New Mexico Geological Society Annual Spring Meeting — Abstracts

Water samples from pools in Lechuguilla Cave, Carlsbad Cavern, two local springs and local precipitation were collected and analyzed for stable isotopes of hydrogen and oxygen as well as the radioisotope chlorine-36. Most of the pools in Lechuguilla Cave had δD-values of -50 to 60% and δ¹⁸O-values of -7 to -8%, while the pools in Carlsbad Cavern had δD-values of -22 to -47%0 and δ¹⁸O-values of -3.6 to -6.8%. The larger range of heavier isotopic values exhibited by pools in Carlsbad Cavern is probably a result of an increased rate of evaporation due to air currents within the cave. The small gated entrance of Lechuguilla Cave inhibits air currents within the cave. Therefore, most of the pools in Lechuguilla experience very little evaporation.

The weighted average isotopic composition of local precipitation (δD-value of -52%, δ¹⁸O-value of -7.0%0) and the stable isotopic composition of a nearby spring (δD-value of 53%, δ¹⁸O-value of -7.6%) fall in the small range of values of Lechuguilla pools. This suggests that waters in Lechuguilla have undergone little change during storage within the cave and therefore accurately represent the isotopic composition of local ground seepage water.

Isotopic values for Lechuguilla pools at depths less than 200 meters were slightly lighter than those of pools at greater depths. There is no chemical evidence that this trend represents greater evaporation rates at greater depths. It may represent a change in the long-tenn average isotopic composition of the regional precipitation. This suggests that younger waters have a slightly lighter isotopic composition than older waters. Some chlorine-36 data supports this hypothesis, however more data is needed. More pool and drip samples from Lechuguilla Cave will be collected in the near future in order to further test this hypothesis.