Ice and Guano Deposits in El Malpais Lava Tubes: Potential Paleoclimate Archives for the Southwest United States

Three ice cores and one guano core were obtained from lava tubes in El Malpais National Monument, New Mexico, in the Southwest United States. A large hiatus in the Bat Cave guano record, resulting from mining activities in the early 1900's, left us with only ~50 years of data (from AD 1955–2006) to use in analysis. δ13C values in guano (–18 to –11.6‰) primarily fall within the range of C4 plants, and likely indicate a tendency of Tadarida brasiliensis to feed on insects over grasslands to the North or South of the park. Variations in δ 13C values for this period fail to reflect local temperature or precipitation influences, and are likely the result of changes in bat foraging range (to include C3 plants within the park). 15N variations in Bat Cave guano (from 5.7 to 8.8‰) likely reflect changes in bat trophic level, and may also be impacted by fire activity in El Malpais. Ice cores from two lava tubes (Cave 91 and Cave 455) did not contain enough organic sediment for dating. However, analysis of LMWLs indicates that moisture from Cave 91 ice came from an arid moisture source, and strong evaporative processes were present during precipitation. The LMWL derived from Cave 455 ice closely resembles the GMWL, therefore it can be assumed that no evaporation happened during precipitation, and that the moisture came from a source of high relative humidity. The presence of abundant charcoal in Cave 29 (indicating that Native Americans melted the ice for drinking water) allowed radiocarbon dating, the results of which suggest the ice core spans the period between AD 138 and 948. Analysis of δ18O from this lava tube (values range from –9.3 to –6.8‰) reveals that the majority of ice accumulation happened during summer monsoon months, with the Gulf of Mexico being the primary moisture source. A lesser degree of winter precipitation, arriving in the form of snow storms form the Pacific, also likely contributed to ice accumulation by being deposited on the surface and percolating into the lava tube after melting in the spring. Variations in δ18 O in Cave 29 ice result from local surface temperature variations, with a decrease in values around AD 450 aligning with a transition from the Roman Warm Period to the Dark Age Cold Period. δ18O values then display an accelerated increase beginning around AD 800, at the onset of the Medieval Warm Period. Considering the fact that ice in El Malpais caves is rapidly ablating under present day climate conditions, we highly recommend that this resource be further examined in paleoclimate studies over the next several years