GEOCHEMICAL ANALYSIS OF CAVE SEDIMENTS AT THREE SOUTH CENTRAL TEXAS CAVES: POTENTIAL IMPLICATIONS FOR CAVE-AIR CO2 PRODUCTION

Robber Baron Cave (RBC), a maze cave, located in south-central Texas, exhibits anomalously high CO₂ levels in summer months. Other caves in the region, Natural Bridge Caverns-north (NBC-n), and Cave Without A Name (CWAN), yield substantially lower CO₂ concentrations. Results of δ13C and δ15N values, trace metal concentrations, and mineralogical compositions of cave sediments suggest microbial activity may contribute to the differences in cave-air CO₂ concentrations found in south central Texas caves. Depleted δ13C and δ15N values, elevated iron and manganese concentrations, and high abundances of goethite, birnessite, and todorokite in elevated shelf deposits in RBC may suggest chemoautotrophic microbial activity as well as both allogenic and autogenic depositional processes. In contrast, slightly heavier δ13C and δ15N values relative to overlying soil organic matter, and elemental and mineralogical similarities to the bedrock provide substantial evidence for the allogenic deposition of sediments in NBC-n and CWAN. While the respiration from microbial activity could be a potential source for CO₂ in RBC there are likely multiple sources and processes that contribute to the elevated CO₂ levels. The analysis of cave sediments from the three caves in this study provides insight into the processes and sources associated with CO₂ production in both hypogenic and epigenic caves.