MONITORING CALCITE GROWTH AND CAVE AIR AND WATER COMPOSITION IN A TROPICAL CAVE WITH IMPLICATIONS FOR SPELEOTHEM PALEOCLIMATE RECORDS

We present preliminary results showing variable CO₂ concentrations in a tropical cave on the island of Guam coupled with changes in drip rates, drip-water geochemistry, and calcite growth. We deployed frosted glass plates at thirteen sites in two caves over three years, weighing them before and after to determine the amount of calcite growth. In addition to calcite growth rates, we also collected monthly drip-water samples and deployed loggers to measure CO₂. The prominent wet and dry seasons of the tropics have impacts on drip rates and drip-water chemistry, including both dissolved constituents and stable isotopic composition. While studies of Texas caves have highlighted the importance of intra-annual temperature changes on cave ventilation in temperate latitudes, cave air exchange and its effect on calcite growth in tropical climates (where seasons are determined by moisture conditions rather than temperature) remains poorly understood. Drip sites from chambers with the highest CO₂ concentrations generally have slower calcite growth rates, suggesting an influence of CO₂ ventilation. Instead of an annual temperature cycle driving density contrasts between cave and surface air, monthly observations of cave air pCO₂ in this study are anticorrelated with seasonal changes in wind intensity. Several drip sites record their fastest growth early in the dry season, when drip rates are still high and regional wind speeds are at or near their maxima.

The strength of intra-annual variability in drip-water chemistry is strongly site-specific. Even within the same cave, some drips show a clear annual cycle, while others are basically invariant. In sites with seasonal variability in drip-water chemistry, δ¹⁸O values track changes in rainfall composition and Mg/Ca and Sr/Ca ratios are driven by prior calcite precipitation during drier months. Previous work has suggested that paleoclimate proxies in speleothems, collected below drips with both annual variability in drip-water chemistry and seasonally variable growth rates, may not accurately represent mean annual conditions. At our study site, seasonal changes in growth rate do not appear to impact speleothem δ¹⁸O, but would alter the Mg/Ca of speleothem calcite from at least one drip site.