DIGGING BENEATH THE SOIL: EXAMINING THE ROLE OF ORGANIC CARBON OXIDATION IN VADOSE ZONES AND WATER TABLES IN KARSTIFICATION OF EOGENETIC LIMESTONE

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 pCO_2. 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.