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  • 1
    Language: English
    In: Chemical Geology, 2011, Vol.290(1), pp.31-49
    Description: To understand the factors that control rare earth elements (REE) release and fractionation during shale weathering, we investigate the REE contents in solid (bedrock, regolith, stream sediments), and natural waters (stream, and pore waters) from a first-order catchment developed entirely on gray shales in central Pennsylvania, USA. Up to 65% of the REE (relative to parent bedrock) is depleted from the weathering profiles in the acidic and organic-rich soils due to chemical leaching. In addition, newly formed fine particles were also lost along with the down-slope movement of soil waters. Weathering profiles on the south-facing slope show less depletion of REE than those on the north-facing slope (33% vs. 45% on average). We hypothesize that the different degrees of REE depletion on the two transects reflect a history of different chemical weathering rates and possible different surface erosion rates, controlled by contrasting slope aspect-induced microclimate conditions. In addition, weathering profiles, natural waters and sequential extractions all show a preferential removal of Middle REE (up to 22% more) relative to Light REE and Heavy REE during shale weathering, due to preferential release of MREE from rhabdophane. Furthermore, the long-term phosphate mineral dissolution rates (e.g., rhabdophane) were estimated at 10 to 10 mol m s under field conditions, based on REE depletion profiles. Strong positive Ce anomalies (average [Ce/Ce*] value: 1.79) observed in the regolith, stream sediments, and regolith extractions point to the fractionation and preferential precipitation of Ce as compared to other REE, due to the generally oxidizing conditions during release, transport, and redistribution of REE in the surface and subsurface environments. Positive Eu anomalies (average [Eu/Eu*] value: 1.30) observed in the natural waters of the catchment are attributed to weathering of plagioclases in the shale bedrock. This study highlights the use of REE as natural tracers for low-temperature geochemical processes. ► Significant REE depletions during formation of acidic and organic-rich soils. ► Slope aspect controls REE depletion by generating different microclimate conditions. ► Dissolution of rhabdophane controls the MREE fractionation during regolith formation. ► Strong Ce anomalies point to prevalent oxidizing conditions. ► Positive Eu anomalies attributed to weathering of plagioclase.
    Keywords: Rare Earth Elements ; Regolith ; Pore Water ; Chemical Weathering ; Phosphate Mineral Dissolution ; Slope Aspect ; Geology
    ISSN: 0009-2541
    E-ISSN: 1872-6836
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  • 2
    Language: English
    In: Chemical Geology, 2010, Vol.278(1), pp.1-14
    Description: 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 O concentration. Increasing LMWOA concentrations from 0 to 10 mM resulted in enhanced REY release. In general, REY release increased in the order: no ligand ≈ salicylate 〈 phthalate ≈ oxalate 〈 citrate. REY–ligand stability constants were only useful for predicting REY release for oxalate reacted with apatite and phthalate reacted with monazite. The role of dissolved oxygen in dissolution of the phosphate minerals was mixed and inconsistent. Mineral type was observed to significantly affect REY pattern development. REY release patterns for apatite range from nearly flat to those exhibiting the lanthanide contraction effect (radius-dependent fractionation); whereas, monazite REY release patterns are best described as exhibiting an M-type lanthanide tetrad effect (radius-independent fractionation). Weathering of apatite in the presence of aliphatic LMWOAs resulted in development of the lanthanide contraction effect fractionation pattern, and the aliphatic LMWOAs further developed MREE and radius-independent fractionation during monazite dissolution. Geochemical and mineral-specific REY signatures may, therefore, have utility for distinguishing the impacts of biota on soil weathering processes on early Earth. The development of such signatures may be mitigated, in part, by accessory mineral composition, the types and concentration of LMWOAs present, and precipitation of secondary minerals.
    Keywords: Apatite ; Dissolution ; Monazite ; Organic Acids ; Rare Earth Elements ; Yttrium ; Geology
    ISSN: 0009-2541
    E-ISSN: 1872-6836
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  • 3
    Language: English
    In: Chemical Geology, 2010, Vol.269(1), pp.62-78
    Description: Chemical weathering of silicate minerals has long been known as a sink for atmospheric CO , and feedbacks between weathering and climate are believed to affect global climate. While warmer temperatures are believed to increase rates of weathering, weathering in cool climates can be accelerated by increased mineral exposure due to mechanical weathering by ice. In this study, chemical weathering of silicate minerals is investigated in a small temperate watershed. The Jamieson Creek watershed is covered by mature coniferous forest and receives high annual precipitation (4000 mm), mostly in the form of rainfall, and is underlain by quartz diorite bedrock and glacial till. Analysis of pore water concentration gradients indicates that weathering in hydraulically unsaturated ablation till is dominated by dissolution of plagioclase and hornblende. However, a watershed scale solute mass balance indicates high relative fluxes of K and Ca, indicating preferential leaching of these solutes possibly from the relatively unweathered lodgement till. Weathering rates for plagioclase and hornblende calculated from a watershed scale solute mass balance are similar in magnitude to rates determined using pore water concentration gradients. When compared to the Rio Icacos basin in Puerto Rico, a pristine tropical watershed with similar annual precipitation and bedrock, but with dissimilar regolith properties, fluxes of weathering products in stream discharge from the warmer site are 1.8 to 16.2-fold higher, respectively, and regolith profile-averaged plagioclase weathering rates are 3.8 to 9.0-fold higher. This suggests that the Arrhenius effect, which predicts a 3.5- to 9-fold increase in the dissolution rate of plagioclase as temperature is increased from 3.4° to 22 °C, may explain the greater weathering fluxes and rates at the Rio Icacos site. However, more modest differences in K and Ca fluxes between the two sites are attributed to accelerated leaching of those solutes from glacial till at Jamieson Creek. Our findings suggest that under conditions of high rainfall and favorable topography, weathering rates of silicate minerals in warm tropical systems will tend to be higher than in cool temperate systems, even if the temperate system is has been perturbed by an episode of glaciation that deposits regolith high in fresh mineral surface area.
    Keywords: Chemical Weathering ; Watershed ; Diorite ; Solute Mass Balance ; Silicates ; Glacial Till ; Geology
    ISSN: 0009-2541
    E-ISSN: 1872-6836
    Source: ScienceDirect Journals (Elsevier)
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  • 4
    Language: English
    In: Chemical geology, 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 (1.25wt.% 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–O₂ 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. ; p. 50-63.
    Keywords: Bedrock ; Clay ; Plagioclase ; Soil Profiles ; Pyrites ; Shale ; Oxidation ; Illite ; Kaolinite ; Organic Matter ; Weathering ; Iron ; Porosity ; Computed Tomography ; Carbon ; Quartz ; Soil Erosion ; Geochemistry ; Climate ; Surface Area
    ISSN: 0009-2541
    Source: AGRIS (Food and Agriculture Organization of the United Nations)
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  • 5
    Language: English
    In: Chemical Geology, 2011, Vol.281(3), pp.167-180
    Description: 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 Fe 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 Fe. 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. ► Batch dissolution experiments conducted using two distinct end-member shales. ► Fe released from clay minerals and Fe oxides in Rose Hill (brown) shale. ► Fe and Mo released from pyrite in Marcellus (black) shale. ► Fe release enhanced by siderophore, isotopically depleted compared to bulk rock. ► Mo released from Marcellus shale isotopically enriched under all conditions.
    Keywords: Shale Weathering ; Iron Isotopes ; Molybdenum Isotopes ; Organic Ligands ; Bacteria ; Geology
    ISSN: 0009-2541
    E-ISSN: 1872-6836
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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
    Language: English
    In: Chemical Geology, 05 September 2017, Vol.466, pp.352-379
    Description: 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.
    Keywords: Dissolution Kinetics ; Sulfides ; Organic Matter ; Sorption ; Redox ; Geology
    ISSN: 0009-2541
    E-ISSN: 1872-6836
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  • 8
    Language: English
    In: Chemical Geology, March 18, Vol.397, p.37(14)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.chemgeo.2015.01.010 Byline: Lin Ma, Fang-Zhen Teng, Lixin Jin, Shan Ke, Wei Yang, Hai-Ou Gu, Susan L. Brantley Abstract: Magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments, but few have focused on the mechanisms that control Mg isotope fractionation during shale weathering. In this study we focus on understanding Mg isotope fractionation in the Shale Hills catchment in central Pennsylvania. Mg isotope ratios were measured systematically in weathering products, along geochemical pathways of Mg during shale weathering: from bedrock to soils and soil pore water on a planar hillslope, and to sediments, stream water, and groundwater on a valley floor. Significant variations of Mg isotopic values were observed: [delta].sup.26Mg values (-0.6a[degrees] to -0.1a[degrees]) of stream and soil pore waters are about ~0.5a[degrees] to 1a[degrees] lighter than the shale bedrock [delta](.sup.26Mg values of +0.4a[degrees]), consistent with previous observations that lighter Mg isotopes are preferentially released to water during silicate weathering. Dissolution of the carbonate mineral ankerite, depleted in the shallow soils but present in bedrock at greater depths, produced higher Mg.sup.2+ concentrations but lower [delta].sup.26Mg values (-1.1a[degrees]) in groundwater, ~1.5a[degrees] lighter than the bedrock. [delta].sup.26Mg values (+0.2a[degrees] to +0.4a[degrees]) of soil samples on the planar hillslope are either similar or up to ~0.2a[degrees] lighter than the bedrock. Hence a heavy Mg isotope reservoir - complementary to the lighter Mg isotopes in soil pore water and stream water - is missing from the residual soils on the hillslope. In addition, soil samples show a slight but systematic decreasing trend in [delta].sup.26Mg values with increasing weathering duration towards the surface. We suggest that the accumulation of light Mg isotopes in surface soils at Shale Hills is due to a combined effect of i) sequestration of isotopically light Mg from soil water during clay dissolution-precipitation reactions; and ii) loss of isotopically heavy particulate Mg in micron-sized particles from the hillslope as suspended sediments. This latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation. Stream sediments ([delta].sup.26Mg values of +0.3a[degrees] to +0.5a[degrees]) accumulated on the valley floor are ~0.2a[degrees] heavier than the bedrock, and are thus consistent with that mobile particulates are the heavy Mg isotope reservoir. Our study provides the first field evidence that changes in clay mineralogy lead to accumulation of lighter Mg isotopes in residual bulk soils. This example also demonstrates that transport of isotopically distinct fine particles from clay-rich systems could be a new and important mechanism to drive the Mg isotope compositions of silicate weathering residuals. This mechanism drives fractionation in an opposite direction as might be expected from previous studies, i.e. residual soils are driven to lighter Mg values and sediments become isotopically heavier. Article History: Received 26 March 2013; Revised 24 October 2014; Accepted 17 January 2015 Article Note: (miscellaneous) Editor: Michael E. Bottcher
    Keywords: Soil Moisture ; Groundwater ; Soils ; Carbonates ; Precipitation (Meteorology) ; Air Pollution ; Silicates ; River Sediments ; Tracers (Biology) ; Geomorphology ; Clay Minerals ; Shales
    ISSN: 0009-2541
    Source: Cengage Learning, Inc.
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  • 9
    Language: English
    In: Chemical Geology, 2010, Vol.269(1), pp.52-61
    Description: Rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations. In the Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock. We test the hypotheses that the organisms at depth are limited by the availability of two nutrients, P and Fe. Many tropical soils are P-limited, rather than N-limited, and dissolution of apatite is the dominant source of P. We document patterns of apatite weathering and of bioavailable Fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7.5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut. Iron isotopic compositions of 0.5 N HCl extracts of soil and saprolite range from about δ Fe = 0 to − 0.1‰ throughout the saprolite except at the surface and at 5 m depth where δ Fe = − 0.26 to − 0.64‰. The enrichment of light isotopes in HCl-extractable Fe in the soil and at the saprolite–bedrock interface is consistent with active Fe cycling and consistent with the locations of high cell densities and Fe(II)-oxidizing bacteria, identified previously. To evaluate the potential P-limitation of Fe-cycling bacteria in the profile, solid-state concentrations of P were measured as a function of depth in the soil, saprolite, and weathering bedrock. Weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6.8 × 10  mol m s was calculated. While surface communities depend on recycled nutrients and atmospheric inputs, deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. While low available P may limit microbial activity within the middle saprolite, fluxes of P from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite.
    Keywords: Phosphorus ; Iron Isotopes ; Saprolite ; Apatite Weathering Rate ; Fe(II)-Oxidizing Bacteria ; Geology
    ISSN: 0009-2541
    E-ISSN: 1872-6836
    Source: ScienceDirect Journals (Elsevier)
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  • 10
    Language: English
    In: Chemical Geology
    Description: We conduct X-ray microprobe, chemical and U-series isotope analyses on an oriented weathering clast collected from the regolith of a weathered Quaternary volcanoclastic debris flow on Basse Terre Island, French Guadeloupe. The sample consists of an unweathered basaltic andesite core surrounded by a weathering rind, and an indurated crust that separates the rind from the overlying soil matrix. U/Th disequilibria dating indicates that rind age increases away from the core-rind boundary to a maximum of 66 ka. This translates to a rind-advance rate of ~0.2 mm kyr , broadly consistent with rind advance rates calculated elsewhere on Basse Terre Island. The overlying indurated crust is 72 ka, indicating a possible minimum duration of the rind formation. Elemental variations are constrained by a bulk chemical analysis along a vertical transects from the core to the overlying soil matrix and parallel electron microprobe analyses. The hierarchy of elemental loss across the core-rind boundary varies in the order Ca 〉 Na ≈ Mg 〉 K 〉 Mn 〉 Si 〉 Al 〉 Ti = 0 〉 P 〉 Fe, consistent with the relative loss of phases in the clast from plagioclase ≈ glass ≈ pyroxene 〉 apatite 〉 ilmenite. The abrupt, 〈900 μm wide, Ca, Na and porosity reaction fronts at the core-rind boundary approximately equal the length of the long dimension of plagioclase phenocrysts observed in the unweathered core. The 〈1000 μm wide reaction front at the rind-soil interface is marked by an indurated horizon with Fe and Mn enrichment that spans into enrichment of Mn, Ba, Al, Mg and K in the soil matrix. Unlike previously studied clasts, the preservation of the rind-soil interface permits characterization of weathering reactions and material exchanges between the weathering core, the rind, and the surrounding soil matrix, shedding insights into communication between the enveloping weathering rind and host regolith. The lack of an enrichment signal of Mn within the weathered rind suggests that weathering processes active within clasts are distinct from surrounding soil formation processes.
    Keywords: Chemical Weathering ; Critical Zone ; Weathering Rinds ; Redox ; French Guadeloupe ; Geology
    ISSN: 0009-2541
    E-ISSN: 1872-6836
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