RELATIONSHIPS BETWEEN SOIL GAS AND REGOLITH GEOCHEMISTRY IN THE APPALACHIAN CRITICAL ZONE: AN EXAMPLE FROM THE CUMBERLAND VALLEY, PA

The pO₂ and pCO₂ in regolith control pH and pe gradients with depth and therefore play an important role in weathering and evolution of the Appalachian Critical Zone. Yet few studies have measured both soil gas with bulk geochemistry to better understand this relationship. We measured soil O₂ and CO₂ along a 4-m thick weathering profile developed on a diabase dike in Cumberland Valley, PA to determine the relationship between gas and regolith geochemistry. Results from eight gas sampling campaigns from August to December (80 gas assays) show that 94% of CO₂ and O₂ profiles increase and decrease with depth, respectively. The mean CO₂ is 4.6±2.4% by vol. The mean O₂ is 16.8±2.1% by vol. Gas concentrations vary little at the saprolite-rock boundary, four meters below the surface. The mean slope of a plot of O₂ % by vol. versus CO₂ % by vol. is -0.86±0.07. This slope, within the range of reported O₂:CO₂ ratios driven by soil respiration, is consistent with root and microbial respiration as the primary mechanisms driving the consumption of O₂ and production of CO₂ within the diabase profile during the period of measure. At the saprolite-bedrock boundary, we found that the ratio of [O_2(aq)] to [CO_2(aq)] in the soil water assuming standard temperature and Henry’s Law is roughly equal to the ratio of the amount of O₂ to CO₂ consumed during weathering. We explore the physical, chemical and biological processes responsible for this relationship.