Hypothesis to be Tested
October 29, 2022
Speleothem carbonates are useful terrestrial paleoclimate archives because they can record high resolution proxies such as δ13C, δ18O, δ44Ca, and trace metals (Hendy, 1971; Wang et al., 2001; Owen et al, 2016). Past studies have examined these proxies as indicators of aridity, temperature, and effective precipitation on decadal or geologic time scales. Proxies such as δ18O and δ13C are dependent on regional rainfall, temperature, and vegetation processes, so speleothems can only be used to measure this proxy after substantial equilibration in the epikarst and with considerations of previous fractionation (Dreybrodt and Scholz, 2011; Fohlmeister et al., 2020). Considerable efforts have been made to discover a proxy that is more specific to regional climate and on a more precise time scale, demonstrating change on an annual or even seasonal scale (Fairchild et al., 2000).
Recent discoveries have revealed hidden seasonal banding patterns in Texas speleothems, as evidenced through CLFM imagery and LA-ICP-MS of various trace elemental concentrations in thick section (Miller et al, 2021). This project seeks to determine present system processes that could have implications on seasonal banding, as well as evaluating to what extent epikarst and in-cave processes are each contributing to fractionation.This project seeks not only to support the hypothesis that observed bands reflect seasonal changes, but also to determine what geologic and in-cave processes could be contributing to trace metal fractionations. In order to gather precise concentration ratios, a solution mode ICP-MS method is developed to compare fractionation sources observed in speleothems.
Multiple geologic and in-cave processes could contribute to potential changes in water chemistry. Secondary carbonate mineral formation preferentially uses calcium in its crystal lattice according to Goldschmidt’s Law, resulting in the enrichment of trace metals (Mg, Ba, Sr) in dripwaters (Fairchild et al., 2000; McDonald and Drysdale, 2004; Musgrove and Banner, 2004). If prior calcite precipitation occurs in the vadose zone of Cave Without a Name, we expect to find variation in TE/Ca ratios increase (Tremaine and Froelich, 2013; Owen et al., 2016; Fairchild et al., 2000). Other factors must also be considered, such as changes in cave temperature, growth rate, drip rate, CO2 concentration, and precipitation, as well as bedrock composition and residence time (Tremaine and Froelich, 2013; Johnson et al., 2006). TE/Ca ratios are observed in speleothems have been used as indications that PCP and an increase in aridity has occurred, but the quantitative to which it has an effect is difficult to discern (Fairchild et al., 2000; Treble et al., 2005; Tremaine and Froelich, 2013). The goal of this method-development study is to better characterize the relationship of trace elemental concentration in drip water to that of plate calcite, and ultimately to understand drivers of stalagmite element enrichment cycles and their relationship to geologic and in-cave processes. If this study is successful, it will not only enable a new understanding of water-rock interaction in central Texas, but it will also provide a baseline for how this method can be used to monitor other caves in regions governed by similar processes.