Both vertical and lateral flows of rock and water occur within eroding hills. Specifically, when considered over geological timeframes, rock advects vertically upward under hilltops in landscapes experiencing uplift and erosion. Once rock particles reach the land surface, they move laterally and down the hillslope because of erosion. At much shorter timescales, meteoric water moves vertically downward until it reaches the regional water table and then moves laterally as groundwater flow. Water can also flow laterally in the shallow subsurface as interflow in zones of permeability contrast. Interflow can be perched or can occur during periods of a high regional water table. The depths of these deep and shallow water tables in hills fluctuate over time. The fluctuations drive biogeochemical reactions between water, CO 2 , O 2 , and minerals and these in turn drive fracturing. The depth intervals of water table fluctuation for interflow and groundwater flow are thus reaction fronts characterized by changes in composition, fracture density, porosity, and permeability. The shallow and deep reaction zones can separate over meters in felsic rocks. The zones act like valves that reorient downward unsaturated water flow into lateral saturated flow. The valves also reorient the upward advection of rock into lateral flow through solubilization. In particular, groundwater removes highly soluble, and interflow removes moderately soluble minerals. As rock and water moves through the system, hills may evolve toward a condition where the weathering advance rate, W, approaches the erosion rate, E. If W = E, the slopes of the deep and shallow reaction zones and the hillsides must allow removal of the most soluble, moderately soluble, and least soluble minerals respectively. A permeability architecture thus emerges to partition each evolving hill into dissolved and particulate material fluxes as it approaches steady state.
a b s t r a c tAccurate measurements of soil CO 2 concentrations (pCO 2 ) are important for understanding carbonic acid reaction pathways for continental weathering and the global carbon (C) cycle. While there have been many studies of soil pCO 2 , most sample or model only one, or at most a few, landscape positions and therefore do not account for complex topography. Here, we test the hypothesis that soil pCO 2 distribution can predictably vary with topographic position. We measured soil pCO 2 at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), Pennsylvania, where controls on soil pCO 2 (e.g., depth, texture, porosity, and moisture) vary from ridge tops down to the valley floor, between planar slopes and slopes with convergent flow (i.e., swales), and between north and south-facing aspects. We quantified pCO 2 generally at 0.1e0.2 m depth intervals down to bedrock from 2008 to 2010 and in 2013. Of the variables tested, topographic position along catenas was the best predictor of soil pCO 2 because it controls soil depth, texture, porosity, and moisture, which govern soil CO 2 diffusive fluxes. The highest pCO 2 values were observed in the valley floor and swales where soils are deep (!0.7 m) and wet, resulting in low CO 2 diffusion through soil profiles. In contrast, the ridge top and planar slope soils have lower pCO 2 because they are shallower ( 0.6 m) and drier, resulting in high CO 2 diffusion through soil profiles. Aspect was a minor predictor of soil pCO 2 : the north (i.e., south-facing) swale generally had lower soil moisture content and pCO 2 than its south (i.e., north-facing) counterpart. Seasonally, we observed that while the timing of peak soil pCO 2 was similar across the watershed, the amplitude of the pCO 2 peak was higher in the deep soils due to more variable moisture content. The high pCO 2 observed in the deeper, wetter topographic positions could lower soil porewater pH by up to 1 pH unit compared to porewaters equilibrated with atmospheric CO 2 alone. CO 2 is generally the dominant acid driving weathering in soils: based on our observations, models of chemical weathering and CO 2 dynamics would be improved by including landscape controls on soil pCO 2 .
Many areas in the world are characterized by shallow soils underlain by weathered bedrock, but root-rock interactions and their implications for regolith weathering are poorly understood. To test the role of tree roots in weathering bedrock, we excavated four pits along a catena in a shale-dominated catchment at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), USA. We measured a variety of biological, physical, and chemical properties including: 1) root density, distribution, and respiration, 2) soil gas, and 3) elemental compositions, mineralogy, and morphology of soil, rock, and rock fracture fill at ridge top, midslope, toe-slope, and valley floor sites. As expected, root density declined rapidly with depth; nevertheless, fine roots were present in rock fractures even in the deepest, least weathered rock sampled (~180 cm below the land surface). Root densities in shale fractures were comparable between the ridge top and mid-slope pits, but were significantly lower in the toe-slope, despite increasing rock fracture densities, which is likely due to a shallower water table depth at the downslope site. Average root respiration (per gram dry weight of root) in rock fractures was comparable to those in the soil. Thus, the total flux of CO 2 from root respiration tracked root length density, decreasing with depth. Potential microbial respiration in the soils, estimated as the laboratory C mineralization potential, was about an order of magnitude lower than measured root respiration in shale fractures. Roots were only observed in large aperture (>50 μm) shale fractures that were filled with particulate material. The fill in these fractures was mineralogically and geochemically similar to the lowest soil horizons with respect to clay composition, element mobility, extractable DOC, and potentially mineralizable C and N, while total C and total N values for the fracture fill were similar to the shale bedrock. In the bulk soil, depletion profiles (Al, Fe, K, Mg, and Si) relative to unweathered shale reflected characteristic weathering of illite 4
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.
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