ISOTOPE PROFILES FROM THREE DEEP WESTERN U.S. CAVES: INSIGHTS INTO ARID-REGION VADOSE-ZONE PROCESSES

We have collected water samples from large caves of the Guadalupe Mountains (southeastern New Mexico) and the Black Hills (western South Dakota). Pools and drips from these caves provide vadose-zone water samples of sufficient volume to enable stable isotope (H-2, C-13, O-18) and radioisotope (H-3, C-14, Cl-36) analyses. Our results provide new evidence on the nature of flow and transport processes in arid-region vadose zones.

Our previous research in Lechuguilla Cave (Carlsbad Caverns National Park, NM) revealed elevated levels of chlorine-36 associated with atmospheric nuclear testing in the 1950s in all near-surface pools, but only a few isolated deep pools. The deep occurrences indicate isolated fast paths for infiltration through the vadose zone, apparently associated with structural features and surface topography. In contrast, fallout-associated tritium is ubiquitous throughout Lechuguilla, indicating significant vapor-phase transport.

Since then, we have analyzed Lechuguilla water samples for carbon-14, and collected pool and drip samples from Carlsbad Cavern (Carlsbad Caverns National Park, NM) and Wind Cave (Wind Cave National Park, SD) for isotope analysis. Although Carlsbad and Lechuguilla are near each other, of similar scale and similar geologic setting, Carlsbad has a large natural entrance that allows evaporation and atmospheric exchange to occur throughout much of the cave while Lechuguilla has no natural entrance. This difference is reflected in markedly different isotopic signatures between the two caves.

Wind Cave is located in the Black Hills, a markedly different geologic, structural, and speleogenetic environment than the Guadalupe Mountains. Samples collected in June 2001 are currently being analyzed, and will be compared to Lechuguilla and Carlsbad results.