2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 335-2
Presentation Time: 1:45 PM


GULLEY, Jason, School of Geosciences, University of South Florida, 4202 E. Fowler Avenue, NES 107, Tampa, FL 33620-5550, MARTIN, Jonathan B., Department of Geological Sciences, University of Florida, 241 Williamson Hall, P.O. Box 112120, Gainesville, FL 32611-2120, MOORE, Paul J., Karst Waters Institute, Leesburg, VA 20177, SPELLMAN, Patricia, Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Dr, Houghton, MI 49931, BROWN, Amy L., Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611 and EZELL, John, Department of Geological Sciences, University of Florida, Gainesville, FL 32611-2120, gulley.jason@gmail.com

A growing body of evidence suggests that subsoil respiration of CO2 may play a previously underappreciated role in the weathering of carbonate landscapes. While canonical views maintain that soils are the primary source of CO2 that fuels dissolution of carbonate bedrock, pCO2 often increases with depth through vadose zones, and the pCO2 of water at water tables often exceeds the pCO2 of soils. Because soil CO2 cannot diffuse against a concentration gradient, soils cannot be the source of this deeper CO2. Possible sources include respiration of deeply rooted trees and microbial oxidation of particulate and dissolved organic carbon. While much work has investigated how subsoil CO2 production may impact precipitation of speleothems and the climate records preserved within them, less work has investigated how subsoil CO2 production may impact karstification.

Here we present the results of a study into the role of subsoil CO2 on dissolution within eogenetic carbonate landscapes. Data from two abandoned well fields in San Salvador Island, Bahamas, demonstrate that the pCO2 of water at the water table varied from less than 0.01 to more than 0.10 atm over distances of less than 30 m. This heterogeneous distribution of CO2 dissolves carbonate bedrock where water flows from regions of low to high pCO2. Such a process would form karst features such as flank margin caves and banana holes that have globular chamber morphologies and lack initial entrances to the surface. While these morphologies have previously been ascribed to mixing dissolution, we use simple geochemical models to show dissolution caused by heterogeneously distributed pCO2 can dissolve 2.5 to 10 times more calcite than the maximum amount possible by mixing of fresh water and seawater. Our results indicate that heterogeneously distributed pCO2 in the vadose zone, rather than mixing dissolution, may be the dominant mechanism for dissolution and distribution of macroporosity in eogenetic karst aquifers and for landscape development in these settings.