PALEOCLIMATE AND HYDROGEOLOGICAL IMPLICATIONS OF WATER SEEPING INTO JUXTLAHUACA CAVE. GUERRERO, MEXICO

The isotopic signature of precisely dated speleothems in Juxtlahuaca Cave, Guerrero, southwestern Mexico are the base for reconstructions of the Mesoamerican monsoon rainfall during the last 22,000 years (Lachniet et al. 2012, 2013). One of the assumptions to use speleothems as a proxy to climate is the preservation of the rainwater isotopic signature after it infiltrates and percolates into the caves. This assumption is tested by comparing the isotopic signature of water seeping into Juxtlahuaca Cave, of rainwater collected during the 2011 wet season in Colotlipa, a town in the proximity to the cave, and of nearby spring water. Additionally, a profile of the cave is elaborated to determine the bedrock thickness above each seepage water sampling point. Juxtlahuaca Cave is ~5 kilometers long, ~43 m deep, and formed mainly as two nearly-horizontal levels beneath 175 to 200 m of Cretaceous limestone.

Rain water, cave seepage water and spring water isotopic values follow closely the Global Meteoric Water Line, thus confirming their meteoric origin. Spring water isotopic values are lower than those of cave seepage water suggesting the former recharges at higher altitudes and more regionally. In contrast, Juxtlahuaca Cave has a local recharge consistent with its topographic location. There is an inverse relationship between the monthly weighted average δ¹⁸O and the precipitation amount of rain water. This amount effect may get transferred into the cave as the 1.5‰ decrease in the δ¹⁸O values of the seepage waters collected in October, the end of the wet season, compared to those collected in June, the beginning of the wet season. Three other controls of the seepage waters isotopic signature were tested. First, pool waters have slightly more negative values than drip waters (δ¹⁸O of -7.6 vs. -7.3 ‰, respectively). Second, water dripping faster than 1 drop/s is isotopically lighter than slower drips (δ¹⁸O of -8.1 vs. -6.8 ‰, respectively). Last, the bedrock thickness above the seepage water does not have a significative effect on the δ¹⁸O values, thus this values overlap between 1s, suggesting a homogeneous, piston-like percolation of water into the cave. Regardless, of these variations, the cave waters average δ¹⁸O is in the range measured for the rain water δ¹⁸O average confirming the speleothems utility as paleotemperature proxies.