Feedbacks among O2 and CO2 in deep soil gas, oxidation of ferrous minerals, and fractures: A hypothesis for steady-state regolith thickness

Abstract

O 2 and CO 2 , the two essential reactants in weathering along with water and minerals, are important in deep regolith development because they diffuse to weathering fronts at depth. We monitored the dynamics of these gas concentrations in the hand-augerable zone on three ridgetops-one on granite and two on diabase-in Virginia (VA) and Pennsylvania (PA), U.S.A. and related the gas chemistry to regolith development. The VA granite and the PA diabase protoliths were more deeply weathered than the VA diabase. We attribute this to high protolith fracture density. The pO 2 and pCO 2 measurements of these more fractured sites displayed the characteristics of aerobic respiration year round. In contrast, the relation of pO 2 versus pCO 2 on the more massive VA diabase is consistent with seasonal changes in the dominant electron acceptor from O 2 to Fe(III), likely regulated by the expansion/contraction of nontronite in the soil BC horizon. These observations suggest that the fracture density is a first order control on deep regolith gas chemistry. However, fractures can be present in protolith but also can be caused by oxidation of ferrous minerals. We propose that subsurface pO 2 and weatheringinduced fracturing can create positive feedbacks in some lithologies that cause regolith to thicken while nonetheless maintaining aerobic respiration at depth. In contrast, in the absence of weathering-induced fracturing and depletion of pO 2 , a negative feedback that may be modulated by soil micro-biota ultimately results in thin regolith. These feedbacks may have been important in weathering systems over much of earth's history.

Soil CO2 and O2 Concentrations Illuminate the Relative Importance of Weathering and Respiration to Seasonal Soil Gas Fluctuations

“…For example, Angert et al (2015) observed that forest soils in humid, temperate systems (with low pH) rarely attained the ARQ ∼1 anticipated by diffusion and aerobic respiration; they posited that the soils in these acidic forests often have significant levels of reduced Fe in the O and A horizons. Kim et al (2017) observed a similar signature of Fe redox in the subsurface of a diabase‐derived soil in the piedmont of Virginia. This oxidation of Fe in both the soil surface and subsurface likely also contributes to the ARQ <1 we observe in the soils of the SSHCZO because high levels of redox‐active metals have been documented in the surface and subsurface soils of Shale Hills (Herndon et al, 2011; Jin et al, 2010; Kraepiel et al, 2015; Yesavage et al, 2012).…”

“…The capacities of minerals in the parent rock to consume O 2 through oxidation and to consume CO 2 through dissolution have been used to clarify the role of soil gases in weathering induced fracturing and regolith development (Brantley et al, 2014). Recently, scientists interested in understanding biotic controls on soil gases have also started using simultaneous measurements of soil CO 2 and O 2 to calculate the apparent respiratory quotient (Angert et al, 2015; Kim et al, 2017; Sánchez‐Cañete et al, 2018; Stinchcomb et al, 2018). The apparent respiratory quotient (ARQ) is the ratio of change of soil CO 2 from atmospheric conditions divided by the change of soil O 2 from atmospheric concentrations, corrected for the difference in diffusion rates between the two gases.…”

“…1). Deviations of ARQ from 1 can be driven by oxidation of reduced metals in primary minerals (Kim et al, 2017; Stinchcomb et al, 2018), anaerobic respiration by heterotrophs, chemoautotrophy, silicate weathering, carbonate equilibrium (Sánchez‐Cañete et al, 2018), or shifts in the dominant C substrate used for respiration (Masiello et al, 2008; Randerson et al, 2006). These controls on ARQ represent chemical and biological processes that may affect estimated rates of soil respiration and regolith weathering (Table 1).…”

“…To account for the different diffusivities of the gases, all plots of CO 2 vs. O 2 in this manuscript contain a theoretical relationship line with a slope of –0.76 and an x intercept of 20.95 (concentration of O 2 in atmospheric air). Plotting O 2 on the x axis and CO 2 on the y axis is a departure from the convention in some past publications (Kim et al, 2017; Weitzman and Kaye, 2018), but we show it this way to make a direct comparison to ARQ. Plotted in this way, the absolute value of the slope of the regression line for CO 2 vs. O 2 yields an estimate of the prevailing ARQ at the site.…”

Nitrogen Budget and Topographic Controls on Nitrous Oxide in a Shale‐Based Watershed

“…To estimate the oxidation ratio a best fit linear equation for all pO 2 versus pCO 2 was determined (adapted from Kim et al, , equation ): where a* represents the oxidation ratio that has been corrected for the relative diffusivity difference between O 2 and CO 2 ; that is, the product of a (the slope of pO 2 versus pCO 2 ) and D O2 / D CO2 (adapted from Angert et al, and Kim et al, , equation ). …”