HIGH-RESOLUTION HYDROCLIMATE RECONSTRUCTION IN STALAGMITES THROUGH LA-ICP-MS 2D MAPPING

Visualization of stalagmite growth morphology is often challenging by conventional petrographic approaches, yet it has high potential as a hydroclimate proxy in ventilation-style caves of the Edwards Plateau karst system, central Texas. Such proxies may provide insight to rapid climate changes impacting this semi-arid region, such as those that occurred during the last deglacial. Regional cave monitoring demonstrates that drip rate and changes in cave-air CO₂ levels exert principal controls on calcite growth and geochemistry (Banner et al. 2007; Miller et al.; 2021; Janelle et al. 2023). During warm months, rising cave-air CO₂ levels inhibit calcite growth. In cool months, dense outside air displaces high-CO₂ cave air accumulated during warm months. Lowering of cave air CO₂ levels rapidly shifts carbonate equilibrium to CaCO₃ precipitation, with faster drip sites accumulating more CaCO₃. For diffuse flow drip sites, the recharge volume stored in the epikarst is a likely control of drip rate. These observations predict that stalagmites accumulate calcite in cool months with faster accumulation during wetter climate periods. In stalagmite records with well-constrained U-series geochronology, stratigraphic visualization of seasonal growth could thus aid hydroclimate reconstruction. 2D elemental mapping by LA-ICP-MS demonstrates that seasonal growth bands can be resolved chemically (Sr, Ba), with thicker couplets (interpreted as cool season deposition) having higher elemental concentrations (Gunn et al., 2021). To be practical, maps must resolve growth bands and be made in a reasonable amount of time. We advanced in-house efficiencies for LA-ICP-MS 2D mapping through integration of a Dual Concentric Injector device and image calculations, using a fine scale aperture (5 x 20 µm). We then tested this approach in thin sections from a well age-constrained stalagmite spanning rapid climate transitions within the last deglaciation (19-10 ka), making 1.5 x 2.0 mm maps in less than 90 minutes that resolve seasonal growth bands as thin as 20 µm. Preliminary findings demonstrate that Sr and Ba concentrations scale with band thickness, suggesting element uptake controls by in-cave processes such as CO₂ degassing rate and/or systematic changes in drip water composition related to residence time of recharge in the epikarst.