ASSESSING THE ROLE OF THE CARBONIC ANHYDRASE ENZYME IN CONTROLLING H2O-CO2 OXYGEN ISOTOPE EXCHANGE RATES DURING SPELEOTHEM GROWTH

The cave at Westcave Preserve in central Texas is a unique location to study speleothem growth due to the consistently low CO₂ concentrations and seasonally-variable temperature in the cave. Average calcite growth rates in the cave are high, ranging seasonally from 8-40 mg/day, as opposed to the 0-15 mg/day rates observed in other central Texas caves. Despite the rapid growth rates and low ambient CO₂, the calcite is growing at or near isotopic equilibrium with its drip water (Feng et al, 2014). We hypothesize that this unexpected equilibrium is caused by the activity of the enzyme carbonic anhydrase (CA) in the drip water. CA catalyzes the hydroxylation of CO₂ to bicarbonate. It is produced by many organisms, especially photosynthesizers, and is found in soil pore waters. CA has not been previously shown to play a role in speleothem formation, but this may be due to a sampling preference for speleothems from dark zones. The speleothems in Westcave are in entrance or twilight zones, which may allow in situ production of CA on the speleothem by photosynthetic microorganisms, altering the kinetic controls of the carbonate system and allowing for the rapid growth of calcite near isotopic equilibrium with its drip water. We allowed CO₂ to equilibrate with cave and deionized water for 0-22 hours at 25 °C. We duplicated this test with drip water from a dark site in Inner Space Cavern. The tests indicated that CO₂ reaches oxygen isotope equilibrium with drip water in Westcave significantly faster than with deionized water at 25 °C (~9 hours versus ~14 hours), but drip water from Inner Space Cavern shows no significant difference from deionized water. While this indicates that the Westcave drip water carbonate kinetics differ from Inner Space drip water carbonate kinetics, it does not directly prove the activity of CA in the cave. Further examination is required to determine whether or not CA is responsible for the fast oxygen-isotopic equilibration rates observed at Westcave, and whether this phenomenon is unique to and/or characteristic of near-entrance cave environments. If further testing confirms these initial results, near-entrance, twilight zone speleothems should not be treated as purely inorganic carbonate systems, but may still be important sources of stable-isotope climate proxies.