122 D4. Paleoclimatology, Sedimentary, and Historical Studies in Caves and Karst Features

Monday, 23 October 2017
1:30 PM - 5:30 PM
Room 612

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

Paper number: 122-10
Program: Technical Sessions
Time: 3:35 PM – 3:50 PM

Authors

Abstract

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, 20th-century speleothem (WC-3). Combining this age-model with 14C measurements of WC-3 and the 20th-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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-307999

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