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  • Wiley (CrossRef)  (22)
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  • 1
    In: Earth Surface Processes and Landforms, December 2013, Vol.38(15), pp.1793-1807
    Description: Landscape curvature evolves in response to physical, chemical, and biological influences that cannot yet be quantified in models. Nonetheless, the simplest models predict the existence of equilibrium hillslope profiles. Here, we develop a model describing steady‐state regolith production caused by mineral dissolution on hillslopes which have attained an equilibrium parabolic profile. When the hillslope lowers at a constant rate, the rate of chemical weathering is highest at the ridgetop where curvature is highest and the ridge develops the thickest regolith. This result derives from inclusion of all the terms in the mathematical definition of curvature. Including these terms shows that the curvature of a parabolic hillslope profile varies with distance from the ridge. The hillslope model (meter‐scale) is similar to models of weathering rind formation (centimeter‐scale) where curvature‐driven solute transport causes development of the thickest rinds at highly curved clast corners. At the clast scale, models fit observations. Here, we similarly explore model predictions of the effect of curvature at the hillslope scale. The hillslope model shows that when erosion rates are small and vertical porefluid infiltration is moderate, the hill weathers at both ridge and valley in the erosive transport‐limited regime. For this regime, the reacting mineral is weathered away before it reaches the land surface: in other words, the model predicts completely developed element‐depth profiles at both ridge and valley. In contrast, when the erosion rate increases or porefluid velocity decreases, denudation occurs in the weathering‐limited regime. In this regime, the reacting mineral does not weather away before it reaches the land surface and simulations predict incompletely developed profiles at both ridge and valley. These predictions are broadly consistent with observations of completely developed element‐depth profiles along hillslopes denuding under erosive transport‐limitation but incompletely developed profiles along hillslopes denuding under weathering limitation in some field settings. Copyright © 2013 John Wiley & Sons, Ltd.
    Keywords: Hillslope Evolution ; Regolith ; Curvature ; Reactive Transport Modeling ; Weathering
    ISSN: 0197-9337
    E-ISSN: 1096-9837
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  • 2
    In: Earth Surface Processes and Landforms, 15 September 2013, Vol.38(11), pp.1280-1298
    Description: Weathering is both an acid‐base and a redox reaction in which rocks are titrated by meteoric carbon dioxide (CO) and oxygen (O). In general, the depths of these weathering reactions are unknown. To determine such depths, cuttings of Rose Hill shale were investigated from one borehole from the ridge and four boreholes from the valley at the Susquehanna Shale Hills Observatory (SSHO). Pyrite concentrations are insignificant to depths of 23 m under the ridge and 8–9 m under the valley. Likewise, carbonate concentrations are insignificant to 22 and 2 m, respectively. In addition, a 5–6 m‐thick fractured layer directly beneath the land surface shows evidence for loss of illite, chlorite, and feldspar. Under the valley, secondary carbonates may have precipited. The limited number of boreholes and the tight folding make it impossible to prove that depth variations result from weathering instead of chemical heterogeneity within the parent shale. However, carbonate depletion coincides with the winter water table observed at ~20 m (ridge) and ~2 m depth (valley). It would be fortuitous if carbonate‐containing strata are found under ridge and valley only beneath the water table. Furthermore, pyrite and carbonate react quickly and many deep reaction fronts for these minerals are described in the literature. We propose that deep transport of O initiates weathering at SSHO and many other localities because pyrite commonly oxidizes autocatalytically to acidify porewaters and open porosity. According to this hypothesis, the mineral distributions at SSHO are nested reaction fronts that overprint protolith stratigraphy. The fronts are hypothesized to lie subparallel to the land surface because O diffuses to the water table and causes oxidative dissolution of pyrite. Pyrite‐derived sulfuric acid (HSO) plus CO also dissolve carbonates above the water table. To understand how reaction fronts record long‐term coupling between erosion and weathering will require intensive mapping of the subsurface. Copyright © 2013 John Wiley & Sons, Ltd.
    Keywords: Weathering ; Shale ; Pyrite ; Carbonates ; Clays
    ISSN: 0197-9337
    E-ISSN: 1096-9837
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  • 3
    In: Journal of Geophysical Research: Earth Surface, March 2017, Vol.122(3), pp.735-757
    Description: We propose models for calculating the rate of chemical weathering of minerals as a function of depth in the weathering zone. A macropore network is assumed to be responsible for the transport of mobile water, which removes soluble weathering products at the interface of that network and the matrix. Conditions of infrequent rainfall (A) and of very frequent rainfall (B) are separately modeled, but both lead to a volumetric weathering rate with the general form , where the amplitude and the equilibration length depend on the pore geometry of regolith and on parameters describing transport across the macropore‐matrix interface (A) or mineral dissolution (B). This is obtained with no assumption of steady state for regolith evolution. Extrapolating these end‐members into intermediate conditions, the model is consistent with the exponential decay of regolith production rate versus depth reported by several authors and yields in a variety of regolith types in the range from centimeters to meters. The velocity of the regolith‐bedrock interface also shows an exponential decay with weathering zone thickness and is enhanced by bedrock fractures when compared to models of unfractured bedrock. When the residence time of fluid in the weathering zone, , is large, the bulk weathering rate is inversely proportional to this time, and for small , reverts to the laboratory rate. Application to compiled data from several sites leads to estimates of a dissolution rate constant and specific surface area consistent with those of albite. Exponential decay of chemical weathering rate with depth is predicted for varying frequencies of rainfall events Effects of porosity, transport coefficients, chemical parameters, and fracture density are explored Applications to depth‐dependent regolith production rates and albite dissolution in several sites are discussed
    Keywords: Chemical Weathering ; Fractured Rock ; Transport And Reaction ; Regolith Production Rate ; Exponential Decay ; Albite Dissolution
    ISSN: 2169-9003
    E-ISSN: 2169-9011
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  • 4
    In: Earth Surface Processes and Landforms, October 2017, Vol.42(13), pp.2090-2108
    Description: We explore the contribution of fractures (joints) in controlling the rate of weathering advance for a low‐porosity rock by using methods of homogenization to create averaged weathering equations. The rate of advance of the weathering front can be expressed as the same rate observed in non‐fractured media (or in an individual block) divided by the volume fraction of non‐fractured blocks in the fractured parent material. In the model, the parent has fractures that are filled with a more porous material that contains only inert or completely weathered material. The low‐porosity rock weathers by reaction‐transport processes. As observed in field systems, the model shows that the weathering advance rate is greater for the fractured as compared to the analogous non‐fractured system because the volume fraction of blocks is 〈 1. The increase in advance rate is attributed both to the increase in weathered material that accompanies higher fracture density, and to the increase in exposure of surface of low‐porosity rock to reaction‐transport. For constant fracture aperture, the weathering advance rate increases when the fracture spacing decreases. Equations describing weathering advance rate are summarized in the ‘List of selected equations’. If erosion is imposed at a constant rate, the weathering systems with fracture‐bounded bedrock blocks attain a steady state. In the erosional transport‐limited regime, bedrock blocks no longer emerge at the air‐regolith boundary because they weather away. In the weathering‐limited (or kinetic) regime, blocks of various size become exhumed at the surface and the average size of these exposed blocks increases with the erosion rate. For convex hillslopes, the block size exposed at the surface increases downslope. This model can explain observations of exhumed rocks weathering in the Luquillo mountains of Puerto Rico. Published 2017. This article is a U.S. Government work and is in the public domain in the USA
    Keywords: Fractured Bedrock ; Weathering ; Erosion ; Reactive Transport Modeling ; Homogenization
    ISSN: 0197-9337
    E-ISSN: 1096-9837
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  • 5
    In: Earth Surface Processes and Landforms, January 2017, Vol.42(1), pp.128-156
    Description: The base of Earth's critical zone (CZ) is commonly shielded from study by many meters of overlying rock and regolith. Though deep CZ processes may seem far removed from the surface, they are vital in shaping it, preparing rock for infusion into the biosphere and breaking Earth materials down for transport across landscapes. This special issue highlights outstanding challenges and recent advances of deep CZ research in a series of articles that we introduce here in the context of relevant literature dating back to the 1500s. Building on several contributions to the special issue, we highlight four exciting new hypotheses about factors that drive deep CZ weathering and thus influence the evolution of life‐sustaining CZ architecture. These hypotheses have emerged from recently developed process‐based models of subsurface phenomena including: fracturing related to subsurface stress fields; weathering related to drainage of bedrock under hydraulic head gradients; rock damage from frost cracking due to subsurface temperature gradients; and mineral reactions with reactive fluids in subsurface chemical potential gradients. The models predict distinct patterns of subsurface weathering and CZ thickness that can be compared with observations from drilling, sampling and geophysical imaging. We synthesize the four hypotheses into an overarching conceptual model of fracturing and weathering that occurs as Earth materials are exhumed to the surface across subsurface gradients in stress, hydraulic head, temperature, and chemical potential. We conclude with a call for a coordinated measurement campaign designed to comprehensively test the four hypotheses across a range of climatic, tectonic and geologic conditions. Copyright © 2016 John Wiley & Sons, Ltd.
    Keywords: Regolith Production ; Near‐Surface Geophysics ; Weathering ; Fractures
    ISSN: 0197-9337
    E-ISSN: 1096-9837
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  • 6
    In: Earth Surface Processes and Landforms, 30 June 2013, Vol.38(8), pp.847-858
    Description: Weathering disaggregates rock into regolith – the fractured or granular earth material that sustains life on the continental land surface. Here, we investigate what controls the depth of regolith formed on ridges of two rock compositions with similar initial porosities in Virginia (USA). , we predicted that the regolith on diabase would be thicker than on granite because the dominant mineral (feldspar) in the diabase weathers faster than its granitic counterpart. However, weathering advanced 20× deeper into the granite than the diabase. The 20 × ‐thicker regolith is attributed mainly to connected micron‐sized pores, microfractures formed around oxidizing biotite at 20 m depth, and the lower iron (Fe) content in the felsic rock. Such porosity allows pervasive advection and deep oxidation in the granite. These observations may explain why regolith worldwide is thicker on felsic compared to mafic rock under similar conditions. To understand regolith formation will require better understanding of such deep oxidation reactions and how they impact fluid flow during weathering. Copyright © 2012 John Wiley & Sons, Ltd.
    Keywords: Regolith Thickness ; Fluid Transport ; Neutron Scattering ; Geomorphology ; Nanoporosity
    ISSN: 0197-9337
    E-ISSN: 1096-9837
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  • 7
    In: FEMS Microbiology Ecology, 2012, Vol. 79(1), pp.218-228
    Description: G eobacter sulfurreducens exists in the subsurface and has been identified in sites contaminated with radioactive metals, consistent with its ability to reduce metals under anaerobic conditions. The natural state of organisms in the environment is one that lacks access to high concentrations of nutrients, namely electron donors and terminal electron acceptors (TEAs). Most studies have investigated G . sulfurreducens under high-nutrient conditions or have enriched for it in environmental systems via acetate amendments. We replicated the starvation state through long-term batch culture of G . sulfurreducens , where both electron donor and TEA were scarce. The growth curve revealed lag, log, stationary, death, and survival phases using acetate as electron donor and either fumarate or iron(III) citrate as TEA. In survival phase, G . sulfurreducens persisted at a constant cell count for as long as 23 months without replenishment of growth medium. G eobacter sulfurreducens demonstrated an ability to acquire a growth advantage in stationary-phase phenotype (GASP), with strains derived from subpopulations from death- or survival phase being able to out-compete mid-log-phase populations when co-cultured. The molecular basis for GASP was not because of any detectable mutation in the rpoS gene (GSU1525) nor because of a mutation in a putative homolog to E scherichia coli lrp , GSU3370.
    Keywords: Dissimilatory Iron Reduction ; Starvation ; Stationary Phase
    ISSN: 01686496
    E-ISSN: 1574-6941
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  • 8
    In: Journal of Geophysical Research: Earth Surface, June 2013, Vol.118(2), pp.722-740
    Description: To investigate the timescales of regolith formation on hillslopes with contrasting topographic aspect, we measured U‐series isotopes in regolith profiles from two hillslopes (north facing versus south facing) within the east‐west trending Shale Hills catchment in Pennsylvania. This catchment is developed entirely on the Fe‐rich, Silurian Rose Hill gray shale. Hillslopes exhibit a topographic asymmetry: The north‐facing hillslope has an average slope gradient of ~20°, slightly steeper than the south‐facing hillslope (~15°). The regolith samples display significant U‐series disequilibrium resulting from shale weathering. Based on the U‐series data, the rates of regolith production on the two ridgetops are indistinguishable (40 ± 22 versus 45 ± 12 m/Ma). However, when downslope positions are compared, the regolith profiles on the south‐facing hillslope are characterized by faster regolith production rates (50 ± 15 to 52 ± 15 m/Ma) and shorter durations of chemical weathering (12 ± 3 to 16 ± 5 ka) than those on the north‐facing hillslope (17 ± 14 to 18 ± 13 m/Ma and 39 ± 20 to 43 ± 20 ka). The south‐facing hillslope is also characterized by faster chemical weathering rates inferred from major element chemistry, despite lower extents of chemical depletion. These results are consistent with the influence of aspect on regolith formation at Shale Hills; we hypothesize that aspect affects such variables as temperature, moisture content, and evapotranspiration in the regolith zone, causing faster chemical weathering and regolith formation rates on the south‐facing side of the catchment. The difference in microclimate between these two hillslopes is inferred to have been especially significant during the periglacial period that occurred at Shale Hills at least ~15 ka before present. At that time, the erosion rates may also have been different from those observed today, perhaps denuding the south‐facing hillslope of regolith but not quite stripping the north‐facing hillslope. An analysis of hillslope evolution and response timescales with a linear mass transport model shows that the current landscape at Shale Hills is not in geomorphologic steady state (i.e., so‐called dynamic equilibrium) but rather is likely still responding to the climate shift from the Holocene periglacial to the modern, temperate conditions. Regolith production rates determined by U-series isotopes Slope aspect controls regolith formation Landscape still responding to distubance in the past
    Keywords: Regolith Production ; Slope Aspect ; U‐Series Isotopes ; Chemical Weathering
    ISSN: 2169-9003
    E-ISSN: 2169-9011
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  • 9
    In: Eos, Transactions American Geophysical Union, 05 November 2013, Vol.94(45), pp.409-411
    Description: Clean drinking water is taken for granted in the United States, but what about groundwater and the water in streams, creeks, and lakes?
    Keywords: Marcellus Shale ; Water Quality ; Natural Gas ; Database
    ISSN: 0096-3941
    E-ISSN: 2324-9250
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  • 10
    In: Journal of Geophysical Research: Earth Surface, September 2013, Vol.118(3), pp.1877-1896
    Description: Regolith‐mantled hillslopes are ubiquitous features of most temperate landscapes, and their morphology reflects the climatically, biologically, and tectonically mediated interplay between regolith production and downslope transport. Despite intensive research, few studies have quantified both of these mass fluxes in the same field site. Here we present an analysis of 87 meteoric Be measurements from regolith and bedrock within the Susquehanna Shale Hills Critical Zone Observatory (SSHO), in central Pennsylvania. Meteoric Be concentrations in bulk regolith samples ( = 73) decrease with regolith depth. Comparison of hillslope meteoric Be inventories with analyses of rock chip samples ( = 14) from a 24 m bedrock core confirms that 〉80% of the total inventory is retained in the regolith. The systematic downslope increase of meteoric Be inventories observed at SSHO is consistent with Be accumulation in slowly creeping regolith (~ 0.2 cm yr). Regolith flux inferred from meteoric Be varies linearly with topographic gradient (determined from high‐resolution light detection and ranging‐based topography) along the upper portions of hillslopes at SSHO. However, regolith flux appears to depend on the product of gradient and regolith depth where regolith is thick, near the base of hillslopes. Meteoric Be inventories at the north and south ridgetops indicate minimum regolith residence times of 10.5 ± 3.7 and 9.1 ± 2.9 ky, respectively, similar to residence times inferred from U‐series isotopes in Ma et al. (2013). The combination of our results with U‐series‐derived regolith production rates implies that regolith production and erosion rates are similar to within a factor of two on SSHO hillcrests. Meteoric 10Be used to quantify regolith flux on soil‐mantled hillslopes Regolith flux depends linearly on hillslope gradient Regolith flux comparable to independently measured regolith production
    Keywords: Meteoric 10be ; Critical Zone ; Regolith Flux
    ISSN: 2169-9003
    E-ISSN: 2169-9011
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