Digging Beneath the Soil: Examining the Generation of Vuggy Porosity in Eogenetic Limestones by Organic Carbon Oxidation in Vadose Zones and Water Tables
A growing body of evidence suggests that subsoil respiration of CO2 may play a previously underappreciated role in the dissolution of carbonate rocks. While canonical views maintain that soils are the primary source of CO2 that fuels carbonate mineral dissolution, pCO2 often increases 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 of CO2 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 the precipitation of speleothems and the climate records preserved within them, less work has investigated how subsoil CO2 production may impact the development of vuggy porosity at and near the water table. Here we present the results of a study into the role of subsoil CO2 on dissolution within eogenetic carbonates. 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 log -2.0 to more than log -1.0 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 likely drives the development of vuggy porosity including meter-scale pores, 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, rather than mixing dissolution, may be the dominant mechanism for the development of vuggy porosity in meteoric settings. Such recognition of pore type development provides a powerful insight on the magnitude and distribution of vuggy porosity in carbonate aquifers and reservoirs.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017