STABLE ISOTOPIC INVESTIGATION OF CAVE POOLS IN CARLSBAD CAVERNS NATIONAL PARK, NM

Water samples from pools in Lechuguilla Cave, Carlsbad Cavern, two local springs and local precipitation have been collected and analyzed for stable isotopes of hydrogen and oxygen as well as the radioisotopes tritium and chlorine-36. The stable isotopic ratios of most Lechuguilla pools (dD-value of -50 to -60 ‰, d¹⁸O-value of -7 to -8‰) fell in a smaller range and were isotopically lighter than those of Carlsbad cavern pools (dD-value of -22‰ to -47 ‰, d¹⁸O-value of -3.6 to -6.8‰), indicating that pools in Lechuguilla experience less evaporation than pools in Carlsbad Cavern. The weighted average isotopic composition of local precipitation (dD-value of -52‰, d¹⁸O-value of -7.0 ‰) and the stable isotopic composition of a nearby spring (dD-value of -53‰, d¹⁸O-value of -7.6 ‰) fall in the 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. Stable isotopic values for Lechuguilla pools exhibit a trend linking the isotopic composition with depth. Pools within 200 meters of the surface were slightly lighter isotopically than 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-term average isotopic composition of the regional precipitation, changing this trend with depth to a trend with age. This suggests that younger waters have a slightly lighter isotopic composition than older waters. Some chlorine-36 data supports this hypothesis, however, there is one inconsistency. More samples will be collected in the near future in order to further test this hypothesis.