MONITORING A UNIQUE KARST SYSTEM OVER TWO-YEARS TO ELUCIDATE HYDROCLIMATE VARIABLES INFLUENCING THE GEOCHEMISTRY OF SURFACE AND SUBSURFACE WATERS

Near entrance temperate latitude stalagmites may record seasonal growth and geochemical variations associated with hydroclimate variability. Monitoring the cave system can establish empirical relationships between hydroclimate variables and geochemical proxies and growth rate. We present results from a two-year (2017-2019 CE) bimonthly monitoring campaign at Sitting Bull Falls (SBF) Cave and its watershed in remote SE New Mexico. SBF Cave is continuously fed by a surface spring/stream that terminates into a surface pool perched 4m above the cave. In this sense, the cave potentially integrates changes in the entire watershed, and thus its stalagmites may provide unique information regarding temporal changes in watershed-scale processes. Surface (spring and stream samples) and subsurface (cave dripwaters) waters were analyzed for trace element concentrations and O and H isotope values. Cave calcite accumulated on plates placed under cave drip sites were analyzed for growth rate. Cave ventilation was monitored via cave-air temperature, CO₂ concentrations, and relative humidity.

Our monitoring results demonstrate year-round cave ventilation and calcite growth in SBF Cave. Dripwater O and H isotope values deviate from the Global Meteoric Water Line consistent with evaporation, likely occurring in the epikarst, on the cave ceiling, or both. Co-variation of ln(Sr/Ca) and ln(Mg/Ca) in surface and subsurface waters is quantitatively accounted for by prior calcite precipitation (PCP). Subsurface waters are more PCP-evolved than surface waters. Co-variation between ln(Sr/Ca) and ln(Mg/Ca) of cave dripwaters, which is commonly used as an aridity proxy in stalagmites, might be a result of PCP occurring in both the watershed and the cave. Lastly, the water-calcite distribution coefficient of Mg for a site nearest to the cave entrance has a strong correlation with seasonal temperature variability (r² = 0.67). Thus, we hypothesize that low/high Mg/Ca ratio couplets may aide in constraining seasonal geochemical cyclicity of near-entrance stalagmites. Taken together, we show processes in the watershed influencing the geochemical variability of cave dripwaters.