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Oxidative weathering of pyrite plays an important role in the biogeochemical cycling of Fe and S in terrestrial environments. While the mechanism and occurrence of biologically accelerated pyrite oxidation under acidic conditions are well established, much less is known about microbially mediated pyrite oxidation at circumneutral pH. Recent work (Percak‐Dennett et al., 2017, Geobiology, 15, 690) has demonstrated the ability of aerobic chemolithotrophic microorganisms to accelerate pyrite oxidation at circumneutral pH and proposed two mechanistic models by which this phenomenon might occur. Here, we assess the potential relevance of aerobic microbially catalyzed circumneutral pH pyrite oxidation in relation to subsurface shale weathering at Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in Pennsylvania, USA. Specimen pyrite mixed with native shale was incubated in groundwater for 3 months at the inferred depth of in situ pyrite oxidation. The colonized materials were used as an inoculum for pyrite‐oxidizing enrichment cultures. Microbial activity accelerated the release of sulfate across all conditions. 16S rRNA gene sequencing and metagenomic analysis revealed the dominance of a putative chemolithoautotrophic sulfur‐oxidizing bacterium from the genus Thiobacillus in the enrichment cultures. Previously proposed models for aerobic microbial pyrite oxidation were assessed in terms of physical constraints, enrichment culture geochemistry, and metagenomic analysis. Although we conclude that subsurface pyrite oxidation at SSCHZO is largely abiotic, this work nonetheless yields new insight into the potential pathways by which aerobic microorganisms may accelerate pyrite oxidation at circumneutral pH. We propose a new "direct sulfur oxidation" pathway, whereby sulfhydryl‐bearing outer membrane proteins mediate oxidation of pyrite surfaces through a persulfide intermediate, analogous to previously proposed mechanisms for direct microbial oxidation of elemental sulfur. The action of this and other direct microbial pyrite oxidation pathways have major implications for controls on pyrite weathering rates in circumneutral pH sedimentary environments where pore throat sizes permit widespread access of microorganisms to pyrite surfaces. [ABSTRACT FROM AUTHOR]

We used seismic refraction to image the P‐wave velocity structure of a shale watershed experiencing regional compression in the Valley and Ridge Province (USA). From estimates showing strong compressional stress, we expected the depth to unweathered bedrock to mirror the hill‐valley‐hill topography ("bowtie pattern") by analogy to seismic velocity patterns in crystalline bedrock in the North American Piedmont that also experience compression. Previous researchers used failure potentials calculated for strong compression in the Piedmont to suggest fractures are open deeper under hills than valleys to explain the "bowtie" pattern. Seismic images of the shale watershed, however, show little evidence of such a "bowtie." Instead, they are consistent with weak (not strong) compression. This contradiction could be explained by the greater importance of infiltration‐driven weathering than fracturing in determining seismic velocities in shale compared to crystalline bedrock, or to local perturbations of the regional stress field due to lithology or structures. Plain Language Summary: Rock mechanic theory suggests that the depth to crystalline bedrock under hill‐valley‐hill landscapes mirrors the land surface when the landscape experiences strong compression. We tested for this in a region of compression for a watershed on shale and found the depth pattern was consistent only with weak compression. This observation may be because infiltration and chemical weathering are more important than mechanical fracturing in controlling density of near‐surface shale. Alternatively, local effects related to the last glacial advance or the differences in rock types might explain the observation. The depth of weathering (depth to bedrock) is apparently not only controlled by fracturing but rather is heavily influenced by hydrogeochemical processes on shale. Key Points: The P‐wave velocity structure of a shale watershed under compression is imagedSeismic images show little evidence of the expected bowtie structureResults are explained by greater importance of chemical weathering than fracturing in determining seismic velocities in shale landscapes [ABSTRACT FROM AUTHOR]

Knowing little about how porosity and permeability are distributed at depth, we commonly develop models of groundwater by treating the subsurface as a homogeneous black box even though porosity and permeability vary with depth. One reason for this depth variation is that infiltrating meteoric water reacts with minerals to affect porosity in localized zones called reaction fronts. We are beginning to learn to map and model these fronts beneath headwater catchments and show how they are distributed. The subsurface landscapes defined by these fronts lie subparallel to the soil‐air interface but with lower relief. They can be situated above, below, or at the water table. These subsurface landscapes of reaction are important because porosity developed from weathering can control subsurface water storage. In addition, porosity often changes at the weathering fronts, and when this affects permeability significantly, the front can act like a valve that re‐orients water flowing through the subsurface. We explore controls on the positions of reaction fronts under headwater landscapes by accounting for the timescales of erosion, chemical equilibration, and solute transport. One strong control on the landscape of subsurface reaction is the land surface geometry, which is in turn a function of the erosion rate. In addition, the reaction fronts, like the water table, are strongly affected by the lithology and water infiltration rate. We hypothesize that relationships among the land surface, reaction fronts, and the water table are controlled by feedbacks that can push landscapes towards an 'ideal hill'. In this steady state, reaction‐front valves partition water volumes into shallow and deep flowpaths. These flows dissolve low‐ and high‐solubility minerals, respectively, allowing their reaction fronts to advance at the erosion rate. This conceptualization could inform better models of subsurface porosity and permeability, replacing the black box. [ABSTRACT FROM AUTHOR]

In weathered bedrock aquifers, groundwater is stored in pores and fractures that open as rocks are exhumed and minerals interact with meteoric fluids. Little is known about this storage because geochemical and geophysical observations are limited to pits, boreholes, or outcrops or to inferences based on indirect measurements between these sites. We trained a rock physics model to borehole observations in a well-constrained ridge and valley landscape and then interpreted spatial variations in seismic refraction velocities. We discovered that P-wave velocities track where a porosity-generating reaction initiates in shale in three boreholes across the landscape. Specifically, velocities of 2.7 ± 0.2 km/s correspond with growth of porosity from dissolution of chlorite, the most reactive of the abundant minerals in the shale. In addition, sonic velocities are consistent with the presence of gas bubbles beneath the water table under valley and ridge. We attribute this gas largely to CO2 produced by 1) microbial respiration in soils as meteoric waters recharge into the subsurface and 2) the coupled carbonate dissolution and pyrite oxidation at depth in the rock. Bubbles may nucleate below the water table because waters depressurize as they flow from ridge to valley and because pores have dilated as the deep rock has been exhumed by erosion. Many of these observations are likely to also describe the weathering and flow path patterns in other headwater landscapes. Such combined geophysical and geochemical observations will help constrain models predicting flow, storage, and reaction of groundwater in bedrock systems. [ABSTRACT FROM AUTHOR]

We present a model of chemical reaction within hills to explore how evolving porosity (and by inference, permeability) affects flow pathways and weathering. The model consists of hydrologic and reactive‐transport equations that describe alteration of ferrous minerals and feldspar. These reactions were chosen because previous work emphasized that oxygen‐ and acid‐driven weathering affects porosity differently in felsic and mafic rocks. A parameter controlling the order of the fronts is presented. In the absence of erosion, the two reaction fronts move at different velocities and the relative depths depend on geochemical conditions and starting composition. In turn, the fronts and associated changes in porosity drastically affect both the vertical and lateral velocities of water flow. For these cases, estimates of weathering advance rates based on simple models that posit unidirectional constant‐velocity advection do not apply. In the model hills, weathering advance rates diminish with time as the Darcy velocities decrease with depth. The system can thus attain a dynamical steady state at any erosion rate where the regolith thickness is constant in time and velocities of both fronts become equal to one another and to the erosion rate. The slower the advection velocities in a system, the faster it attains a steady state. For example, a massive rock with relatively fast‐dissolving minerals such as diabase – where solute transport across the reaction front mainly occurs by diffusion – can reach a steady state more quickly than granitoid rocks in which advection contributes to solute transport. The attainment of a steady state is controlled by coupling between weathering and hydrologic processes that force water to flow horizontally above reaction fronts where permeability changes rapidly with depth and acts as a partial barrier to fluid flow. Published 2020. This article is a U.S. Government work and is in the public domain in the USA. [ABSTRACT FROM AUTHOR]

The flux of solutes from the chemical weathering of the continental crust supplies a steady supply of essential nutrients necessary for the maintenance of Earth's biosphere. Promotion of weathering by microorganisms is a well-documented phenomenon and is most often attributed to heterotrophic microbial metabolism for the purposes of nutrient acquisition. Here, we demonstrate the role of chemolithotrophic ferrous iron [Fe(II)]-oxidizing bacteria in biogeochemical weathering of subsurface Fe(II)-silicateminerals at the Luquillo Critical Zone Observatory in Puerto Rico. Under chemolithotrophic growth conditions, mineral-derived Fe(II) in the Rio Blanco Quartz Diorite served as the primary energy source for microbial growth. An enrichment in homologs to gene clusters involved in extracellular electron transfer was associated with dramatically accelerated rates of mineral oxidation and adenosine triphosphate generation relative to sterile diorite suspensions. Transmission electron microscopy and energy-dispersive spectroscopy revealed the accumulation of nanoparticulate Fe-oxyhydroxides on mineral surfaces only under biotic conditions. Microbially oxidized quartz diorite showed greater susceptibility to proton-promoted dissolution, which has important implications for weathering reactions in situ. Collectively, our results suggest that chemolithotrophic Fe(II)-oxidizing bacteria are likely contributors in the transformation of rock to regolith. [ABSTRACT FROM AUTHOR]

The Rio Icacos watershed in the Luquillo Mountains (Puerto Rico) is unique due to its extremely rapid weathering rates. The watershed is incised into a quartz diorite that has developed a large knickzone defining the river profile. Regolith thickness within the watershed generally decreases from 20 to 30 m at the ridges to several meters in the quartz diorite‐dominated valley to tens of centimeters near the major river knickpoint, as determined from previous studies. Above the knickzone, we observe spheroidal corestones, but below this weathering is much less apparent. Measured erosion rates from previous studies are also high in the knickzone compared with upper elevations within the river profile. A suite of near‐surface geophysical methods (i.e. ground penetrating radar and terrain conductivity) capable of fast data acquisition in rugged landscapes, was deployed at kilometer scales to characterize critical zone structure. Concentrations of chaotic ground penetrating radar (GPR) reflections and diffraction hyperbolas with low electrical conductivity were observed in vertical zones that outcrop at the land surface as areas of intense fracturing and spheroidally weathered corestones. The width of these fractured and weathered zones showed an increase with proximity to the knickpoint, and was attributed to dilation of these sub‐vertical fractures near the knickpoint, as postulated theoretically by a stress model calculated for the topographic variability across the knickzone in the Rio Icacos, and that shows a release of compressive stress near the knickpoint. We hypothesize that erosion rates increase in the knickzone because of this inferred dilation of fractures. Specifically, opened fractures could enhance access of water and in turn promote spalling, erosion, and spheroidal weathering. This study shows that ground‐based hydrogeophysical methods used at the landscape‐scale (traditionally applied at smaller scales) can be used to explore critical zone architecture at the scales needed to explain the extreme variability in erosion rates across river profiles. © 2018 John Wiley & Sons, Ltd. A suite of near‐surface geophysical methods was deployed at kilometer scales to characterize fracturing zones across the Rio Icacos Watershed in the Luquillo Mountains (Puerto Rico). Results were combined with stress models calculated for the topographic variability across the knickzone and showed an increase in fracturing width with proximity to the knickpoint that is indicative of a release in compressive stress near the knickpoint. [ABSTRACT FROM AUTHOR]

Abstract Models for weathering and regolith formation are generally built on the assumption of constant rates of water advection in the zone of water-saturated pores, and constant water content of those pores, but it is common that weathering occurs in the water-unsaturated zone where lateral flow occurs. Thus, water content in pores varies with depth. Here we model mineral weathering profiles while accounting for depth-dependent water content. Like previous models, a mineral equilibrates with water over a length ξ that depends on dissolution and advection rates, but a new lengthscale λ is introduced to describe the decrease of water content with depth. Steady states of the regolith thickness can be attained for any finite λ and non-zero velocity v E of erosion at the land surface. The type of mineral depletion profile developed over geological timescales depends on coupling between weathering and erosion: for slow erosion, a completely developed profile (CDP) is observed, in which the mineral-depleted zone at the top of the regolith has thickness of order λ ; as v E increases, there is a transition to an incompletely developed profile (IDP), in which partial mineral depletion at the land surface is constrained to a narrow range of velocities; when v E exceeds the advance rate of weathering under far from equilibrium conditions, the profile transitions to an unstable regime that exposes bedrock. In general, the reaction front (RF) thickness and the velocity where CPD transitions to IDP depend on the interplay of both water infiltration and chemical equilibration over the timescale of water residence in regolith. The RF thickness roughly equals a correlation length χ defined as half the harmonic average of ξ and λ. In cases of limited water infiltration, water-mineral equilibration is achieved within the length λ , so that the RF thickness is controlled by hydrological properties and is independent of dissolution and advection rates. In the opposite endmember, water infiltrates to large depths and the effects of physical and chemical parameters on RF thickness are the same as in previous geochemical models. The relaxation time for reaching a steady state is shown to be ∼ χ / v E . We discuss the effects of physical and chemical parameters in CDPs and IDPs in those endmembers and show an application to a CDP in granitic regolith. [ABSTRACT FROM AUTHOR]

Mineral weathering, a major control on long term atmospheric CO 2 , can be limited by processes such as reaction kinetics, supply of fresh mineral, or water throughput in the weathering zone. In these cases, weathering fluxes increase with temperature, erosion, and runoff, respectively, and we refer to the regimes as kinetically limited (KL), erosive transport limited (ETL), and runoff limited (RL). Larger watersheds can exhibit mixtures of these behaviors in a transition regime (TR). To understand these weathering regimes, we focused on the lithology (shale) that is one of the most widespread lithologies exposed on continents globally. We identified and analyzed 142 shale watersheds across the United States, spanning a diverse range of climatic (mean annual temperature (MAT): 3–17 °C; mean annual precipitation (MAP): 220–1975 mm yr−1; potential evapotranspiration (PET): 908–2059 mm yr−1), topographic (mean catchment elevation: 78–2346 m; mean catchment slope: 0.4–28°), and hydrologic (annual runoff (q): 0.002–1.3 m yr−1) conditions. For these sites, we calculated silica fluxes (i.e., F SiO2) using SiO 2 concentrations and river discharge measurements. We observed that when the humidity index (HI; MAP/PET) is <0.55, F SiO2 increases with MAP and q; however, when HI > 0.55 F SiO2 , does not increase with MAP nor q. We posit that sites with HI < 0.55 are RL. The river chemistry for the RL sites is consistent with equilibrium between smectite and kaolinite. Fitting the data for sites with HI > 0.55 to an exponential temperature dependence, we find that the sensitivity of weathering to temperature is consistent with an apparent activation energy, Ea, of 56 ± 8 kJ mol−1. If sites with HI > 0.55 but average slopes <10°are removed, the value increases to 92 ± 10 kJ mol−1. This value is consistent with the Ea measured experimentally for the dissolution of the common shale clay, chlorite. To extrapolate from our measurements, we use geospatial-based definitions to classify HUC12 watersheds across the United States. This reveals that 48%, 37%, 9%, and 7% of US shale watersheds are RL, ETL, TR, and KL, respectively. Globally, we estimate that silicates weathering in shales consume 2.6 × 1012 mol CO 2 yr−1, which is 22% of the total CO 2 sequestered by silicate weathering. [ABSTRACT FROM AUTHOR]

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 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

ABSTRACT 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. [ABSTRACT FROM AUTHOR]

ABSTRACT Weathering is both an acid-base and a redox reaction in which rocks are titrated by meteoric carbon dioxide (CO2) and oxygen (O2). 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 O2 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 O2 diffuses to the water table and causes oxidative dissolution of pyrite. Pyrite-derived sulfuric acid (H2SO4) plus CO2 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. [ABSTRACT FROM AUTHOR]

The base of the Critical Zone includes the mantle of altered soil and rock--regolith--that changes in response to chemical, physical, and biological processes occurring at Earth's surface. These processes are recorded in the chemistry of the regolith, and this long-term record can often be deciphered. For example, on eroding ridgetops where flows are generally downward for water and upward for earth material, element concentrations vary with depth to constitute depletion, addition, depletion-enrichment, and biogenic profiles. Models can be used to explore the records of mineral dissolution, atmospheric input, coupled dissolution-precipitation, and biolifting documented in these profiles. These models enable interpretation of the effects of time, climate, rates of erosion, and human and other biotic impacts on the profile patterns. By testing quantitative models against the long-term record of information in regolith, we will learn to project changes arising from human and natural perturbations of the Critical Zone. [ABSTRACT FROM AUTHOR]

Quartz diorite bedrock underlying the Luquillo Mountains of eastern Puerto Rico undergoes weathering at one of the fastest documented rates for granitic rocks in the world. Although tropical temperatures and precipitation promote rapid weathering in this location, increased bacterial densities in the regolith immediately above the bedrock suggest that microorganisms contribute to mineral weathering as well. Deep saprolite and saprock samples were obtained at the bedrock interface in an upland location (Guaba Ridge) in the Rio Icacos watershed for examination by environmental scanning electron microscopy (ESEM). In ESEM images, mineral nanotubes were observed to occur frequently in association with coccus- and rod-shaped structures resembling bacteria. These nanotubes (50-140-nm width and 150-2700-nm length) were identified as halloysite using transmission electron microscopy. Observations of multiple nanotubes on the surfaces of an individual cell are consistent with the cell's exterior functional groups interacting with Si in pore water to facilitate halloysite nucleation. We propose that one mechanism by which bacteria contribute to the rapid weathering of quartz diorite minerals in this regolith is by lowering the free energy for secondary mineral formation. The presence of bacterial surfaces may result in more rapid removal of Si from solution, thereby increasing the dissolution rates of primary minerals. [ABSTRACT FROM AUTHOR]

Abstract: The primary objective of this research was to investigate the effects of aliphatic and aromatic low molecular weight organic acids (LMWOAs) on rare earth element and yttrium (REY) release from the phosphate minerals apatite and monazite. Since prior studies have shown that redox status can affect REY partitioning during incongruent dissolution, a secondary objective was to assess the influence of dissolved O2 concentration. Increasing LMWOA concentrations from 0 to 10mM resulted in enhanced REY release. In general, REY release increased in the order: no ligand≈salicylate

Abstract: In natural weathering systems, both the chemistry and the topography of mineral surfaces change as rocks and minerals equilibrate to surface conditions. Most geochemical research has focused on changes in solution chemistry over time; however, temporal changes in surface topography may also yield information about rates and mechanisms of dissolution. We use stochastic dissolution simulations of a regular 2-D lattice with reaction mechanisms defined in terms of nearest neighbor interactions to elucidate how the surface area and reactivity of a crystal evolve during dissolution. Despite the simplicity of the model, it reproduces key features observed or inferred for mineral dissolution. Our model results indicate that: (i) dissolving surfaces reach a steady-state conformation after sufficient dissolution time, (ii) linear defects cause surface area and dissolution rate to vary in concert with one another, (iii) sigmoidal and non-sigmoidal rate vs. free-energy of reaction (ΔG rxn) behavior can be rationalized in terms of the multiple steps occurring during dissolution, and (iv) surface roughness as a function of ΔG rxn is highly sensitive to the reaction mechanism. When simulated times to reach steady-state are compared to published time series rate data using suitable scaling, good agreement is found for silicate minerals while the model significantly over-predicts the duration of the transient for Fe and Al oxides. The implication of our simple model is that many aspects of mineral dissolution behavior, including approach to steady-state, sigmoidal vs. non-sigmoidal rate vs. ΔG rxn behavior, and development of rougher surfaces in conditions further from equilibrium can be explained by nearest neighbor interactions and simple Kossel-type models where reactivity of a surface is defined in terms of perfect surface, step, and kink sites. [Copyright &y& Elsevier]

Provides information on a study which investigated mineral-water reaction rates in specific surface areas of several primary silicate materials used to estimate rates of weathering. Methods used; Results and discussion; Conclusions.

The reaction mechanism of weathering of chlorite, an important rock-forming phyllosilicate, is not well understood in natural settings. In this work we investigated the weathering of Fe-rich chlorite from deep protolith to saprock to soil across a small shale-underlain watershed in the Appalachian Mountains, USA (Shale Hills). We found that oxidation of Fe(II) in chlorite always occurs prior to dissolution of the interlayers of the mineral. The oxidation of pyrite and chlorite commence near the water table across narrow depth intervals under the upper-catchment ridges, but well below the water table across wide depth intervals under the valley. We hypothesize that these patterns can be explained by hydrological and geochemical differences between the ridge and the valley: oxygenated water descends sub-vertically (1D flow) under the ridge, while under the valley, oxygen-depleted water moves upward to the stream and laterally out of the watershed in the subsurface (3D flow). Geochemical and mineralogical characterization indicates that the transformation of Fe-rich chlorite at Shale Hills is initiated by the oxidation of Fe(II). Next, the interlayer hydroxide sheet dissolves to form hydroxy-interlayered vermiculite and then vermiculite. During the transformation, Mg and Fe are released into solution and Fe is reprecipitated as goethite in pore space. Delivery of oxygen to the deep subsurface by infiltration of meteoric water is thought to control the initial transformation of chlorite at Shale Hills. It is possible that weathering of many Fe(II)-rich minerals is initiated by oxidation as mediated by rates of subsurface oxygen delivery. [ABSTRACT FROM AUTHOR]

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 weathering-induced 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. [ABSTRACT FROM AUTHOR]

Bulk chemical composition and mineralogy were examined in three soil profiles and a deeper 11-meter profile weathering on a granite ridgeline under subtropical climate conditions in south China. The weathering sequence is delineated by mineralogy and major element variations. Apatite, biotite, hornblende and plagioclase dissolve early during weathering, resulting in nearly 100% Ca and Na loss and significant Mg, Fe and P depletion at depth. The K-feldspar reaction front begins at the depth of depletion of plagioclase, leading to a loss of ~ 80% of the K at the land surface near the bottom of the ridgeline and almost 100% at the top. Dissolution of quartz and other silicates releases about 60% of Si in the profiles. Kaolinite is the dominant clay mineral and it transforms to gibbsite in the uppermost layer. The soil horizon (upper 100 cm) is the zone dominated by pedogenic processes, including active biological activity, physical erosion and influx of high concentrations of atmospheric components (especially CO 2 and O 2 ). The pedogenic processes are characterized by low pH (~ 4.54 to 5.85), high clay content (kaolinite: 6–16 wt.%; gibbsite: 2–8 wt.%) and total organic carbon content (0.13–3.93%) and intensive fracturing and dissolution of quartz and K-feldspar compared with the lower horizons. Accumulation of organic material and resistant minerals downslope is attributed to down-ridge movement of water (termed here, interflow) and weathering products in the uppermost 100 cm down the ridgeline. Using a mass balance model calculation for the catena that assumes steady-state soil thickness at all sites, the bottom profile shows the highest apparent total chemical weathering loss rate (~ 14 g m − 2 y − 1 ) whereas the middle position shows the highest physical erosion loss rate (~ 44 g m − 2 y − 1 ). SiO 2 accounts for about 84% of the chemical weathering outflux from the soil horizons along the ridgeline hillslope. Chemical weathering rates at the bottom of the hillslope may be accelerated by high concentrations of organic material and by dissolution of kaolinite. This study demonstrates that mineral reaction fronts in granite become separated over depth intervals of meters and that elemental fluxes and release mechanisms vary with position along a ridgeline catena. [ABSTRACT FROM AUTHOR]

The Rio Icacos basin, in Puerto Rico, is the site of the highest measured chemical solute fluxes for a catchment on granodiorite; this is partly attributable to high annual rainfall (4300 mm), high average temperature (23 °C), and moderate relief. The bulk of these fluxes is contributed by dissolution of plagioclase and amphiboles in zones of partially weathered rock (0.5–1.5 m thick) underlying saprolite. These zones are characterized by systems of onion-skin “rindlets” (each 3–10 cm thick) in which porosity development is dominated by weathering of plagioclase to kaolinite. Fe-bearing aluminosilicate minerals hornblende, augite, and biotite persist in the weathered rock after plagioclase has weathered to completion, but hornblende and augite disappear between the porous rindlet systems and saprolite. A new watershed-scale solute mass balance corroborates earlier studies and yields an overall reaction stoichiometry that closely resembles complete weathering of the rock (except quartz and biotite) to kaolinite and Fe3+ oxides.Our findings indicate that steady-state assumptions are reasonable and useful in studying weathering at this site. Estimated steady-state sediment yields (based on net solute fluxes and average bedrock composition) of 3.24×10−9 to 3.60×10−9 kg m−2 s−1 are smaller than measured values, indicating that sediment generation is currently accelerated with respect to steady-state; however, sediment generation has recently been closer to steady-state. Since weathering within the rindlet systems occurs on multiple fronts moving upward and downward, two new steady-state conceptual models are developed to account for (1) multiple parallel weathering fronts, and (2) differential rates of advance of weathering fronts due to porosity development. Based on these models, in situ weathering rates for plagioclase at two sample locations of 2.7×10−15 and 5.0×10−15 mol Na g−1 s−1 are calculated, values comparable to BET surface area normalized rates of 2.7×10−16 to 5.0×10−15 mol Na m−2 s−1. [Copyright &y& Elsevier]

Weathering plays a crucial role in a number of environmental processes, and the microstructure and evolution of multi-scale pore space is a critically important factor in weathering. In igneous rocks the infiltration of meteoric water into initially relatively dry material can initiate disaggregation, increasing porosity and surface area, and allowing further disaggregation and weathering. These processes, in turn, allow biota to colonize the rock, further enhancing the weathering rate. In some rocks this may be driven by primary mineral oxidation. One such mineral, biotite, has been repeatedly mentioned as a cause of cracking during oxidation. However, the scale-dependence of the processes by which this occurs are poorly understood. We cannot, therefore, accurately extrapolate laboratory reaction rates to the field in predictive numerical models. In order to better understand the effects of oxidation and test the hypothesis that fracture and disaggregation are initiated by swelling of oxidizing biotites, we reacted granite cores in a selenic acid-rich aqueous solution at 200 °C for up to 438 days. Elevated temperatures and selenic acid were used to provide relatively fast reaction rates and highly oxidizing conditions in sealed reaction vessels. These experiments were analyzed using a combination of imaging, X-ray diffraction, Mössbauer spectroscopy, and small- and ultra-small angle neutron scattering to interrogate porosity and microfracture formation. The experimental results show little observable biotite swelling, but significantly more observable fractures and growth of iron oxides and/or clays along grain boundaries. Pyrite disappeared from the reacted sample. Significant increases in porosity were also observed at the sample rim, likely associated with feldspar alteration. Fractures and transport were observed throughout the core, suggesting that stresses due to crystallization pressures caused by the growing iron phases may be the initiating factors in granite weathering, possibly followed by biotite swelling after sufficient permeability is achieved. [ABSTRACT FROM AUTHOR]

We systematically investigated six soil profiles developed on a climosequence of gray shale to constrain the mobility and fractionation of rare earth elements (REE) during chemical weathering processes. In addition, one site developed on black shale (Marcellus Formation) was included to document REE behaviors in organic-rich versus organic-poor shale end members under the same environmental conditions. Our study shows that REE are mobilized intensively during shale weathering and the extent of depletion is larger under warm/humid climates. However, the integrated release rates calculated from six soil profiles are not directly correlated to mean annual precipitation or temperature. Instead, the primary control might be the REE concentrations in the most reactive minerals. REE-bearing phases in shale (sulfides, phosphates and organic matter) probably react quickly at first, mobilizing REE. Following that, REE are then released more slowly during dissolution reactions of clay minerals. Consistent with this interpretation, black shale weathers much faster and releases more REE than gray shale under the same climate conditions, due to the higher organic matter and sulfide contents and lower soil pH. REE are not 100% depleted in any of the investigated soil sites; in northern sites, depletion is minimal whereas in the southern (warm and humid) sites, surface depletion is higher and re-deposition is observed at depth. Retention of REE is likely caused by adsorption to mineral surfaces as pH increases and dissolved organic matter content decreases with depth. This case study quantified loss, redistribution and fractionation of REE during shale weathering, improved our understanding of REE mobility in surficial environments, and contributed to the exploration of REE as strategic mineral resources. [ABSTRACT FROM AUTHOR]

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) in central Pennsylvania. 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, mid-slope, 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 shale sampled (~ 180 cm below the land surface). Root densities in shale fractures were comparable between the ridge top and mid-slope pits. However, they 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 mass of dry root tissue) in rock fractures was comparable to rates in the soil. Thus, the total flux of CO 2 from root respiration tracked root densities, decreasing with depth. Potential microbial respiration, estimated as the laboratory C mineralization potential, was about an order of magnitude lower than measured root respiration in both the soil and 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 dissolved organic C (DOC), inorganic N-species, 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 and vermiculized chlorite to kaolinite and are similar between soils and fracture fill. Such similarities indicate that the fracture coatings are likely the result of pedogenic processes that occur at depth in the fractures rather than translocation of soil particles downward into the fractures. Overall, our data suggest that roots and fill in shale fractures down to ~ 180 cm are qualitatively similar to those in surface soil horizons. Thus, the deepest manifestation of the chemical depletion profiles observed in the pits consists of the rock fracture fill, and this fill is present at low concentrations with similarly low concentrations of fine roots. [ABSTRACT FROM AUTHOR]

The hydrologic connectivity between hillslopes and streams impacts the geomorphological evolution of catchments. Here, we propose a conceptual model for hydrogeomorphological evolution of the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), a first-order catchment developed on shale in central Pennsylvania, U.S.A. At SSHCZO, the majority of available water (the difference between incoming meteoric water and outgoing evapotranspiration) flows laterally to the catchment outlet as interflow, while the rest is transported by regional groundwater flow. Interflow, shallow hillslope flow, is limited to the upper 5 to 8 m of highly fractured bedrock, thought to have formed during periglacial conditions in the late Pleistocene. In contrast, groundwater flowpaths are influenced by the primary permeability of the varying stratigraphic units. Both flowpaths respond to weathering-related secondary permeability. O2-rich interflow mixes with deep O2-poor groundwater under the catchment outlet at depths of 5–8 m. Penetration of this oxygenated interflow under the valley results in pyrite oxidation, release of sulfuric acid, dissolution of minerals, and weakening of bedrock. This is hypothesized to enhance channel incision and, in turn, to promote drainage of deep groundwater from the ridges. Drainage subsequently lowers the catchment water table, advancing the cascade of reactions that produce regolith. Weathering in the catchment is characterized by both sharp and diffuse reaction fronts. Relatively sharp fronts (pyrite, carbonate) mark where vertical, unsaturated flow changes to horizontal, saturated flow, while diffuse fronts (illite, chlorite, feldspar) mark where flow is largely vertical and unsaturated. According to this model, catchment morphology reflects subsurface pyrite reactions due to mixing of interflow and groundwater flow under the valley floor that ultimately results in clay weathering and regolith production nearer the land surface. [ABSTRACT FROM AUTHOR]

The martian surface has undergone chemical and physical weathering in the past, and these processes may continue intermittently today. To explore whether martian rocks are likely to retain features indicative of weathering, we investigated how basaltic material weathers on Earth. Specifically, we investigated weathering of a Quaternary-aged basaltic flow at the Sverrefjell volcano in Svalbard, above the Arctic Circle. This flow weathered since deglaciation under cold, dry (<400 mm/yr) conditions. We analyzed a ∼75-cm core of regolith for chemical loss and then characterized the mineralogical and morphological properties using electron microscopy (EM), X-ray diffraction (XRD), infrared (IR) spectroscopy and selective chemical dissolution. In addition, we ran colloidal dispersion, wetting/drying, and freeze/thaw experiments. In the regolith, we observed concentrations of short-range ordered (SRO) phases similar to those observed in warmer, wetter volcanic ash soils. IR and EM analyses of the clay-sized fraction were consistent with allophane as the predominant secondary phase. Selective chemical extractions targeting SRO phases indicated lower Al/Si ratios than those observed in volcanic soils reported in warmer localities, which we attribute to Si-rich allophane and/or abundant Si-rich rock coatings. The oxic circumneutral-pH colloidal dispersion experiments mobilized Al, Fe and Ti primarily as 260–415 nm particles and Ca, Mg and Na as solutes. Si was lost both in the colloidal and dissolved forms. Dispersed colloids likely contain allophane and ferrihydrite. Under anoxic conditions, dissolution of Fe oxide cements also released fines. The experiments help to explain elemental loss from the clay-sized regolith fraction at Svalbard: observed depletions in Ca, K, Mg and Na were likely due to solute loss, while particle-reactive Al, Fe, Si and Ti were mostly retained. Wetting/drying was observed to be as effective as freeze/thaw in driving material loss. It is thus possible that cyclic adsorption of water onto basaltic rocks in this dry climate may result in high physical spalling rates that in turn promote chemical leaching. Many observations at Sverrefjell are similar to inferences from Mars: the presence of SRO phases, Si-rich coatings, and/or Si-rich allophane, as well as the persistence of olivine. Given these similarities, it is inferred that Sverrefjell volcano is a good analog for martian weathering and that other processes operating at Sverrefjell may also have occurred on Mars, including Na leaching, surface spalling, and precipitation of Si-rich layers. Such processes could have occurred on Mars wherever basalts were exposed to water at circumneutral pH for thousands to tens of thousands of years. [ABSTRACT FROM AUTHOR]

Shale covers about 25% of the land surface, and is therefore an important rock type that consumes CO 2 during weathering. We evaluated the potential of gray shale to take up CO 2 from the atmosphere by investigating the evolution of dissolved inorganic carbon (DIC) concentrations and its carbon isotopic ratio (δ 13 C DIC ) 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 2 , is in chemical and isotopic equilibrium with CO 2 in the soil atmosphere where pCO 2 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 2 . 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 2+ and Mg 2+ ) 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 13 C DIC values of these groundwaters document mixing between the ankerite and soil CO 2 . The major element concentrations, DIC, and δ 13 C DIC 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 2 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 13 C DIC slightly above the expected mixing values. At the catchment scale, the stream SO 4 2− is also dominantly derived from wet deposition, as stream has δ 34 S SO4 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 2 at a rate of ∼44 and ∼42–48 mol m −2 ky −1 respectively, while degradation of ancient organic matter releases CO 2 at a rate of ∼1.3 mol m −2 ky −1 . Silicate dissolution at the shallow soils is facilitated by low pH and high soil pCO 2 . As ankerite dissolution and organic matter oxidation are shown to occur early during shale alteration, CO 2 consumption by shale weathering is thus limited by initiation of rock disintegration (e.g., fractures) and exposure of fresh surface area to infiltrating CO 2 - and O 2 -rich water. [ABSTRACT FROM AUTHOR]

Abstract: 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 ∼15cm 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 (δ56Fe, relative to IRMM-014) of bulk Fe and 0.5N HCl-extracted Fe (operationally designed to remove amorphous Fe (oxyhydr)oxides) range between −0.3‰ and +0.3‰, with Δ56Febulk-extractable values between ∼0.2‰ and 0.4‰. Throughout the soils along the catena, δ56Fe 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Isotopic fractionation of Fe and Mo during weathering could contribute toward the isotopic signatures of river and ocean waters. To investigate weathering processes, batch experiments were carried out at pH 6 under oxygenated conditions to investigate the influence of a nitrogen-fixing soil bacterium and organic ligands on the extent and isotopic signature of Fe and Mo release from two Pennsylvania shales with different mineralogies: an olive to grey shale (Rose Hill) and a black shale (Marcellus). Results of these studies showed that Fe and Mo were mainly released from illite, chlorite/vermiculite, and Fe oxides in the RHS and from pyrite in the MS. Dissolution rates of the clays estimated from our batch experiments are broadly consistent with published estimates. Release of Fe from both shales was only enhanced by the hydroxamate siderophore desferrioxamine B (DFAM). In contrast, none of the treatments enhanced release of Mo. Furthermore, release of this metal was measurable only for the black shale. For both shales, Fe that was released in the presence of DFAM was depleted in 56Fe as compared to the bulk rock. In contrast, leachate solutions were enriched in the heavier Mo isotope as compared to the bulk Marcellus in all experiments with and without organic ligands and the bacterium. The isotopic fractionations in Fe and Mo are broadly consistent with previously reported fractionation effects during sorption onto precipitating Fe,Mn oxides. The observed Mo isotope fractionation is also consistent with all published studies of rivers, i.e., riverine Mo is enriched in heavy Mo isotopes. The observed Fe isotope fractionation is likewise consistent with published studies for some rivers, i.e., riverine Fe is sometimes depleted in 56Fe. Our experimental results are consistent with sorption of Fe and Mo onto Fe (oxyhydr)oxides during weathering as a possible explanation for the isotopic signatures of these metals in many rivers. [Copyright &y& Elsevier]

Abstract: Changes in the biomass and structure of soil microbial communities have the potential to impact ecosystems via interactions with plants and weathering minerals. Previous studies of forested long-term (1000s – 100,000s of years) chronosequences suggest that surface microbial communities change with soil age. However, significant gaps remain in our understanding of long-term soil microbial community dynamics, especially for non-forested ecosystems and in subsurface soil horizons. We investigated soil chemistry, aboveground plant productivity, and soil microbial communities across a grassland chronosequence (65,000–226,000 yrs old) located near Santa Cruz, CA. Aboveground net primary productivity (ANPP) initially increased to a maximum and then decreased for the older soils. We used polar lipid fatty acids (PLFA) to investigate microbial communities including both surface (<0.1 m) and subsurface (≥0.2 m) soil horizons. PLFAs characteristic of Gram-positive bacteria and actinobacteria increased as a fraction of the microbial community with depth while the fungal fraction decreased relative to the surface. Differences among microbial communities from each chronosequence soil were found primarily in the subsurface where older subsurface soils had smaller microbial community biomass, a higher proportion of fungi, and a different community structure than the younger subsurface soil. Subsurface microbial community shifts in biomass and community structure correlated with, and were likely driven by, decreasing soil P availability and Ca concentrations, respectively. Trends in soil chemistry as a function of soil age led to the separation of the biological (≤1 m depth) and geochemical (>1 m) cycles in the old, slowly eroding landscape we investigated, indicating that this separation, commonly observed in tropical and subtropical ecosystems, can also occur in temperate climates. This study is the first to investigate subsurface microbial communities in a long-term chronosequence. Our results highlight connections between soil chemistry and both the aboveground and belowground parts of an ecosystem. [Copyright &y& Elsevier]

Where Martian rocks have been exposed to liquid water, chemistry versus depth profiles could elucidate both Martian climate history and potential for life. The persistence of primary minerals in weathered profiles constrains the exposure time to liquid water: on Earth, mineral persistence times range from ∼10 k.y. (olivine) to ∼250 k.y. (glass) to ∼1 m.y. ( pyroxene) to ∼5 m.y. (plagioclase). Such persistence times suggest mineral persistence minima on Mars. However, Martian solutions may have been more acidic than on Earth. Relative mineral weathering rates observed for basalt in Svalbard (Norway) and Costa Rica demonstrate that laboratory pH trends can be used to estimate exposure to liquid water both qualitatively (mineral absence or presence) and quantitatively (using reactive transport models). Qualitatively, if the Martian solution pH >∼2, glass should persist longer than olivine; therefore, persistence of glass may be a pH indicator. With evidence for the pH of weathering, the reactive transport code CrunchFlow can quantitatively calculate the minimum duration of exposure to liquid water consistent with a chemical profile. For the profile measured on the surface of the exposed Martian rock known as Humphrey in Gusev Crater, the calculated exposure time is 22 k.y., which is a minimum due to physical erosion. If correct, this estimate is consistent with short-term, episodic alteration accompanied by ongoing surface erosion. More of these depth profiles should be measured to illuminate the weathering history of Mars. [ABSTRACT FROM AUTHOR]

Weathering rind thicknesses were measured on ∼ 200 basaltic clasts collected from three regionally extensive alluvial fill terraces (Qt 1, Qt 2, and Qt 3) preserved along the Pacific coast of Costa Rica. Mass balance calculations suggest that conversion of unweathered basaltic core minerals (plagioclase and augite) to authigenic minerals in the porous rind (kaolinite, allophane, gibbsite, Fe oxyhydroxides) is iso-volumetric and Ti and Zr are relatively immobile. The hierarchy of cation mobility (Ca &ap; Na > K &ap; Mg > Si > Al > Fe &ap; P) is similar to other tropical weathering profiles and is indicative of differential rates of mineral weathering (anorthite > albite &ap; hypersthene > orthoclase ≫ apatite). Alteration profiles across the cm-thick rinds document dissolution of plagioclase and augite and the growth of kaolinite, with subsequent dissolution of kaolinite and precipitation of gibbsite as weathering rinds age. The rate of weathering rind advance is evaluated using a diffusion-limited model which predicts a parabolic rate law for weathering rind thickness, rr, as a function of time, t(rr =κt), and an interface-limited model which predicts a linear rate law for weathering rind thickness as a function of time (rr = kappt). In these rate laws, κ is a diffusion parameter and kapp is an apparent rate constant. The rate of advance is best fit by the interface model.Terrace exposures are confined to the lower reaches of streams draining the Pacific slope near the coast where the stream gradient is less than ∼3 m/km, and terrace deposition is influenced by eustatic sea level fluctuations. Geomorphological evidence is consistent with terrace deposition coincident with sea level maxima when the stream gradient would be lowest. Assigning the most weathered regionally extensive terrace Qt 1 (mean rind thickness 6.9 ± 0. 6cm) to oxygen isotope stage (OIS) 7 (ca. 240 ka), and assuming that at time = 0 rind thickness = 0, it is inferred that terrace Qt 2 (rr = 2.9 ± 0.1 cm) is coincident with stage 5e (ca. 125 ka) and that Qt 3 (rr = 0.9 ± 0.1 cm) is consistent with OIS 3 (ca. 37 ka). These assignments yield a value of kapp of 8.6 × 10−13 cm s−1 (R2 = 0.99). Only this value satisfies both the existing age controls and yields ages coincident with sea level maxima. Using this value, elemental weathering release fluxes across a weathering rind from Qt 2 range from 6.0 × 10−9 mol Si m−2 s−1 to 2.5 × 10−11 mol K m−2 s−1. The rate of rind advance for the Costa Rican terraces is 2.8 × 10−7 m yr−1. Basalt rind formation rates in lower temperature settings described in the literature are also consistent with interface-controlled weathering with an apparent activation energy of about 50 kJ mol−1. Rates of rind formation in Costa Rica are an order of magnitude slower than reported for global averages of soil formation rates. [Copyright &y& Elsevier]

To further develop boron isotopes as a tool for understanding shale weathering, we explored patterns of boron concentrations and isotopes across the forested Susquehanna Shale Hills Critical Zone Observatory (CZO). We present boron measurements for all watershed components that provided a foundation for examining water-rock interactions in a shale dominated watershed, including water compartments (e.g., precipitation, stream water, groundwater) and solid compartments (e.g., soil, bedrock, stream sediments, suspended load, and leaf litter). Results show boron isotopes (δ11B) in the bedrock (− 4.6‰) and soil (− 5.9 to - 4.2‰) were very similar. All waters were enriched in 11B by comparison: precipitation (7.2 to 22.6‰), stream (10.3 to 15.5‰), and groundwater (2.2 to 17.4‰). Modeling revealed that isotopic fractionation observed in the surface water and groundwater could mainly be explained by water-rock interactions including clay mineral dissolution (e.g., chlorite) and coprecipitation/adsorption processes (e.g., coatings on illite particles), likely in the near surface soils (~2 m deep). We found that leaching, the loss of boron from vegetation to stream water, plays a secondary role. Specifically, such leaching likely contributes the equivalent of 10 to 26% of the B fluxes from the watershed outlet. Boron mass balance between bedrock and precipitation inputs and the exported flux of dissolved and solid pools identified a "missing" isotopically light solid flux (δ11B of −12.2 ± 5.3‰ at ~4.4 ± 3.8 mol/ha/y of B; uncertainty reported as 2 SD). We did not sample any pool with this isotopic signature. Here our data suggest the composition of this pool is more likely related to precipitation of secondary clays rather than adsorption or (co)precipitation on Fe oxides. We propose two hypotheses to explain the missing light B pool: 1) a significant portion of the particles carrying the missing 10B are not sampled because they enter groundwater at depth and are transported out of the catchment under the stream; and/or 2) the inputs and outputs of boron are not operating at steady state in the catchment today, suggesting that the missing boron particles were lost in the past in proportions higher than today. When this B budget is paired with studies of δ26Mg and δ56Fe from Shale Hills, both of which also show missing isotopic pools, the pattern indicates a fundamental gap in understanding of shale weathering. We concluded that light B particles, presumably generated in the upper soils, are likely transported deep beneath the surface in the groundwater system or episodically in the past through riverine fluxes. • Boron isotopes and concentrations at Shale Hills are controlled by clay mineral weathering, while biotic processesplay a secondary role • Boron mass balance revealed that this shale system is characterized by an unconstrained or "missing" pool of isotopically light boron • The boron isotopic composition is best explained by a "missing" pool of particles comprised of clay minerals or coatings on primary minerals [ABSTRACT FROM AUTHOR]