T135 Karst Critical Zone Evolution: The Rapid Responses of Carbonate Systems to Changes in Climate, Sea Level, Groundwater Pumping, and Land Cover/Land-Use

Monday, 26 September 2016
1:30 PM - 5:30 PM
Room 504

A SUBSOIL CARBON SOURCE TO CAVE-AIR CO2 AND SPELEOTHEM CALCITE IN CENTRAL TEXAS

Paper number: T135-4
Program: Technical Sessions
Time: 2:05 PM – 2:25 PM

Authors

Abstract

The canonical model for speleothem formation invokes soil respiration as the dominant carbon source. However, a number of lines of evidence point to a deeper, older carbon source to cave air CO2 in many karst regions. Here we use the oxidative ratio (OR, the number of moles of O2 consumed per mole of CO2 produced during respiration) to distinguish between soil and subsoil carbon sources. Because O2 and CO2 have different diffusion coefficients in air, measured ‘apparent’ OR values are controlled by diffusion in addition to the actual OR during respiration. We compared apparent OR values in caves and soils at two localities in central Texas: Natural Bridge Caverns (NB) and Inner Space Cavern (IS). At NB, soil gas samples follow the trend expected for respiration + diffusion (OR = 0.67 ± 0.05) whereas cave air samples follow a trend expected for respiration without subsequent diffusion (OR = 1.09 ± 0.06). At IS, most soil gas samples follow the respiration + diffusion trend, but some samples likely record substantial CO2 solution into water in a partly closed system resulting in a higher average OR value (OR = 1.01 ± 0.08). Cave air samples at IS also follow a trend expected for respiration followed by CO2 solution into water in a partly closed system (OR = 2.40 ± 0.14). These observations indicate that soil respiration is not the dominant carbon source to cave air CO2 at NB. In addition, these observations indicate that the majority of CO2 is advected into these caves as a gas, as opposed to diffusing into the caves or being transported by water. We suggest that 1) microbial oxidation of aged organic carbon in the bedrock fracture network is the dominant carbon source to cave air and speleothems and 2) bedrock fractures ventilate, advecting CO2 into caves. Radiocarbon activities of CO2 in cave air and soil gas support these interpretations and the chemistry of drip waters suggest that these interpretations also apply to the carbon source for speleothems.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-279385

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