IT'S OKAY . . . WE'RE IN THE BAND – IMPORTANCE OF ORIENTED BAND FABRIC IMAGERY FOR ESTABLISHING HIGH-RESOLUTION TRACE ELEMENT TIME-SERIES IN SLOW-GROWTH SPELEOTHEMS BY LA-ICP-MS

U-series dating of central Texas speleothems of the Edwards aquifer karst system indicates that calcite grew in response to glacial-interglacial climatic transitions spanning the past 70 ky. Modern geochemical monitoring studies of drip waters and associated calcite (ongoing in this system and elsewhere) have demonstrated seasonality in trace element concentrations, suggesting that continuous speleothem growth band fabrics may hold value as long-term, subannual, archives of terrestrial paleoclimate. Establishing high-resolution trace element time series in speleothems by LA-ICP-MS requires unaliased sampling at sub-annual spatial resolution, while obtaining sufficient signal-to-noise to discern potential season-to-season variations. Obtaining sub-annual sampling resolution in central Texas speleothems is challenged by: (1) markedly slow growth rates, which rarely exceeded 25-50 µm/year, and (2) the fact that growth bands are seldom revealed by conventional petrographic approaches. Provided adequate signal-to-noise, the former challenge can be met by using a thin rectangular aperture (5x150 µm) and slow scan rate (1µm/sec) for laser ablation line scans. Maintaining the rectangular “slit” aperture parallel to growth bands throughout the length of LA-ICP-MS line scans, the second challenge, is accomplished through the precise overlapping of UV-fluorescence imagery, which reveals the orientation and preservational status of growth bands. Georeferencing of imagery is facilitated by establishing a grid of small laser spots (400 µm spacing), which is visible on thin sections in the ablation cell and in UV-fluorescence imagery. Using this approach, in conjunction with ablation parameters optimized for speleothem calcite, yielded trace element time series that closely correspond to UV fluorescent growth band variations in most samples. Because UV-fluorescence character typically follows organic content, fluorescent growth band couplets in temperate climates are thought to correspond to winter discharge, when organic acids derived from the overlying soil and epikarst zone were likely to be most concentrated to adsorb onto or co-precipitate with growing calcite. The derived trace element time-series can thus be matched to fluorescence character to infer seasonality.