. Drip waters from six caves from central Texas to eastern New Mexico display a spatial gradient in which their drip-water δ18O values are lower for caves located further inland from the Gulf of Mexico. This likely reflects a combination of continental and temperature effects on precipitation above the caves. The observed spatial gradient in drip-water δ18O values emphasizes the utility of comparing speleothem paleoclimate records from regionally distributed caves. Past variations in such a gradient may imply changes in prevailing storm paths or moisture source region.