Susquehanna Shale Hills Critical Zone Research Articles

Dual-phase mass balance modeling of small mineral particle losses from sedimentary rock-derived soils

Losses of small mineral particles can be a significant physical process that affects the elemental composition of soils derived from sedimentary rocks. Shales, in particular, contain abundant clay-sized minerals that can be mobilized by simple disaggregation, and solutional weathering is limited because the parent rock is composed primarily of recalcitrant minerals previously subjected to continental weathering. Here, the dual-phase mass balance model is employed to quantify losses of small mineral particles as water dispersible colloids (WDCs) from three previously studied soil profiles along a hill slope at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). WDCs were isolated from soil in the laboratory to determine their mineralogical and elemental compositions. Clay minerals dominated WDCs, including illite, vermiculite, and chlorite inherited from the parent shale, along with neoformed kaolinite. Quartz present in bulk soil was generally excluded from WDCs. Elements of low solubility and/or bound in recalcitrant forms, like Rb in illite, were employed in tracer ratios in the dual-phase model. Aluminum, Ga, and Rb were enriched in WDCs, and Zr and Hf were partially excluded. Six different combinations of elements into tracer ratios (Al/Zr, Ga/Zr, Rb/Zr, Al/Hf, Ga/Hf, Rb/Hf) each yielded similar model results. Mass losses of WDCs were large, ranging from −68±7% to −15±5% relative to soil parent material in different parts of the profiles. Mass losses via solution were smaller, ranging from −7±2% to a gain of 6±1% in part of one profile. Losses of WDCs account for >90% of total mass loss, surpassing chemical dissolution, and therefore dominate the weathering portion of denudation at SSHO. Zirconium concentrations were 97–158ppm in the generally ≤1μm WDCs, suggesting colloidal, Zr-bearing phases. Model-quantified losses of Zr via WDCs were large, with a median loss of 41% relative to parent material. Such losses indicate systematic underestimates of weathering by traditional mass balance that uses Zr as an index element. Losses of Ca, Mg, and K via WDCs exceeded losses via solution, countering assumptions of base cation losses primarily via mineral dissolution. The results illustrate a geochemical fingerprint of physical weathering and the ability of the dual-phase model to quantify that weathering process.

Weathering of rock to regolith: The activity of deep roots in bedrock fractures

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 (~180cm 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 CO2 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 (>50μm) 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 ~180cm 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.

Understanding watershed hydrogeochemistry: 2. Synchronized hydrological and geochemical processes drive stream chemostatic behavior

AbstractWhy do solute concentrations in streams remain largely constant while discharge varies by orders of magnitude? We used a new hydrological land surface and reactive transport code, RT‐Flux‐PIHM, to understand this long‐standing puzzle. We focus on the nonreactive chloride (Cl) and reactive magnesium (Mg) in the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO). Simulation results show that stream discharge comes from surface runoff (Qs), soil lateral flow (QL), and deeper groundwater (QG), with QL contributing >70%. In the summer, when high evapotranspiration dries up and disconnects most of the watershed from the stream, Cl is trapped along planar hillslopes. Successive rainfalls connect the watershed and mobilize trapped Cl, which counteracts dilution effects brought about by high water storage (Vw) and maintains chemostasis. Similarly, the synchronous response of clay dissolution rates (Mg source) to hydrological conditions, maintained largely by a relatively constant ratio between “wetted” mineral surface area Aw and Vw, controls Mg chemostatic behavior. Sensitivity analysis indicates that cation exchange plays a secondary role in determining chemostasis compared to clay dissolution, although it does store an order‐of‐magnitude more Mg on exchange sites than soil water. Model simulations indicate that dilution (concentration decrease with increasing discharge) occurs only when mass influxes from soil lateral flow are negligible (e.g., via having low clay surface area) so that stream discharge is dominated by relatively constant mass fluxes from deep groundwater that are unresponsive to surface hydrological conditions.

Understanding watershed hydrogeochemistry: 1. Development of RT‐Flux‐PIHM

AbstractModel development in hydrology and geochemistry has been advancing separately with limited integration. We developed a watershed hydrogeochemical code RT‐Flux‐PIHM to understand complex interactions between hydrological processes (PIHM), land‐surface processes (FLUX—Noah Land Surface Model), and multicomponent subsurface reactive transport (RT). The RT module simulates geochemical processes including aqueous complexation, surface complexation, mineral dissolution and precipitation, and cation exchange. The RT module is verified against the widely used reactive transport code CrunchFlow. The code uses semidiscrete finite volume method and irregular gridding and offers data harvesting capabilities from national databases. The application of RT‐Flux‐PIHM is demonstrated in the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO). We aim to understand key processes that govern hydrogeochemical dynamics of the nonreactive chloride and reactive magnesium. Simulation results indicate that watershed characteristics, in particular topography, dictate the spatial distributions of water content and soil dissolution rates. Ion exchange provides buffering capacities and leads to a hysteresis loop of concentration and discharge relationship of magnesium, which differs from the open hysteresis of chloride. RT‐Flux‐PIHM offers physics‐based modeling capabilities to integrate the vast amount of water and chemistry data that have now become available, to differentiate the relative importance of competing processes, and to test hypotheses at the interface of hydrology and geochemistry.

Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment

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.

Atmospheric inputs of organic matter to a forested watershed: Variations from storm to storm over the seasons

The objectives of this study were to determine the quantity and chemical composition of precipitation inputs of dissolved organic carbon (DOC) to a forested watershed; and to characterize the associated temporal variability. We sampled most precipitation that occurred from May 2012 through August 2013 at the Susquehanna Shale Hills Critical Zone Observatory (Pennsylvania, USA). Sub-event precipitation samples (159) were collected sequentially during 90 events; covering various types of synoptic meteorological conditions in all climatic seasons. Precipitation DOC concentrations and rates of wet atmospheric DOC deposition were highly variable from storm to storm, ranging from 0.3 to 5.6 mg C L−1 and from 0.5 to 32.8 mg C m−2 h−1, respectively. Seasonally, storms in spring and summer had higher concentrations of DOC and more optically active organic matter than in winter. Higher DOC concentrations resulted from weather types that favor air advection, where cold frontal systems, on average, delivered more than warm/stationary fronts and northeasters. A mixed modeling statistical approach revealed that factors related to storm properties, emission sources, and to the chemical composition of the atmosphere could explain more than 60% of the storm to storm variability in DOC concentrations. This study provided observations on changes in dissolved organic matter that can be useful in modeling of atmospheric oxidative chemistry, exploring relationships between organics and other elements of precipitation chemistry, and in considering temporal changes in ecosystem nutrient balances and microbial activity.

Designing a suite of measurements to understand the critical zone

Abstract. Many scientists have begun to refer to the earth surface environment from the upper canopy to the depths of bedrock as the critical zone (CZ). Identification of the CZ as an integral object worthy of study implicitly posits that the study of the whole earth surface will provide benefits that do not arise when studying the individual parts. To study the CZ, however, requires prioritizing among the measurements that can be made – and we do not generally agree on the priorities. Currently, the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) is expanding from a small original focus area (0.08 km2, Shale Hills catchment), to a larger watershed (164 km2, Shavers Creek watershed) and is grappling with the prioritization. This effort is an expansion from a monolithologic first-order forested catchment to a watershed that encompasses several lithologies (shale, sandstone, limestone) and land use types (forest, agriculture). The goal of the project remains the same: to understand water, energy, gas, solute, and sediment (WEGSS) fluxes that are occurring today in the context of the record of those fluxes over geologic time as recorded in soil profiles, the sedimentary record, and landscape morphology. Given the small size of the Shale Hills catchment, the original design incorporated measurement of as many parameters as possible at high temporal and spatial density. In the larger Shavers Creek watershed, however, we must focus the measurements. We describe a strategy of data collection and modeling based on a geomorphological and land use framework that builds on the hillslope as the basic unit. Interpolation and extrapolation beyond specific sites relies on geophysical surveying, remote sensing, geomorphic analysis, the study of natural integrators such as streams, groundwaters or air, and application of a suite of CZ models. We hypothesize that measurements of a few important variables at strategic locations within a geomorphological framework will allow development of predictive models of CZ behavior. In turn, the measurements and models will reveal how the larger watershed will respond to perturbations both now and into the future.

Reliance on shallow soil water in a mixed-hardwood forest in central Pennsylvania.

We investigated depth of water uptake of trees on shale-derived soils in order to assess the importance of roots over a meter deep as a driver of water use in a central Pennsylvania catchment. This information is not only needed to improve basic understanding of water use in these forests but also to improve descriptions of root function at depth in hydrologic process models. The study took place at the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania. We asked two main questions: (i) Do trees in a mixed-hardwood, humid temperate forest in a central Pennsylvania catchment rely on deep roots for water during dry portions of the growing season? (ii) What is the role of tree genus, size, soil depth and hillslope position on the depth of water extraction by trees? Based on multiple lines of evidence, including stable isotope natural abundance, sap flux and soil moisture depletion patterns with depth, the majority of water uptake during the dry part of the growing season occurred, on average, at less than ∼60 cm soil depth throughout the catchment. While there were some trends in depth of water uptake related to genus, tree size and soil depth, water uptake was more uniformly shallow than we expected. Our results suggest that these types of forests may rely considerably on water sources that are quite shallow, even in the drier parts of the growing season.

Topographic controls on the depth distribution of soil CO2 in a small temperate watershed

Accurate measurements of soil CO2 concentrations (pCO2) are important for understanding carbonic acid reaction pathways for continental weathering and the global carbon (C) cycle. While there have been many studies of soil pCO2, most sample or model only one, or at most a few, landscape positions and therefore do not account for complex topography. Here, we test the hypothesis that soil pCO2 distribution can predictably vary with topographic position. We measured soil pCO2 at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), Pennsylvania, where controls on soil pCO2 (e.g., depth, texture, porosity, and moisture) vary from ridge tops down to the valley floor, between planar slopes and slopes with convergent flow (i.e., swales), and between north and south-facing aspects. We quantified pCO2 generally at 0.1–0.2 m depth intervals down to bedrock from 2008 to 2010 and in 2013. Of the variables tested, topographic position along catenas was the best predictor of soil pCO2 because it controls soil depth, texture, porosity, and moisture, which govern soil CO2 diffusive fluxes. The highest pCO2 values were observed in the valley floor and swales where soils are deep (≥0.7 m) and wet, resulting in low CO2 diffusion through soil profiles. In contrast, the ridge top and planar slope soils have lower pCO2 because they are shallower (≤0.6 m) and drier, resulting in high CO2 diffusion through soil profiles. Aspect was a minor predictor of soil pCO2: the north (i.e., south-facing) swale generally had lower soil moisture content and pCO2 than its south (i.e., north-facing) counterpart. Seasonally, we observed that while the timing of peak soil pCO2 was similar across the watershed, the amplitude of the pCO2 peak was higher in the deep soils due to more variable moisture content. The high pCO2 observed in the deeper, wetter topographic positions could lower soil porewater pH by up to 1 pH unit compared to porewaters equilibrated with atmospheric CO2 alone. CO2 is generally the dominant acid driving weathering in soils: based on our observations, models of chemical weathering and CO2 dynamics would be improved by including landscape controls on soil pCO2.

Topographic stress and rock fracture: a two‐dimensional numerical model for arbitrary topography and preliminary comparison with borehole observations

AbstractTheoretical calculations indicate that elastic stresses induced by surface topography may be large enough in some landscapes to fracture rocks, which in turn could influence slope stability, erosion rates, and bedrock hydrologic properties. These calculations typically have involved idealized topographic profiles, with few direct comparisons of predicted topographic stresses and observed fractures at specific field sites. We use a numerical model to calculate the stresses induced by measured topographic profiles and compare the calculated stress field with fractures observed in shallow boreholes. The model uses a boundary element method to calculate the stress distribution beneath an arbitrary topographic profile in the presence of ambient tectonic stress. When applied to a topographic profile across the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania, the model predicts where shear fractures would occur based on a Mohr–Coulomb criterion, with considerable differences in profiles of stresses with depth beneath ridgetops and valley floors. We calculate the minimum cohesion required to prevent shear failure, Cmin, as a proxy for the potential for fracturing or reactivation of existing fractures. We compare depth profiles of Cmin with structural analyses of image logs from four boreholes located on the valley floor, and find that fracture abundance declines sharply with depth in the uppermost 15 m of the bedrock, consistent with the modeled profile of Cmin. In contrast, Cmin increases with depth at comparable depths below ridgetops, suggesting that ridgetop fracture abundance patterns may differ if topographic stresses are indeed important. Thus, the present results are consistent with the hypothesis that topography can influence subsurface rock fracture patterns and provide a basis for further observational tests. Copyright © 2014 John Wiley & Sons, Ltd.

The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory

Shale covers about 25% of the land surface, and is therefore an important rock type that consumes CO2 during weathering. We evaluated the potential of gray shale to take up CO2 from the atmosphere by investigating the evolution of dissolved inorganic carbon (DIC) concentrations and its carbon isotopic ratio (δ13CDIC) 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 CO2, is in chemical and isotopic equilibrium with CO2 in the soil atmosphere where pCO2 varies seasonally to as high as 40 times that of the atmosphere. The degradation of ancient organic matter is negligible in contributing to soil CO2. 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 (∼2500μmol/L) and divalent cations (Ca2+ and Mg2+) 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 δ13CDIC values of these groundwaters document mixing between the ankerite and soil CO2. The major element concentrations, DIC, and δ13CDIC 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 CO2 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 δ13CDIC slightly above the expected mixing values. At the catchment scale, the stream SO42− is also dominantly derived from wet deposition, as stream has δ34SSO4 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 CO2 at a rate of ∼44 and ∼42–48molm−2ky−1 respectively, while degradation of ancient organic matter releases CO2 at a rate of ∼1.3molm−2ky−1. Silicate dissolution at the shallow soils is facilitated by low pH and high soil pCO2. As ankerite dissolution and organic matter oxidation are shown to occur early during shale alteration, CO2 consumption by shale weathering is thus limited by initiation of rock disintegration (e.g., fractures) and exposure of fresh surface area to infiltrating CO2- and O2-rich water.

Aspect-dependent variations in regolith creep revealed by meteoric 10Be

Although variations in insolation and emergent feedbacks among soil moisture, vegetation, and soil cohesion are commonly invoked to explain topographic asymmetry that depends on aspect, few studies have directly quantified the efficiency of regolith transport along hillslopes of opposing aspect. We utilize meteoric 10 Be concentrations in regolith (n = 74) to determine mass flux along equatorial-facing and polar-facing hillslopes in three forested upland watersheds in and adjacent to the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania (USA). In combination with regolith depth measurements and high-resolution topography, these fluxes allow us to evaluate transport rate laws and the efficiency of regolith creep. Concentrations of meteoric 10 Be in regolith along six separate transects imply that regolith flux is similar along all hillslopes, despite differences in topographic gradient and regolith thickness. Comparison of flux with regolith depth and topographic gradient reveals that transport depends on regolith depth, and that regolith creep is twice as efficient along low-gradient, south-facing slopes with thin regolith as compared to steep, north-facing slopes mantled with thicker regolith. We suggest that the observed topographic asymmetry in these watersheds has evolved over geologic time as a result of differences in the frequency of freeze-thaw events between hillslopes of opposing aspect.

Regolith production and transport at the Susquehanna Shale Hills Critical Zone Observatory, Part 2: Insights from meteoric 10Be

Regolith‐mantled hillslopes are ubiquitous features of most temperate landscapes, and their morphology reflects the climatically, biologically, and tectonically mediated interplay between regolith production and downslope transport. Despite intensive research, few studies have quantified both of these mass fluxes in the same field site. Here we present an analysis of 87 meteoric 10Be measurements from regolith and bedrock within the Susquehanna Shale Hills Critical Zone Observatory (SSHO), in central Pennsylvania. Meteoric 10Be concentrations in bulk regolith samples (n = 73) decrease with regolith depth. Comparison of hillslope meteoric 10Be inventories with analyses of rock chip samples (n = 14) from a 24 m bedrock core confirms that >80% of the total inventory is retained in the regolith. The systematic downslope increase of meteoric 10Be inventories observed at SSHO is consistent with 10Be accumulation in slowly creeping regolith (~ 0.2 cm yr−1). Regolith flux inferred from meteoric 10Be varies linearly with topographic gradient (determined from high‐resolution light detection and ranging‐based topography) along the upper portions of hillslopes at SSHO. However, regolith flux appears to depend on the product of gradient and regolith depth where regolith is thick, near the base of hillslopes. Meteoric 10Be inventories at the north and south ridgetops indicate minimum regolith residence times of 10.5 ± 3.7 and 9.1 ± 2.9 ky, respectively, similar to residence times inferred from U‐series isotopes in Ma et al. (2013). The combination of our results with U‐series‐derived regolith production rates implies that regolith production and erosion rates are similar to within a factor of two on SSHO hillcrests.

Climate dependence of feldspar weathering in shale soils along a latitudinal gradient

Although regolith, the mantle of physically, chemically, and biologically altered material overlying bedrock, covers much of Earth’s continents, the rates and mechanisms of regolith formation are not well quantified. Without this knowledge, predictions of the availability of soil to sustain Earth’s growing population are problematic. To quantify the influence of climate on regolith formation, a transect of study sites has been established on the same lithology – Silurian shale – along a climatic gradient in the northern hemisphere as part of the Susquehanna Shale Hills Critical Zone Observatory, Pennsylvania, USA. The climate gradient is bounded by a cold/wet end member in Wales and a warm/wet end member in Puerto Rico; in between, mean annual temperature (MAT) and mean annual precipitation (MAP) increase to the south through New York, Pennsylvania, Virginia, Tennessee and Alabama. The site in Puerto Rico does not lie on the same shale formation as the Appalachian sites but is similar in composition. Soils and rocks were sampled at geomorphologically similar ridgetop sites to compare and model shale weathering along the transect. Focusing on the low-concentration, non-nutrient element Na, we observe that the extent and depth of Na depletion is greater where mean annual temperature (MAT) and precipitation (MAP) are higher. Na depletion, a proxy for feldspar weathering, is the deepest reaction documented in the augerable soil profiles. This may therefore be the reaction that initiates the transformation of high bulk-density bedrock to regolith of low bulk density. Based on the shale chemistry along the transect, the time-integrated Na release rate (QNa) increases exponentially as a function of MAT and linearly with MAP. NY, the only site with shale-till parent material, is characterized by a QNa that is 18 times faster than PA, an observation which is attributed to the increased surface area of minerals due to grinding of the glacier and kinetically limited weathering in the north. A calculated apparent Arrhenius-type temperature dependence across the transect (excluding NY) for the dissolution of feldspar (Na depletion) is 99±15kJmol−1, a value similar to field-measured values of the activation energy (14–109kJmol−1) or laboratory-measured values of the enthalpy of the albite reaction (79.8kJmol−1). Observations from this transect document that weathering losses of Na from Silurian shale can be understood with models of regolith formation based on chemical and physical factors such that weathering progresses from kinetically limited sites (Wales to AL) to the transport-limited site in Puerto Rico. Significant advances in our ability to predict regolith formation will be made as we apply more quantitative models to such transect studies on shales and other rocks types.

Regolith production and transport in the Susquehanna Shale Hills Critical Zone Observatory, Part 1: Insights from U‐series isotopes

AbstractTo investigate the timescales of regolith formation on hillslopes with contrasting topographic aspect, we measured U‐series isotopes in regolith profiles from two hillslopes (north facing versus south facing) within the east‐west trending Shale Hills catchment in Pennsylvania. This catchment is developed entirely on the Fe‐rich, Silurian Rose Hill gray shale. Hillslopes exhibit a topographic asymmetry: The north‐facing hillslope has an average slope gradient of ~20°, slightly steeper than the south‐facing hillslope (~15°). The regolith samples display significant U‐series disequilibrium resulting from shale weathering. Based on the U‐series data, the rates of regolith production on the two ridgetops are indistinguishable (40 ± 22 versus 45 ± 12 m/Ma). However, when downslope positions are compared, the regolith profiles on the south‐facing hillslope are characterized by faster regolith production rates (50 ± 15 to 52 ± 15 m/Ma) and shorter durations of chemical weathering (12 ± 3 to 16 ± 5 ka) than those on the north‐facing hillslope (17 ± 14 to 18 ± 13 m/Ma and 39 ± 20 to 43 ± 20 ka). The south‐facing hillslope is also characterized by faster chemical weathering rates inferred from major element chemistry, despite lower extents of chemical depletion. These results are consistent with the influence of aspect on regolith formation at Shale Hills; we hypothesize that aspect affects such variables as temperature, moisture content, and evapotranspiration in the regolith zone, causing faster chemical weathering and regolith formation rates on the south‐facing side of the catchment. The difference in microclimate between these two hillslopes is inferred to have been especially significant during the periglacial period that occurred at Shale Hills at least ~15 ka before present. At that time, the erosion rates may also have been different from those observed today, perhaps denuding the south‐facing hillslope of regolith but not quite stripping the north‐facing hillslope. An analysis of hillslope evolution and response timescales with a linear mass transport model shows that the current landscape at Shale Hills is not in geomorphologic steady state (i.e., so‐called dynamic equilibrium) but rather is likely still responding to the climate shift from the Holocene periglacial to the modern, temperate conditions.

Regolith production and transport in the Susquehanna Shale Hills Critical Zone Observatory, Part 1: Insights from U-series isotopes

Regolith production and transport in the Susquehanna Shale Hills Critical Zone Observatory, Part 1: Insights from U-series isotopes

Regolith production and transport at the Susquehanna Shale Hills Critical Zone Observatory, part 2: Insights from meteoric10Be

Regolith production and transport at the Susquehanna Shale Hills Critical Zone Observatory, part 2: Insights from meteoric10Be

Opening the “Black Box”: Water Chemistry Reveals Hydrological Controls on Weathering in the Susquehanna Shale Hills Critical Zone Observatory

We investigated variations of Mg concentration, δD and δO in precipitation, soil water along a planar hillslope, groundwater, and first‐order stream water at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Water flows vertically in the unsaturated zone of the hillslope, but hydrological saturation periodically causes lateral flow along interfaces of permeability contrast between the A–B and B–C soil horizons. Changes in soil water Mg concentration respond to hydrological changes and are ultimately controlled by the kinetics of clay mineral dissolution, but are buffered by the soil exchange capacity. Clay dissolution predominantly occurs within the A and B horizons, and Mg released from these zones of “low‐flow” diffuses or flows into the “high‐flow” zones at horizon interfaces. The Mg concentrations are low in these high‐flow zones because fresher (younger) water flows in through macropores. The amplitude of seasonal variations in water isotopes (data from 2008–2010) decreases in the following order: precipitation (δD: 286‰) >> soil water (δD: 86‰) > shallow groundwater (δD: 26‰), indicating water becomes progressively older along the flowpath. Fractures and preferential high‐flow paths make the watershed hydrologically responsive: the average time water stays in the shallow subsurface is inferred to be <2 yr. The stream water chemistry is affected by inputs of old groundwater that is relatively high in Mg concentration but relatively limited in range in δD, as well as by inputs from young soil water that is relatively low in Mg concentration with a wide range in δD. The relative contributions of these two sources to the stream change seasonally.

Preliminary estimates of regolith generation and mobility in the Susquehanna Shale Hills Critical Zone Observatory, Pennsylvania, using meteoric 10Be

Abstract This study seeks to quantify the rate and timing of regolith generation in the Critical Zone at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Meteoric 10 Be depth profiles were determined using measurements from 30 hillslope soil and bedrock core samples in an effort to constrain 10 Be inventories. The SSHO is located in the temperate climate zone of central Pennsylvania and comprises a first-order watershed developed entirely on a Fe-rich, organic-poor, Silurian-aged shale. Two major perturbations to the landscape have occurred at SSHO in the geologically recent past, including significant and sustained periglacial activity until after the retreat of the Laurentide ice sheet (∼21 ka) and deforestation during early colonial land-use. Bulk soil samples ( n = 16) were collected at three locations along a planar hillslope on the southern ridge of the catchment, representing the ridge top, mid-slope and valley floor. Rock chip samples ( n = 14) were also collected from a 24 m deep core drilled into the northern ridge top. All meteoric 10 Be concentration profiles show a declining trend with depth, with most of the 10 Be retained in the uppermost decimeters of the soil. Meteoric 10 Be inventories are higher at the mid-slope and valley floor sample sites, at 3.71 ± 0.02 × 10 10 at/cm 2 and 3.69 ± 0.02 × 10 10 at/cm 2 , than at the ridge top site (1.90 ± 0.01 × 10 10 at/cm 2 ). The 10 Be inventory at the convex ridge top site implies a minimum residence time of ∼10.6 ka, or if erosion is steady, an erosion rate of 19.4 ± 0.2 m/My.