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  • Chemical Weathering
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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
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(23), pp.7644-7667
    Description: Saprolite formation rates influence many important geological and environmental issues ranging from agricultural productivity to landscape evolution. Here we investigate the chemical and physical transformations that occur during weathering by studying small-scale “saprolites” in the form of weathering rinds, which form on rock in soil or saprolite and grow in thickness without physical disturbance with time. We compare detailed observations of weathered basalt clasts from a chronosequence of alluvial terraces in Costa Rica to diffusion-reaction simulations of rind formation using the fully coupled reactive transport model CrunchFlow. The four characteristic features of the weathered basalts which were specifically used as criteria for model comparisons include (1) the mineralogy of weathering products, (2) weathering rind thickness, (3) the coincidence of plagioclase and augite reaction fronts, and (4) the thickness of the zones of mineral reaction, i.e. reaction fronts. Four model scenarios were completed with varying levels of complexity and degrees of success in matching the observations. To fit the model to all four criteria, however, it was necessary to (1) treat diffusivity using a threshold in which it increased once porosity exceeded a critical value of 9%, and (2) treat mineral surface area as a fitting factor. This latter approach was presumably necessary because the mineral-water surface area of the connected (accessible) porosity in the Costa Rica samples is much less than the total porosity ( ). The model-fit surface area, here termed reacting surface area, was much smaller than the BET-measured surface area determined for powdered basaltic material. In the parent basalt, reacting surface area and diffusivity are low due to low pore connectivity, and early weathering is therefore transport controlled. However, as pore connectivity increases as a result of weathering, the reacting surface area and diffusivity also increase and weathering becomes controlled by mineral reaction kinetics. The transition point between transport and kinetic control appears to be related to a critical porosity (9%) at which pore connectivity is high enough to allow rapid transport. Based on these simulations, we argue that the rate of weathering front advance is controlled by the rate at which porosity is created in the weathering interface, and that this porosity increases because of mineral dissolution following a rate that is largely surface-reaction controlled.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 3
    Language: English
    In: Earth and Planetary Science Letters, 2010, Vol.297(1), pp.211-225
    Description: In the Critical Zone where rocks and life interact, bedrock equilibrates to Earth surface conditions, transforming to regolith. The factors that control the rates and mechanisms of formation of regolith, defined here as material that can be augered, are still not fully understood. To quantify regolith formation rates on shale lithology, we measured uranium-series (U-series) isotopes ( U, U, and Th) in three weathering profiles along a planar hillslope at the Susquehanna/Shale Hills Observatory (SSHO) in central Pennsylvania. All regolith samples show significant U-series disequilibrium: ( U/ U) and ( Th/ U) activity ratios range from 0.934 to 1.072 and from 0.903 to 1.096, respectively. These values display depth trends that are consistent with fractionation of U-series isotopes during chemical weathering and element transport, i.e., the relative mobility decreases in the order U 〉 U 〉 Th. The activity ratios observed in the regolith samples are explained by i) loss of U-series isotopes during water–rock interactions and ii) re-deposition of U-series isotopes downslope. Loss of U and Th initiates in the meter-thick zone of “bedrock” that cannot be augered but that nonetheless consists of up to 40% clay/silt/sand inferred to have lost K, Mg, Al, and Fe. Apparent equivalent regolith production rates calculated with these isotopes for these profiles decrease exponentially from 45 m/Myr to 17 m/Myr, with increasing regolith thickness from the ridge top to the valley floor. With increasing distance from the ridge top toward the valley, apparent equivalent regolith residence times increase from 7 kyr to 40 kyr. Given that the SSHO experienced peri-glacial climate ∼ 15 kyr ago and has a catchment-wide averaged erosion rate of ∼ 15 m/Myr as inferred from cosmogenic Be, we conclude that the hillslope retains regolith formed before the peri-glacial period and is not at geomorphologic steady state. Both chemical weathering reactions of clay minerals and translocation of fine particles/colloids are shown to contribute to mass loss of U and Th from the regolith, consistent with major element data at SSHO. This research documents a case study where U-series isotopes are used to constrain the time scales of chemical weathering and regolith production rates. Regolith production rates at the SSHO should be useful as a reference value for future work at other weathering localities.
    Keywords: U-Series Isotopes ; Regolith Formation ; Chemical Weathering ; Erosion ; Critical Zone ; Geology ; Physics
    ISSN: 0012-821X
    E-ISSN: 1385-013X
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  • 4
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 November 2017, Vol.217, pp.421-440
    Description: Shale formations account for 25% of the land surface globally and contribute a large proportion of the natural gas used in the United States. One of the most productive shale-gas formations is the Marcellus, a black shale that is rich in organic matter and pyrite. As a first step toward understanding how Marcellus shale interacts with water in the surface or deep subsurface, we developed a reactive transport model to simulate shale weathering under ambient temperature and pressure conditions, constrained by soil and water chemistry data. The simulation was carried out for 10,000 years since deglaciation, assuming bedrock weathering and soil genesis began after the last glacial maximum. Results indicate weathering was initiated by pyrite dissolution for the first 1000 years, leading to low pH and enhanced dissolution of chlorite and precipitation of iron hydroxides. After pyrite depletion, chlorite dissolved slowly, primarily facilitated by the presence of CO and organic acids, forming vermiculite as a secondary mineral. A sensitivity analysis indicated that the most important controls on weathering include the presence of reactive gases (CO and O ), specific surface area, and flow velocity of infiltrating meteoric water. The soil chemistry and mineralogy data could not be reproduced without including the reactive gases. For example, pyrite remained in the soil even after 10,000 years if O was not continuously present in the soil column; likewise, chlorite remained abundant and porosity remained small if CO was not present in the soil gas. The field observations were only simulated successfully when the modeled specific surface areas of the reactive minerals were 1–3 orders of magnitude smaller than surface area values measured for powdered minerals. Small surface areas could be consistent with the lack of accessibility of some fluids to mineral surfaces due to surface coatings. In addition, some mineral surface is likely interacting only with equilibrated pore fluids. An increase in the water infiltration rate enhanced weathering by removing dissolution products and maintaining far-from-equilibrium conditions. We conclude from these observations that availability of reactive surface area and transport of H O and gases are the most important factors affecting rates of Marcellus shale weathering of the in the shallow subsurface. This weathering study documents the utility of reactive transport modeling for complex subsurface processes. Such modelling could be extended to understand interactions between injected fluids and Marcellus shale gas reservoirs at higher temperature, pressure, and salinity conditions.
    Keywords: Chemical Weathering ; Reactive Transport Modeling ; Critical Zone ; Marcellus Shale ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 5
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 December 2016, Vol.195, pp.29-67
    Description: Inside soil and saprolite, rock fragments can form weathering clasts (alteration rinds surrounding an unweathered core) and these weathering rinds provide an excellent field system for investigating the initiation of weathering and long term weathering rates. Recently, uranium-series (U-series) disequilibria have shown great potential for determining rind formation rates and quantifying factors controlling weathering advance rates in weathering rinds. To further investigate whether the U-series isotope technique can document differences in long term weathering rates as a function of precipitation, we conducted a new weathering rind study on tropical volcanic Basse-Terre Island in the Lesser Antilles Archipelago. In this study, for the first time we characterized weathering reactions and quantified weathering advance rates in multiple weathering rinds across a steep precipitation gradient. Electron microprobe (EMP) point measurements, bulk major element contents, and U-series isotope compositions were determined in two weathering clasts from the Deshaies watershed with mean annual precipitation (MAP) = 1800 mm and temperature (MAT) = 23 °C. On these clasts, five core-rind transects were measured for locations with different curvature (high, medium, and low) of the rind-core boundary. Results reveal that during rind formation the fraction of elemental loss decreases in the order: Ca ≈ Na 〉 K ≈ Mg 〉 Si ≈ Al 〉 Zr ≈ Ti ≈ Fe. Such observations are consistent with the sequence of reactions after the initiation of weathering: specifically, glass matrix and primary minerals (plagioclase, pyroxene) weather to produce Fe oxyhydroxides, gibbsite and minor kaolinite. Uranium shows addition profiles in the rind due to the infiltration of U-containing soil pore water into the rind as dissolved U phases. U is then incorporated into the rind as Fe-Al oxides precipitate. Such processes lead to significant U-series isotope disequilibria in the rinds. This is the first time that multiple weathering clasts from the same watershed were analyzed for U-series isotope disequlibrian and show consistent results. The U-series disequilibria allowed for the determination of rind formation ages and weathering advance rates with a U-series mass balance model. The weathering advance rates generally decreased with decreasing curvature: ∼0.17 ± 0.10 mm/kyr for high curvature, ∼0.12 ± 0.05 mm/kyr for medium curvature, and ∼0.11 ± 0.04, 0.08 ± 0.03, 0.06 ± 0.03 mm/kyr for low curvature locations. The observed positive correlation between the curvature and the weathering rates is well supported by predictions of weathering models, i.e., that the curvature of the rind-core boundary controls the porosity creation and weathering advance rates at the clast scale. At the watershed scale, the new weathering advance rates derived on the low curvature transects for the relatively dry Deshaies watershed (average rate of 0.08 mm/kyr; MAP = 1800 mm and MAT = 23 °C) are ∼60% slower than the rind formation rates previously determined in the much wetter Bras David watershed (∼0.18 mm/kyr, low curvature transect; MAP = 3400 mm and MAT = 23 °C) also on Basse-Terre Island. Thus, a doubling of MAP roughly correlates with a doubling of weathering advance rate. The new rind study highlights the effect of precipitation on weathering rates over a time scale of ∼100 kyr. Weathering rinds are thus a suitable system for investigating long-term chemical weathering across environmental gradients, complementing short-term riverine solute fluxes.
    Keywords: U-Series Isotopes ; Weathering Rinds ; Weathering Rates ; Precipitation ; French Guadeloupe ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 6
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 March 2012, Vol.80, pp.92-107
    Description: To quantify rates of rind formation on weathering clasts under tropical and humid climate and to determine factors that control weathering reactions, we analyzed Uranium series isotope compositions and trace element concentrations in a basaltic andesite weathering clast collected from Basse-Terre Island in Guadeloupe. U, Th, and Ti elemental profiles reveal that Th and Ti behave conservatively during rind formation, but that U is added from an external source to the rind. In the rind, weathering reactions include dissolution of primary minerals such as pyroxene, plagioclase, and glass matrix, as well as formation of Fe oxyhydroxides, gibbsite and minor kaolinite. Rare earth element (REE) profiles reveal a significant Eu negative anomaly formed during clast weathering, consistent with plagioclase dissolution. Significant porosity forms in the rind mostly due to plagioclase dissolution. The new porosity is inferred to allow influx of soil water carrying externally derived, dissolved U. Due to this influx, U precipitates along with newly formed clay minerals and oxyhydroxides in the rind. The conservative behavior of Th and the continuous addition of U into the rind adequately explain the observed systematic trends of ( U/ Th) and ( Th/ Th) activity ratios in the rind. Rind formation rates, determined from the measured U-series activity ratios with an open system U addition model, increase by a factor of ∼1.3 (0.18–0.24 mm/kyr) from a low curvature to a high curvature section (0.018–0.12 mm ) of the core–rind boundary, revealing that curvature affects rates of rind formation as expected for diffusion-limited rind formation. U-series geochronometry thus provides the first direct evidence that the curvature of the interface controls the rate of regolith formation at the clast scale. The weathering rates determined at the clast scale can be reconciled with the weathering rates determined at the watershed or soil profile scale if surface roughness equals values of approximately 1300–2200.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 7
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 December 2012, Vol.99, pp.18-38
    Description: During weathering, Fe in primary minerals is solubilized by ligands and/or reduced by bacteria and released into soil porewaters. Such Fe is then removed or reprecipitated in soils. To understand these processes, we analyzed Fe chemistry and isotopic composition in regolith of the Shale Hills watershed, a Critical Zone Observatory in central Pennsylvania overlying iron-rich shale of the Rose Hill Formation. Elemental concentrations were measured in soil from a well-drained catena on a planar hillslope on the south side of the catchment. Based upon X-ray diffraction and bulk elemental data, loss of Fe commences as clay begins to weather ∼15 cm below the depth of auger-refusal. More Fe(III) was present than Fe(II) in all soil samples from the ridge top to the valley floor. Both total and ferrous iron are depleted from the land surface of catena soils relative to the bedrock. Loss of ferrous Fe is attributed mostly to abiotic or biotic oxidation. Loss of Fe is most likely due to transport of micron-sized particles that are not sampled by porous-cup lysimeters, but which are sampled in stream and ground waters. The isotopic compositions (δ Fe, relative to IRMM-014) of bulk Fe and 0.5 N HCl-extracted Fe (operationally designed to remove amorphous Fe (oxyhydr)oxides) range between −0.3‰ and +0.3‰, with Δ Fe values between ∼0.2‰ and 0.4‰. Throughout the soils along the catena, δ Fe signatures of both bulk Fe and HCl-extracted Fe become isotopically lighter as the extent of weathering proceeds. The isotopic trends are attributed to one of two proposed mechanisms. One mechanism involves Fe fractionation during mobilization of Fe from the parent material due to either Fe reduction or ligand-promoted dissolution. The other mechanism involves fractionation during immobilization of Fe (oxyhydr)oxides. If the latter mechanism is true, then shale – which comprises one quarter of continental rocks – could be an important source of isotopically heavy Fe for rivers.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 8
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 May 2013, Vol.109, pp.400-413
    Description: During weathering, rocks release nutrients and store water vital for growth of microbial and plant life. Thus, the growth of porosity as weathering advances into bedrock is a life-sustaining process for terrestrial ecosystems. Here, we use small-angle and ultra small-angle neutron scattering to show how porosity develops during initial weathering under tropical conditions of two igneous rock compositions, basaltic andesite and quartz diorite. The quartz diorite weathers spheroidally while the basaltic andesite does not. The weathering advance rates of the two systems also differ, perhaps due to this difference in mechanism, from 0.24 to 100 mm kyr , respectively. The scattering data document how surfaces inside the feldspar-dominated rocks change as weathering advances into the protolith. In the unaltered rocks, neutrons scatter from two types of features whose dimensions vary from 6 nm to 40 μm: pores and bumps on pore–grain surfaces. These features result in scattering data for both unaltered rocks that document multi-fractal behavior: scattering is best described by a mass fractal dimension ( ) and a surface fractal dimension ( ) for features of length scales greater than and less than ∼1 μm, respectively. In the basaltic andesite, is approximately 2.9 and is approximately 2.7. The mechanism of solute transport during weathering of this rock is diffusion. Porosity and surface area increase from ∼1.5% to 8.5% and 3 to 23 m g respectively in a relatively consistent trend across the mm-thick plagioclase reaction front. Across this front, both fractal dimensions decrease, consistent with development of a more monodisperse pore network with smoother pore surfaces. Both changes are consistent largely with increasing connectivity of pores without significant surface roughening, as expected for transport-limited weathering. In contrast, porosity and surface area increase from 1.3% to 9.5% and 1.5 to 13 m g respectively across a many cm-thick reaction front in the spheroidally weathering quartz diorite. In that rock, is approximately 2.8 and is approximately 2.5 prior to weathering. These two fractals transform during weathering to multiple surface fractals as micro-cracking reduces the size of diffusion-limited subzones of the matrix. Across the reaction front of plagioclase in the quartz diorite, the specific surface area and porosity change very little until the point where the rock disaggregates into saprolite. The different patterns in porosity development of the two rocks are attributed to advective infiltration plus diffusion in the rock that spheroidally fractures versus diffusion-only in the rock that does not. Fracturing apparently diminishes the size of the diffusion-limited parts of the spheroidally weathering rock system to promote infiltration of meteoric fluids, therefore explaining the faster weathering advance rate into that rock.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 9
    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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  • 10
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 October 2014, Vol.142, pp.260-280
    Description: Shale covers about 25% of the land surface, and is therefore an important rock type that consumes CO during weathering. We evaluated the potential of gray shale to take up CO from the atmosphere by investigating the evolution of dissolved inorganic carbon (DIC) concentrations and its carbon isotopic ratio (δ C ) along water flow paths in a well-characterized critical zone observatory (Susquehanna Shale Hills catchment). In this catchment, chemical weathering in shallow soils is dominated by clay transformation as no carbonates are present, and soil pore waters are characterized by low DIC and pH. In shallow soil porewaters, the DIC, dominated by dissolved CO , is in chemical and isotopic equilibrium with CO in the soil atmosphere where pCO varies seasonally to as high as 40 times that of the atmosphere. The degradation of ancient organic matter is negligible in contributing to soil CO . The chemistry of groundwater varies along different flowpaths as soil pore water recharges to the water table and then dissolves ankerite or secondary calcite under the valley floor. Weathering of carbonate leads to much higher concentrations of DIC (∼2500 μmol/L) and divalent cations (Ca and Mg ) in groundwaters than soil waters. The depth to the ankerite weathering front is hypothesized to be roughly coincident with the water table but it varies due to heterogeneities in the protolith composition. Groundwater chemistry therefore shows different saturation indices with respect to ankerite depending upon location along the valley. The δ C values of these groundwaters document mixing between the ankerite and soil CO . The major element concentrations, DIC, and δ C in the first-order stream incising the valley of the catchment are derived from groundwater and soil waters in proportions that vary both spatially and temporally. The CO degassed slightly in the stream but little evidence of C isotopic equilibration with the atmosphere is observed, due to the short length of the stream and short contact time with air. The ankerite reaction front also lies close to the pyrite dissolution front. Pyrite oxidation in bedrock likely released sulfuric acid and played a minor role in the ankerite dissolution, shifting groundwater δ C slightly above the expected mixing values. At the catchment scale, the stream SO is also dominantly derived from wet deposition, as stream has δ S values around 3‰, well within the range of acid deposition. A mass balance calculation shows that silicate and ankerite dissolution of the Rose Hill shale at Shale Hills consumes CO at a rate of ∼44 and ∼42–48 mol m ky respectively, while degradation of ancient organic matter releases CO at a rate of ∼1.3 mol m ky . Silicate dissolution at the shallow soils is facilitated by low pH and high soil pCO . As ankerite dissolution and organic matter oxidation are shown to occur early during shale alteration, CO consumption by shale weathering is thus limited by initiation of rock disintegration (e.g., fractures) and exposure of fresh surface area to infiltrating CO - and O -rich water.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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