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

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.  d¹³C_DIC values of the tidally influenced pool of Crescent Top Cave average -5.86 ‰, and were more depleted than Crescent Pond, which averages -4.28 ‰.  d¹³C_DIC of cave atmospheric water vapor has a value of -45.70 ‰, whereas d¹³C_CO2 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 d¹³C_DIC of permanent water bodies average -5.14 ‰, with a d¹³C_DOC value of -25.85 ‰.  d¹³C_DIC 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.