ASSESSING POTENTIAL SEASONAL PALEOCLIMATE SIGNALS IN A LATE PLEISTOCENE STALAGMITE FROM CENTRAL TEXAS

The late Pleistocene climate of central Texas is relatively well understood on long-term time scales, indicating that during the last glacial period this region was wetter and cooler. By contrast, little is known about short-term changes in central Texas’ climate. This necessitates a climate proxy with higher temporal resolution (i.e., seasonal to decadal), as has been found in some speleothems in other regions. Seasonal variations observed in the composition of modern-day cave drip waters in central Texas suggest the potential for seasonal proxies in speleothems here. A stalagmite from Double Decker Cave in central Texas exhibits well developed visible and UV fluorescent laminae. A previous U-series geochronologic study demonstrates that the sample grew over the past 70 kyr, with significant variations in growth rates. We used laser ablation inductively coupled mass spectrometry (LA-ICP-MS) to resolve high-frequency trace element cycles in a fast-growing, 16,000-year-old section of this stalagmite. Two parallel ~6mm-long LA-ICP-MS transects, spaced 30 microns apart, analyzed the concentrations of Mg, Sr, Ca, and Ba along the growth direction of the stalagmite. Regular cycles were observed in Ba/Ca, Sr/Ca, and Mg/Ca, with an average wavelength determined by qualitatively counting peaks. Additionally, we used fluorescence microscopy to characterize the mean width of fluorescent banding (light/dark couplets) along the laser transects. Based on the assumption that the trace element cycles and fluorescent bands are annual, growth rates can be calculated. Both 1) mean trace element wavelength, and 2) mean fluorescent band width yield growth rates that are within error of the U-series growth rate of 29 microns/year for the studied time interval. The close agreement of all three parameters suggests that chemical and UV fluorescent variations may both be responses to seasonal variations in temperature and/or rainfall. These results demonstrate the applicability of central Texas speleothems as potential climate proxies with seasonal resolution.