Relating Soil Gas to Weathering Using Rock and Regolith Geochemistry
American Journal of Science
Weathering-induced fracturing (WIF) has been posited to be a mechanism that develops secondary porosity when mineral reaction fronts separate over depth intervals in regolith, and, in particular, when oxidation (which can promote porosity development through volume expansion) occurs deeper than dissolution (which grows porosity through material removal). If this is true, then the protolith's capacity to reduce O2 [for example, the Fe(II) content] and O2availability should affect WIF. This study explores the hypothesis that if the ratio of pO2 to pCO2, in soil water, R'(aq), is greater than the ratio of the capacity of the protolith to consume O2 and CO2, R0, then WIF is more likely to occur, and regolith will become thicker. We evaluated this hypothesis by measuring the bulk geochemistry of regolith and rock and monitoring soil gas at three sites, encompassing a wide range of FeO concentrations and regolith thickness: a Pennsylvania (PA) diabase (10.15%; 3.8 m), a Virginia (VA) diabase (10.49%; 1.4 m), and a VA granite (1.45%; 20 m). We inferred soil water O2concentrations from calculated equilibrium with the measured soil gas pO2. We observed WIF in the VA granite and PA diabase where R'(aq) > R0, while at the site that lacked WIF – the VA diabase – R'(aq) < R0, particularly during the wet season. In the VA diabase, the presence of swelling clays (smectite) limits the ability of the oxidant (O2) to diffuse deeper into the weathering profile during the wet season and microbially accelerated iron oxidation rapidly consumes O2, limiting O2 availability for WIF. Smectite has little to no observable effect on O2consumption in the PA diabase because the PA diabase is more fractured. A compilation of dissolved soil gas oxidation ratios, the stoichiometric ratio of O2consumed to CO2 produced, shows that for unsaturated conditions, the mean is −1.45 ± 0.88, which is consistent with aerobic root and microbial respiration and the oxidation of organic matter. For near-saturated conditions, the mean oxidation ratio of the compilation is −3.46 ± 1.79, which is consistent with Fe redox and microbial metabolism under reducing conditions. The consistency between the VA and PA data presented here and the compilation suggests that soil water surplus drives coupled Fe-redox reactions that may act as a negative feedback, limiting O2 supply and WIF under wetter soil moisture conditions.
We defined Rz, the ratio of O2 consumption to CO2 consumption during weathering for each depth interval, z. For all profiles, R'(aq) > Rz near the surface but R'(aq) approaches Rz in the saprolite. We suggest that R'(aq) > Rz in the soil reflects consumption of O2 and production of CO2 due to biotic processes whereas R'(aq) approaching Rz suggests that low fluxes at depth are at least partly dictated by rock and regolith composition, notably tortuosity of pores. In the VA diabase, we observed R'(aq) < Rz occasionally during the wet season in the lowermost soil and saprolite. Thus, at times the O2 availability may be less than the O2 consumption at that depth, consistent with Fe(II) loss and a lack of WIF. Mass-balance calculations show Fe loss in the VA diabase. The influence of rock composition on aqueous O2/CO2 concentrations in saprolite is consistent with the hypothesis that the protolith's capacity to consume O2 and CO2 has some effect on oxidation and acid consumption deep in the weathering profile.
Stinchcomb, Gary E., Hyojin Kim, Elizabeth A. Hasenmueller, Pamela L. Sullivan, Peter B. Sak, and Susan L. Brantley. "Relating Soil Gas to Weathering Using Rock and Regolith Geochemistry." American Journal of Science 318 (2018): 727-763. http://www.ajsonline.org/content/318/7/727.abstract?sid=22110060-d6c3-4e03-b110-802652a9e322