USING A CONCEPTUAL MODEL TO ELUCIDATE CONTROLS ON HIGH FREQUENCY MG/CA AND SR/CA SPELEOTHEM VARIATIONS

We use a conceptual model to generate cave drip-water and speleothem (mineral cave deposits) chemical time series that result from a given rainfall record under drip sites with contrasting hydrologic and cave-meteorologic characteristics. Time series are simulated for drip sites characterized by the following variables 1) conduit vs. diffuse vadose flow-path supplying drip sites, and 2) the presence vs. absence of seasonal calcite growth variations. Conduit-supplied drip sites have the potential to reflect short-term (sub-annual) rainfall variations, whereas diffuse-supplied drip sites have the potential to reflect more muted, long-term (> 4 years) rainfall variations. Seasonal variations in rainfall and/or cave-air CO₂ ventilation can affect seasonal calcite growth rates and drip-water Mg/Ca and Sr/Ca values at both conduit- and diffuse-supplied drip sites. This conceptual model illustrates the link between high frequency (as measured by laser ablation ICP-MS) speleothem Mg/Ca and Sr/Ca patterns and the processes responsible for such variations. Furthermore, the model highlights the differences that result in drip-water and speleothem time series from varying combinations of conduit vs. diffuse flow paths and seasonal vs. constant calcite growth. For example, a speleothem under a diffuse supplied drip site with seasonal calcite growth may produce a time series with high frequency, cyclical variations in Mg/Ca and Sr/Ca variations. The high frequency, cyclical Mg/Ca and Sr/Ca variations produced by our model share features with high frequency, cyclical Mg/Ca and Sr/Ca variations observed in both speleothem and modern drip-water time series. These features include 1) variability of cycle peak shape, including irregular asymmetry and occasional double peaks; and 2) compression and truncation of cycle troughs. The consistency between the modeled speleothem time series, observed speleothem time series, and observed drip-water time series shows the important role of conduit vs. diffuse flow and constant vs. seasonal calcite growth in linking climate variations to speleothem geochemistry.