SEASONAL VARIATIONS IN MODERN SPELEOTHEM CALCITE GROWTH

Use of the growth rate and geochemistry of cave calcite deposits, or speleothems, as terrestrial paleoenvironmental records has grown considerably in the past decade. These applications typically postulate a correlation between speleothem calcite growth rates and climate but do not directly investigate the relationship between surface climate and subsurface speleothem growth. Changes in growth rates of speleothem calcite may reflect changes in a wide range of regional and local environmental variables, including atmospheric temperature and precipitation, vadose flow routes, drip-water composition, cave meteorology, soil microbial activity, and the rate of carbon dioxide delivery to the subsurface. In order to investigate the applicability of growth rates as a paleoenvironmental proxy in central Texas, we quantified the growth rates of modern calcite on artificial substrates placed under active drips in three caves across the regional Edwards aquifer. These growth rates are compared to concurrent environmental measurements taken at the drip sites and at the surface.

Eight drip sites were studied over three years, during which growth rates ranged from less than 1 to 75 mg/day on a substrate of approximately 100 cm². Mean growth rate at different drip sites correlates directly to mean drip rate. Within an individual cave, different drip sites exhibit similar temporal cycles in growth rates, including sites with large differences in their mean growth rate. These cycles correspond to seasonal changes above the cave, with the lowest growth rates occurring during hot, dry summers, and peak growth rates occurring in fall and winter. Drip sites from caves 140 km apart exhibit similar cycles in growth rates, indicating a controlling mechanism on at least this regional scale. This seasonal correspondence of growth rates likely reflects the control of temperature and effective precipitation on soil carbon dioxide productivity and attendant output to vadose waters. These results indicate that growth rate variations in ancient speleothems may serve as a proxy with seasonal resolution. Using the approach outlined here, speleothem growth rate proxies and, moreover, geochemical proxies may be evaluated and calibrated for individual and regional cave systems.