2009 Portland GSA Annual Meeting (18-21 October 2009)
Paper No. 178-3
Presentation Time: 9:00 AM-6:00 PM

INVESTIGATING CAVE VENTILATION IN CENTRAL TEXAS USING SEASONAL VARIATIONS IN THE δ13C VALUE AND CONCENTRATION OF CAVE-AIR CO2

QUINN, Ashley E.1, BREECKER, Dan O.2, BANNER, Jay L.2, and COWAN, Brian D.1, (1) Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, atheair@mail.utexas.edu, (2) Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712

Speleothem calcite growth rate is a common proxy for paleoclimate. Seasonal variations in cave CO2 are known to control speleothem growth. Thus, it is important to understand the mechanisms of CO2 input and removal from cave systems in order to determine the factors that control speleothem calcite growth rate. To this end, spatial and temporal variations in the atmosphere of several central Texas caves were studied over the course of a year. Endmembers that contribute to seasonal variations in cave-air CO2 concentrations were determined through analyzing δ13C values of air CO2 samples collected along spatial transects within the caves.

CO2 concentrations (pCO2) consistently increase and δ13C values consistently decrease with distance from the cave entrance. The values along these transects shift seasonally towards higher δ13C values and lower pCO2 in the winter months (October-March). A linear relationship between δ13C and 1/CO2 suggests that air in the cave is a mixture between atmospheric air (~380 ppm CO2, -8‰ δ13C, PDB) and a cave-air endmember with high pCO2 and low δ13C. The cave-air endmember is calculated to find the δ13C value of respired CO2 by correcting for atmospheric mixing and diffusion in the soil. The data are consistent with dominantly C3 vs. C4 vegetation above the caves, with δ13C values closer to -27‰ than to -13‰ δ13C. The δ13C value of respired CO2 is generally higher in the summer and lower in the winter, which is consistent with seasonal changes in the relative productivity of C3 vs. C4 plants. The seasonal- and regionally-consistent isotopic and pCO2 variations support density-driven ventilation, controlled by seasonal temperature changes, as the major influence on cave-air CO­2.

2009 Portland GSA Annual Meeting (18-21 October 2009)
General Information for this Meeting
Session No. 178--Booth# 205
Toward Integrating Cave and Karst Science (Posters)
Oregon Convention Center: Hall A
9:00 AM-6:00 PM, Tuesday, 20 October 2009

Geological Society of America Abstracts with Programs, Vol. 41, No. 7, p. 471

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