HYDROLOGIC CONDITIONS AND CARBON CYCLING DYNAMICS RECORDED IN THE CARBON-ISOTOPE VARIATIONS OF A NEAR-ENTRANCE SPELEOTHEM

Near-entrance speleothems have traditionally been avoided for stable-isotope paleoclimate reconstructions, due to concerns about kinetic isotope effects, but the high growth rates of speleothems in such well-ventilated cave environments can allow for precise dating of high-resolution speleothem records. At Westcave Preserve in central Texas, we use seasonal variations in calcite δ¹⁸O values and [Mg] to develop a seasonally-resolved age model for a near-entrance, 20^th-century speleothem (WC-3). Combining this age-model with ¹⁴C measurements of WC-3 and the 20^th-century atmospheric bomb pulse record, we determined the relative size and turnover times of three carbon pools that best reproduce the ¹⁴C pulse recorded in the speleothem. We present this 3-pool model of epikarst carbon cycling here, along with a record of δ¹³C in WC-3 calcite. The carbon-cycling model informs our interpretations of speleothem δ¹³C, allowing us to account for decade-to-century-scale trends in atmospheric carbon as well as the effects of pCO₂-dependency of carbon isotope fractionation in plant material on WC-3 δ¹³C. We have removed these effects, and compared the resulting residual δ¹³C record to local instrumental records (NCDC and daily onsite precipitation records). We find that the 36-month running Palmer Drought Severity Index (PDSI) explains 25% of the variation of a 3-year mean of residual δ¹³C. For most of the record, troughs in 36-month PDSI (dry periods) correspond to maxima in the running mean of residual δ¹³C. Multiple processes can explain this correspondence between aridity and δ¹³C. These include: 1) increases in drip intervals allowing for more degassing of CO₂ before the water reaches the stalagmite; 2) decreases in δ¹³C fractionation between plant matter and atmospheric CO₂ during periods of water stress; and 3) decreases in belowground pCO₂ and resultant increases in drip water pH pushing DIC speciation away from carbonic acid. We also find that the timing of high-resolution δ¹³C peaks varies throughout the record, with carbon isotope maxima recorded semiannually to biannually. The controls on δ¹³C variations at this scale are likely complicated, depending on seasonal-scale variations in epikarst aridity or water residence time, the timing and magnitude of precipitation events, or local wind speed and direction.