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Berlin Brandenburg

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  • Brantley, S
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  • 1
    Language: English
    In: Earth and Planetary Science Letters, 2010, Vol.297(1), pp.211-225
    Description: In the Critical Zone where rocks and life interact, bedrock equilibrates to Earth surface conditions, transforming to regolith. The factors that control the rates and mechanisms of formation of regolith, defined here as material that can be augered, are still not fully understood. To quantify regolith formation rates on shale lithology, we measured uranium-series (U-series) isotopes ( U, U, and Th) in three weathering profiles along a planar hillslope at the Susquehanna/Shale Hills Observatory (SSHO) in central Pennsylvania. All regolith samples show significant U-series disequilibrium: ( U/ U) and ( Th/ U) activity ratios range from 0.934 to 1.072 and from 0.903 to 1.096, respectively. These values display depth trends that are consistent with fractionation of U-series isotopes during chemical weathering and element transport, i.e., the relative mobility decreases in the order U 〉 U 〉 Th. The activity ratios observed in the regolith samples are explained by i) loss of U-series isotopes during water–rock interactions and ii) re-deposition of U-series isotopes downslope. Loss of U and Th initiates in the meter-thick zone of “bedrock” that cannot be augered but that nonetheless consists of up to 40% clay/silt/sand inferred to have lost K, Mg, Al, and Fe. Apparent equivalent regolith production rates calculated with these isotopes for these profiles decrease exponentially from 45 m/Myr to 17 m/Myr, with increasing regolith thickness from the ridge top to the valley floor. With increasing distance from the ridge top toward the valley, apparent equivalent regolith residence times increase from 7 kyr to 40 kyr. Given that the SSHO experienced peri-glacial climate ∼ 15 kyr ago and has a catchment-wide averaged erosion rate of ∼ 15 m/Myr as inferred from cosmogenic Be, we conclude that the hillslope retains regolith formed before the peri-glacial period and is not at geomorphologic steady state. Both chemical weathering reactions of clay minerals and translocation of fine particles/colloids are shown to contribute to mass loss of U and Th from the regolith, consistent with major element data at SSHO. This research documents a case study where U-series isotopes are used to constrain the time scales of chemical weathering and regolith production rates. Regolith production rates at the SSHO should be useful as a reference value for future work at other weathering localities.
    Keywords: U-Series Isotopes ; Regolith Formation ; Chemical Weathering ; Erosion ; Critical Zone ; Geology ; Physics
    ISSN: 0012-821X
    E-ISSN: 1385-013X
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  • 2
    Language: English
    In: Earth and Planetary Science Letters, 2007, Vol.261(1), pp.321-334
    Description: Weathering of silicate minerals impacts many geological and ecological processes. For example, the weathering of basalt contributes significantly to consumption of atmospheric carbon dioxide (CO ) and must be included in global calculations of such consumption over geological timeframes. Here we compare weathering advance rates for basalt ( ), where and indicate the scale at which the rate is determined and surface area measured, respectively, from the laboratory to the watershed scales. Data collected at the laboratory, weathering rind, soil profile and watershed scales show that weathering advance rate of basalt is a fractal property that can be described by a fractal dimension ( ≈ 2.3). By combining the fractal description of rates with an Arrhenius relationship for basalt weathering, we derive the following equation to predict weathering advance rates at any spatial scale from weathering advance rates measured at the BET scale: Here, is the pre-exponential factor (1.29 × 10  mm mm yr ), is the activation energy (70 kj mol ), and is a spatial constant related to the scale of measurement of BET surface area (10  mm). The term, , is the roughness. The roughness fractal dimension can be conceptualized as a factor related to both the thickness of the reaction front and the specific surface area within the reaction front. However, the above equation can also be written in terms of a surface fractal dimension and the hypothetical average grain radius. These fractal dimensions provide insight into reaction front geometry and should vary with lithology. Once the surface area discrepancy has been accounted for using this method, we find a one to two order of magnitude range in weathering advance rates measured at any scale or temperature that can be attributed to factors such as changes in erosional regime, parent lithology, mechanism, climate, composition of reacting fluid, and biological activity. Our scaled equation, when used to predict global basalt CO consumption based upon global lithologic maps, yields an uptake flux (1.75 × 1013 mol CO yr ) within the predicted error of fluxes estimated based upon riverine measurements.
    Keywords: Basalt ; Weathering ; Fractal Dimension ; Fractals ; Surface Area ; Cation Denudation Rates ; Weathering Advance Rates ; Scaling ; Geology ; Physics
    ISSN: 0012-821X
    E-ISSN: 1385-013X
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  • 3
    In: Journal of Geophysical Research: Earth Surface, June 2013, Vol.118(2), pp.722-740
    Description: To investigate the timescales of regolith formation on hillslopes with contrasting topographic aspect, we measured U‐series isotopes in regolith profiles from two hillslopes (north facing versus south facing) within the east‐west trending Shale Hills catchment in Pennsylvania. This catchment is developed entirely on the Fe‐rich, Silurian Rose Hill gray shale. Hillslopes exhibit a topographic asymmetry: The north‐facing hillslope has an average slope gradient of ~20°, slightly steeper than the south‐facing hillslope (~15°). The regolith samples display significant U‐series disequilibrium resulting from shale weathering. Based on the U‐series data, the rates of regolith production on the two ridgetops are indistinguishable (40 ± 22 versus 45 ± 12 m/Ma). However, when downslope positions are compared, the regolith profiles on the south‐facing hillslope are characterized by faster regolith production rates (50 ± 15 to 52 ± 15 m/Ma) and shorter durations of chemical weathering (12 ± 3 to 16 ± 5 ka) than those on the north‐facing hillslope (17 ± 14 to 18 ± 13 m/Ma and 39 ± 20 to 43 ± 20 ka). The south‐facing hillslope is also characterized by faster chemical weathering rates inferred from major element chemistry, despite lower extents of chemical depletion. These results are consistent with the influence of aspect on regolith formation at Shale Hills; we hypothesize that aspect affects such variables as temperature, moisture content, and evapotranspiration in the regolith zone, causing faster chemical weathering and regolith formation rates on the south‐facing side of the catchment. The difference in microclimate between these two hillslopes is inferred to have been especially significant during the periglacial period that occurred at Shale Hills at least ~15 ka before present. At that time, the erosion rates may also have been different from those observed today, perhaps denuding the south‐facing hillslope of regolith but not quite stripping the north‐facing hillslope. An analysis of hillslope evolution and response timescales with a linear mass transport model shows that the current landscape at Shale Hills is not in geomorphologic steady state (i.e., so‐called dynamic equilibrium) but rather is likely still responding to the climate shift from the Holocene periglacial to the modern, temperate conditions. Regolith production rates determined by U-series isotopes Slope aspect controls regolith formation Landscape still responding to distubance in the past
    Keywords: Regolith Production ; Slope Aspect ; U‐Series Isotopes ; Chemical Weathering
    ISSN: 2169-9003
    E-ISSN: 2169-9011
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  • 4
    In: Journal of Geophysical Research: Earth Surface, September 2013, Vol.118(3), pp.1877-1896
    Description: Regolith‐mantled hillslopes are ubiquitous features of most temperate landscapes, and their morphology reflects the climatically, biologically, and tectonically mediated interplay between regolith production and downslope transport. Despite intensive research, few studies have quantified both of these mass fluxes in the same field site. Here we present an analysis of 87 meteoric Be measurements from regolith and bedrock within the Susquehanna Shale Hills Critical Zone Observatory (SSHO), in central Pennsylvania. Meteoric Be concentrations in bulk regolith samples ( = 73) decrease with regolith depth. Comparison of hillslope meteoric Be inventories with analyses of rock chip samples ( = 14) from a 24 m bedrock core confirms that 〉80% of the total inventory is retained in the regolith. The systematic downslope increase of meteoric Be inventories observed at SSHO is consistent with Be accumulation in slowly creeping regolith (~ 0.2 cm yr). Regolith flux inferred from meteoric Be varies linearly with topographic gradient (determined from high‐resolution light detection and ranging‐based topography) along the upper portions of hillslopes at SSHO. However, regolith flux appears to depend on the product of gradient and regolith depth where regolith is thick, near the base of hillslopes. Meteoric Be inventories at the north and south ridgetops indicate minimum regolith residence times of 10.5 ± 3.7 and 9.1 ± 2.9 ky, respectively, similar to residence times inferred from U‐series isotopes in Ma et al. (2013). The combination of our results with U‐series‐derived regolith production rates implies that regolith production and erosion rates are similar to within a factor of two on SSHO hillcrests. Meteoric 10Be used to quantify regolith flux on soil‐mantled hillslopes Regolith flux depends linearly on hillslope gradient Regolith flux comparable to independently measured regolith production
    Keywords: Meteoric 10be ; Critical Zone ; Regolith Flux
    ISSN: 2169-9003
    E-ISSN: 2169-9011
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  • 5
    Language: English
    In: Anthropocene, September 2014, Vol.7, pp.16-29
    Description: During the early industrial revolution, mining and smelting of ores and coal combustion released significant amounts of lead (Pb) into the atmosphere. While many researchers have documented high Pb concentrations in topsoils due to gasoline combustion between 1940s and 1980s, little work has focused on the extent of Pb and other heavy metal deposition into soils during the early industrial period. Here, we report Pb, cadmium (Cd), and zinc (Zn) concentrations and Pb isotope ratios of soils, sediments, parent bedrock, and waters collected from a small, currently pristine watershed (Shale Hills Critical Zone Observatory) in Pennsylvania (United States of America). Our results show that Pb in the soil comprises an addition profile, i.e. more Pb is present in the soil than is present in the equivalent parent bedrock. All three investigated soil profiles at Shale Hills on the same hillslope have Pb inventories (∼400–600 μg cm ) attributed to atmospheric deposition. Cd and Zn concentrations in these soils show similar addition profiles due to atmospheric deposition. Based on Pb isotopic ratios, the most likely source of the added Pb is coal burning and ore smelting during local iron production in the early 19th century, roughly coincident with the construction of the U.S. transcontinental railroad. Mass balance and diffusive transport modeling were used to quantify Pb deposition rates and redistribution. These model results are consistent with the hypothesis that from ∼1850s to 1920s, coal burning and ore smelting in local iron blasting furnaces significantly increased the local Pb emissions so that Pb deposition rates in soils were in the range of 6–10 μg cm yr . These values are comparable to Pb deposition rates found in other areas with early and intensive industrial activities (e.g. since the ∼1860s in Australia). Our new Pb, Cd, and Zn concentrations and Pb isotope results, in combination with the previously observed manganese (Mn) enrichment at Shale Hills, document that early industrial point sources contaminated local soils with metals that remain even today in topsoils with large sorption capacities. Where these metals are retained, their depth profiles provide a mean to infer the history of metal additions and redistributions.
    Keywords: Heavy Metal Pollution ; Industrial Revolution ; Pb Isotopes and Concentration ; Soils ; Critical Zone
    ISSN: 2213-3054
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  • 6
    Language: English
    In: Chemical Geology, 09 October 2013, Vol.356, pp.50-63
    Description: Soils developed on the Oatka Creek member of the Marcellus Formation in Huntingdon, Pennsylvania were analyzed to understand the evolution of black shale matrix porosity and the associated changes in elemental and mineralogical composition during infiltration of water into organic-rich shale. Making the reasonable assumption that soil erosion rates are the same as those measured in a nearby location on a less organic-rich shale, we suggest that soil production rates have on average been faster for this black shale compared to the gray shale in similar climate settings. This difference is attributed to differences in composition: both shales are dominantly quartz, illite, and chlorite, but the Oatka Creek member at this location has more organic matter (1.25 wt.% organic carbon in rock fragments recovered from the bottom of the auger cores and nearby outcrops) and accessory pyrite. During weathering, the extremely low-porosity bedrock slowly disaggregates into shale chips with intergranular pores and fractures. Some of these pores are either filled with organic matter or air-filled but remain unconnected, and thus inaccessible to water. Based on weathering bedrock/soil profiles, disintegration is initiated with oxidation of pyrite and organic matter, which increases the overall porosity and most importantly allows water penetration. Water infiltration exposes fresh surface area and thus promotes dissolution of plagioclase and clays. As these dissolution reactions proceed, the porosity in the deepest shale chips recovered from the soil decrease from 9 to 7% while kaolinite and Fe oxyhydroxides precipitate. Eventually, near the land surface, mineral precipitation is outcompeted by dissolution or particle loss of illite and chlorite and porosity in shale chips increases to 20%. As imaged by computed tomographic analysis, weathering causes i) greater porosity, ii) greater average length of connected pores, and iii) a more branched pore network compared to the unweathered sample. This work highlights the impact of shale–water–O interactions in near-surface environments: (1) black shale weathering is important for global carbon cycles as previously buried organic matter is quickly oxidized; and (2) black shales weather more quickly than less organic- and sulfide-rich shales, leading to high porosity and mineral surface areas exposed for clay weathering. The fast rates of shale gas exploitation that are ongoing in Pennsylvania, Texas and other regions in the United States may furthermore lead to release of metals to the environment if reactions between water and black shale are accelerated by gas development activities in the subsurface just as they are by low-temperature processes in our field study.
    Keywords: Pyrite Dissolution ; Organic Matter ; Shale Gas ; Trace Metals ; Neutron Scattering ; Computed Tomography ; Geology
    ISSN: 0009-2541
    E-ISSN: 1872-6836
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  • 7
    In: Water Resources Research, March 2017, Vol.53(3), pp.2346-2367
    Description: Why 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 (Q), soil lateral flow (Q), and deeper groundwater (Q), with Q 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 (V) 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 A and V, 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. A new watershed hydrogeochemistry code, RT‐Flux‐PIHM, allows deeper understanding of chemostatic behavior in stream discharge Chemostasis is driven by synchronized geochemical (clay dissolution and Cl mobilization) and hydrological processes Dilution occurs when stream discharge is dominated by relatively constant and hydrologically unresponsive deep groundwater influxes
    Keywords: Watershed Hydrogeochemistry ; Reactive Transport ; Concentration‐Discharge Relationship
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 8
    Language: English
    In: Geochimica et Cosmochimica Acta, 2006, Vol.70(1), pp.56-70
    Description: This study documents the first example of in vitro solid-phase mineral oxide reduction by enzyme-containing membrane fractions. Previous in vitro studies have only reported the reduction of aqueous ions. Total membrane (TM) fractions from iron-grown cultures of were isolated and shown to catalyze the reduction of goethite, hematite, birnessite, and ramsdellite/pyrolusite using formate. In contrast, nicotinamide adenine dinucleotide (NADH) and succinate cannot function as electron donors. The significant implications of observations related to this cell-free system are: (i) both iron and manganese mineral oxides are reduced by the TM fraction, but aqueous U(VI) is not; (ii) TM fractions from anaerobically grown, but not aerobically grown, cells can reduce the mineral oxides; (iii) electron shuttles and iron chelators are not needed for this in vitro reduction, documenting conclusively that reduction can occur by direct contact with the mineral oxide; (iv) electron shuttles and EDTA stimulate the in vitro Fe(III) reduction, documenting that exogenous molecules can enhance rates of enzymatic mineral reduction; and (v) multiple membrane components are involved in solid-phase oxide reduction. The membrane fractions, consisting of liposomes of cytoplasmic and outer membrane segments, contain at least 100 proteins including the enzyme that oxidizes formate, formate dehydrogenase. Mineral oxide reduction was inhibited by the addition of detergent Triton X-100, which solubilizes membranes and their associated proteins, consistent with the involvement of multiple electron carriers that are disrupted by detergent addition. In contrast, formate dehydrogenase activity was not inhibited by Triton X-100. The addition of anthraquinone-2,6-disulfonate (AQDS) and menaquinone-4 was unable to restore activity; however, menadione (MD) restored 33% of the activity. The addition of AQDS and MD to reactions without added detergent increased the rate of goethite reduction. The Michaelis–Menten values of 71 ± 22 m /L for hematite and 50 ± 16 m /L for goethite were calculated as a function of surface area of the two insoluble minerals. was determined to be 123 ± 14 and 156 ± 13 nmol Fe(II)/min/mg of TM protein for hematite and goethite, respectively. These values are consistent with in vivo rates of reduction reported in the literature. These observations are consistent with our conclusion that the enzymatic reduction of mineral oxides is an effective probe that will allow elucidation of molecular chemistry of the membrane–mineral interface where electron transfer occurs.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 9
    Language: English
    In: Geochimica et Cosmochimica Acta, 2005, Vol.69(22), pp.5233-5246
    Description: The isotopic composition of dissolved Cu and solid Cu-rich minerals [δ Cu (‰) = ( Cu/ Cu / Cu/ Cu ) - 1)*1000] were monitored in batch oxidative dissolution experiments with and without . Aqueous copper in leach fluids released during abiotic oxidation of both chalcocite and chalcopyrite was isotopically heavier (δ Cu = 5.34‰ and δ Cu = 1.90‰, respectively, [±0.16 at 2σ]) than the initial starting material (δ Cu = 2.60 ± 0.16‰ and δ Cu = 0.58 ± 0.16‰, respectively). Isotopic mass balance between the starting material, aqueous copper, and secondary minerals precipitated in these experiments explains the heavier isotopic values of aqueous copper. In contrast, aqueous copper from leached chalcocite and chalcopyrite inoculated with was isotopically similar to the starting material. The lack of fractionation of the aqueous copper in the biotic experiments can best be explained by assuming a sink for isotopically heavy copper present in the bacteria cells with δ Cu = 5.59 ± 0.16‰. Consistent with this inference, amorphous Cu-Fe oxide minerals are observed surrounding cell membranes of grown in the presence of dissolved Cu and Fe. Extrapolating these experiments to natural supergene environments implies that release of isotopically heavy aqueous Cu from oxidative leach caps, especially under abiotic conditions, should result in precipitates in underlying enrichment blankets that are isotopically heavy. Where iron-oxidizing cells are involved, isotopically heavy oxidized Cu entrained in cellular material may become associated with leach caps, causing the released aqueous Cu to be less isotopically enriched in the heavy isotope than predicted for the abiotic system. Rayleigh fractionation trends with fractionation factors calculated from our experiments for both biotic and abiotic conditions are consistent with large numbers of individual abiotic or biotic leaching events, explaining the supergene chalcocites in the Morenci and Silver Bell porphyry copper deposits.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 10
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 February 2019, Vol.247, pp.1-26
    Description: To quantify chemical weathering processes, it is essential to develop and utilize new geochemical tools that can provide information about chemical weathering in the field. U-series isotopes have emerged as a useful chronometer to directly constrain the rates and duration of chemical weathering. However, the conventional solution-based MC-ICPMS method involves a long and expensive sample processing procedure that restricts the numbers of measurements of samples by U-series analysis that can be completed. Here, we report measurements of U-series disequilibria obtained with laser ablation (LA)-MC-ICPMS on weathering rinds collected from the tropical island of Basse-Terre in the archipelago of French Guadeloupe. We characterized two weathering rinds for U-series isotope compositions and elemental distributions with LA-MC-ICPMS and LA-Q-ICPMS. The measurements of U-series disequilibria were consistent with the previous bulk measurements obtained by conventional solution MC-ICPMS despite the larger analytical uncertainties. The LA technique allowed a greater number of measurements that accelerated sample throughput and improved spatial resolution of measurement. The rind formation age, weathering rates, and U-series mobility parameters modeled in this study are comparable to the results from previous studies conducted on the same clasts, and also reveal new insights on rind formation such as the impact of micro-fractures on weathering history and U-series ratios. The improved spatial resolution available with LA Q-ICPMS helps distinguish between linear and power law rind thickness-age relationships that were unresolvable using conventional solution-based MC-ICPMS. measurements with LA-Q-ICPMS in these weathering rinds also elucidates the sequences of mineral reactions during chemical weathering. The LA-Q-ICPMS maps of major and trace elements and elemental ratios reveal details about the rind formation processes at the weathering interfaces of clasts such as dissolution of primary phases, formation of new phases, development of porosity, and mobility behavior of U. This study demonstrates a new analytical method for determining weathering rates in rinds rapidly and accurately that can be used in a large number of rinds, providing key information at the clast scale.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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