2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 20
Presentation Time: 8:00 AM-4:45 PM

Tracing CO2 in Cave Systems: Testing a Biologic Model of H2CO3-Driven Cave Dissolution Using Stable Isotopes of Carbon

MCGEE, Dorien K., WYNN, Jonathan G., HARRIES, Peter J. and ONAC, Bogdon P., Department of Geology, University of South Florida, 4202 E. Fowler Avenue, SCA 528, Tampa, FL 33620, dmcgee@cas.usf.edu

Temperature, saturation state, and pH are often investigated as factors governing dissolution rates in speleogenetic research.  New studies, however, suggest that a biotic component, specifically bacteria, may contribute significantly to cave formation.  H2CO3 is a common corrosive agent in limestone settings, however the origin of the CO2 utilized in the production of H2CO3 is rarely investigated.  In this study, d13C of DIC and DOC collected from a variety of sources were used as tracers to determine sources and contributions of CO2 in two geographically distinct caves where H2CO3-driven dissolution has been documented.  Crescent Top Cave in the northeastern region of San Salvador Island, The Bahamas has a single, low entrance and is hydrologically influenced by two dominant sources: 1) surface percolation and 2) a tidally-influenced marine pool connected to the hypersaline Crescent Pond fed by subsurface conduits to the ocean.  d13CDIC values of the tidally influenced pool of Crescent Top Cave average -5.86 ‰, and were more depleted than Crescent Pond, which averages -4.28 ‰.  d13CDIC of cave atmospheric water vapor has a value of -45.70 ‰, whereas d13CCO2 of the cave atmosphere measures -16.07 ‰.  In contrast, Thornton's Cave in West Central Florida is a shallow cave with numerous entrances and permanent as well as ephemeral pools influenced by inputs from a variety of freshwater sources. Preliminary d13C analyses suggest that d13CDIC of permanent water bodies average -5.14 ‰, with a d13CDOC value of -25.85 ‰.  d13CDIC of pore water extracted from walls of the cave average -3.33 ‰.  Given the relatively enriched isotopic values of the abiotic sources that may influence CO2 in both cave settings, the depleted d13C results of this pilot study indicate that biotic activity influences the carbon cycling in cave settings, and is a likely contributor to H2CO3-driven dissolution.