USING UV FLUORESCENT IMAGERY TO ASSESS SPELEOTHEM GROWTH RATE VARIATIONS DURING AN ABRUPT WARMING EPISODE

Speleothems are potentially useful high-resolution proxies of terrestrial climate that may record global climate events. The central Texas Edwards Plateau karst region contains a 70 ky record of speleothem formation, with first order growth associated with oscillations between glacial and interglacial periods and higher order growth variations related to abrupt climate transitions and seasonality of drivers of calcite precipitation. In this system, we characterized stalagmite (CWN4) growth-rate variations associated with the Bølling-Allerød excursion (BAE) – a globally recognized interval of abrupt warming that occurred ~14.7 ka during the last deglaciation. High-resolution δ¹⁸O analyses (IRMS micromill, SIMS), coupled with high-precision U-series ages identify the main BAE as a 1.5‰ decrease in δ¹⁸O that occurred within 400 years. Growth bands for this analysis were imaged by confocal fluorescent laser microscopy (CFLM) in a polished CWN4 slab, along two 1.5 cm-long, growth-parallel traverses. Measured growth band (fluorescent-non-fluorescent) couplet thicknesses closely correspond to the U-series growth rate estimates, strongly suggesting that growth bands are seasonal. Overall, the BAE represents a distinct period of reduced seasonal growth (median ~22 µm/yr), marking a transition from moderate (median ~38 µm/yr) to substantially faster (median ~72 µm/yr) seasonal growth. Because growth band thickness can be influenced by changes in seasonal drip water supply and duration of seasonal ventilation, and because Edwards Aquifer speleothems grew fastest during colder glacial periods, central Texas may have experienced more arid and warmer conditions during the BAE. Three dispersed cryptic zones, with unclear banding precede the BAE and may indicate prior climatic instability. The unique perspective of growth band morphology, realized by CFLM, also sheds light on the challenge of obtaining representative sampling via techniques such as micromill-IRMS, SIMS, and LA-ICP-MS.