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  • 1
    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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  • 2
    Language: English
    In: Geochimica et Cosmochimica Acta, March 1, 2012, Vol.80, p.92(16)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2011.11.038 Byline: Lin Ma (a)(b)(c), Francois Chabaux (b), Eric Pelt (b), Mathieu Granet (b), Peter B. Sak (d), Jerome Gaillardet (e), Marina Lebedeva (a), Susan L. Brantley (a) Abstract: To quantify rates of rind formation on weathering clasts under tropical and humid climate and to determine factors that control weathering reactions, we analyzed Uranium series isotope compositions and trace element concentrations in a basaltic andesite weathering clast collected from Basse-Terre Island in Guadeloupe. U, Th, and Ti elemental profiles reveal that Th and Ti behave conservatively during rind formation, but that U is added from an external source to the rind. In the rind, weathering reactions include dissolution of primary minerals such as pyroxene, plagioclase, and glass matrix, as well as formation of Fe oxyhydroxides, gibbsite and minor kaolinite. Rare earth element (REE) profiles reveal a significant Eu negative anomaly formed during clast weathering, consistent with plagioclase dissolution. Significant porosity forms in the rind mostly due to plagioclase dissolution. The new porosity is inferred to allow influx of soil water carrying externally derived, dissolved U. Due to this influx, U precipitates along with newly formed clay minerals and oxyhydroxides in the rind. The conservative behavior of Th and the continuous addition of U into the rind adequately explain the observed systematic trends of (.sup.238U/.sup.232Th) and (.sup.230Th/.sup.232Th) activity ratios in the rind. Rind formation rates, determined from the measured U-series activity ratios with an open system U addition model, increase by a factor of [approximately equal to]1.3 (0.18-0.24mm/kyr) from a low curvature to a high curvature section (0.018-0.12mm.sup.-1) of the core-rind boundary, revealing that curvature affects rates of rind formation as expected for diffusion-limited rind formation. U-series geochronometry thus provides the first direct evidence that the curvature of the interface controls the rate of regolith formation at the clast scale. The weathering rates determined at the clast scale can be reconciled with the weathering rates determined at the watershed or soil profile scale if surface roughness equals values of approximately 1300-2200. Author Affiliation: (a) Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA (b) Laboratoire d'Hydrologie et de Geochimie de Strasbourg, EOST, University of Strasbourg and CNRS, Strasbourg, France (c) Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968-0555, USA (d) Department of Earth Sciences, Dickinson College, Carlisle, PA 17013, USA (e) Laboratoire de Geochimie et Cosmochimie, Institue de Physique du Globe de Paris, Paris, France Article History: Received 28 March 2011; Accepted 21 November 2011 Article Note: (miscellaneous) Associate editor: Eric H. Oelkers
    Keywords: Clay Minerals -- Analysis ; Basalt -- Analysis ; Soil Moisture -- Analysis ; Uranium -- Analysis ; Porosity -- Analysis
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 3
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 December 2016, Vol.195, pp.29-67
    Description: Inside soil and saprolite, rock fragments can form weathering clasts (alteration rinds surrounding an unweathered core) and these weathering rinds provide an excellent field system for investigating the initiation of weathering and long term weathering rates. Recently, uranium-series (U-series) disequilibria have shown great potential for determining rind formation rates and quantifying factors controlling weathering advance rates in weathering rinds. To further investigate whether the U-series isotope technique can document differences in long term weathering rates as a function of precipitation, we conducted a new weathering rind study on tropical volcanic Basse-Terre Island in the Lesser Antilles Archipelago. In this study, for the first time we characterized weathering reactions and quantified weathering advance rates in multiple weathering rinds across a steep precipitation gradient. Electron microprobe (EMP) point measurements, bulk major element contents, and U-series isotope compositions were determined in two weathering clasts from the Deshaies watershed with mean annual precipitation (MAP) = 1800 mm and temperature (MAT) = 23 °C. On these clasts, five core-rind transects were measured for locations with different curvature (high, medium, and low) of the rind-core boundary. Results reveal that during rind formation the fraction of elemental loss decreases in the order: Ca ≈ Na 〉 K ≈ Mg 〉 Si ≈ Al 〉 Zr ≈ Ti ≈ Fe. Such observations are consistent with the sequence of reactions after the initiation of weathering: specifically, glass matrix and primary minerals (plagioclase, pyroxene) weather to produce Fe oxyhydroxides, gibbsite and minor kaolinite. Uranium shows addition profiles in the rind due to the infiltration of U-containing soil pore water into the rind as dissolved U phases. U is then incorporated into the rind as Fe-Al oxides precipitate. Such processes lead to significant U-series isotope disequilibria in the rinds. This is the first time that multiple weathering clasts from the same watershed were analyzed for U-series isotope disequlibrian and show consistent results. The U-series disequilibria allowed for the determination of rind formation ages and weathering advance rates with a U-series mass balance model. The weathering advance rates generally decreased with decreasing curvature: ∼0.17 ± 0.10 mm/kyr for high curvature, ∼0.12 ± 0.05 mm/kyr for medium curvature, and ∼0.11 ± 0.04, 0.08 ± 0.03, 0.06 ± 0.03 mm/kyr for low curvature locations. The observed positive correlation between the curvature and the weathering rates is well supported by predictions of weathering models, i.e., that the curvature of the rind-core boundary controls the porosity creation and weathering advance rates at the clast scale. At the watershed scale, the new weathering advance rates derived on the low curvature transects for the relatively dry Deshaies watershed (average rate of 0.08 mm/kyr; MAP = 1800 mm and MAT = 23 °C) are ∼60% slower than the rind formation rates previously determined in the much wetter Bras David watershed (∼0.18 mm/kyr, low curvature transect; MAP = 3400 mm and MAT = 23 °C) also on Basse-Terre Island. Thus, a doubling of MAP roughly correlates with a doubling of weathering advance rate. The new rind study highlights the effect of precipitation on weathering rates over a time scale of ∼100 kyr. Weathering rinds are thus a suitable system for investigating long-term chemical weathering across environmental gradients, complementing short-term riverine solute fluxes.
    Keywords: U-Series Isotopes ; Weathering Rinds ; Weathering Rates ; Precipitation ; French Guadeloupe ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 4
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 March 2012, Vol.80, pp.92-107
    Description: To quantify rates of rind formation on weathering clasts under tropical and humid climate and to determine factors that control weathering reactions, we analyzed Uranium series isotope compositions and trace element concentrations in a basaltic andesite weathering clast collected from Basse-Terre Island in Guadeloupe. U, Th, and Ti elemental profiles reveal that Th and Ti behave conservatively during rind formation, but that U is added from an external source to the rind. In the rind, weathering reactions include dissolution of primary minerals such as pyroxene, plagioclase, and glass matrix, as well as formation of Fe oxyhydroxides, gibbsite and minor kaolinite. Rare earth element (REE) profiles reveal a significant Eu negative anomaly formed during clast weathering, consistent with plagioclase dissolution. Significant porosity forms in the rind mostly due to plagioclase dissolution. The new porosity is inferred to allow influx of soil water carrying externally derived, dissolved U. Due to this influx, U precipitates along with newly formed clay minerals and oxyhydroxides in the rind. The conservative behavior of Th and the continuous addition of U into the rind adequately explain the observed systematic trends of ( U/ Th) and ( Th/ Th) activity ratios in the rind. Rind formation rates, determined from the measured U-series activity ratios with an open system U addition model, increase by a factor of ∼1.3 (0.18–0.24 mm/kyr) from a low curvature to a high curvature section (0.018–0.12 mm ) of the core–rind boundary, revealing that curvature affects rates of rind formation as expected for diffusion-limited rind formation. U-series geochronometry thus provides the first direct evidence that the curvature of the interface controls the rate of regolith formation at the clast scale. The weathering rates determined at the clast scale can be reconciled with the weathering rates determined at the watershed or soil profile scale if surface roughness equals values of approximately 1300–2200.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 5
    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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  • 6
    Language: English
    In: Chemical Geology, 2010, Vol.276(3), pp.129-143
    Description: A clast of low porosity basaltic andesite collected from the B horizon of a soil developed on a late Quaternary volcaniclastic debris flow in the Bras David watershed on Basse-Terre Island, Guadeloupe, exhibits weathering like that observed in many weathered clasts of similar composition in other tropical locations. Specifically, elemental profiles measured across the core–rind interface document that primary minerals and glass weather to Fe oxyhydroxides, gibbsite and minor kaolinite in the rind. The earliest reaction identified in the core is oxidation of Fe in pyroxene but the earliest reaction that creates significant porosity is plagioclase dissolution. Elemental loss varies in the order Ca ≈ Na 〉 K ≈ Mg 〉 Si 〉 Al 〉 Fe ≈ P ≫ Ti, consistent with the relative reactivity of phases in the clast from plagioclase ≈ pyroxene ≈ glass 〉 apatite 〉 ilmenite. The rind surrounds a core of unaltered material that is more spherical than the original clast. The distance from the core–rind boundary to a visually prominent rind layer, , was measured as a proxy for the rind thickness at 36 locations on a slab cut vertically through the nominal center of the clast. This distance averaged 24.4 ± 3.1 mm. Maximum and minimum values for , 35.8 and 20.6 mm, were observed where curvature of the core–rind boundary is greatest (0.12 mm ) and smallest (0.018 mm ) respectively. Extrapolating from other rinds in other locations, the rate of rind formation is estimated to vary by a factor of about 2 (from ∼ 4 to 7 × 10 m s ) from low to high curvature. The observation of a higher rate of rind formation for a higher curvature interface is consistent with a diffusion-limited model for weathering rind formation. The diffusion-limited model predicts that, like rind thickness, values of the thickness of the reaction front ( ) for a given reaction, defined as the zone over which a parent mineral such as plagioclase completely weathers to rind material, should also increase with curvature. Values of were quantified as a function of interface curvature using bulk chemical analysis (500 〈 〈 2000 μm). Values of were also quantified by measuring loss of matrix glass and increase in porosity as a function of curvature. In contrast to rind thickness, shows no consistent increase with curvature. This contradiction is attributed to the mm-scale roughness of the interface which is related to phenocryst grain size. Therefore, the overall rind formation rate is strongly affected by curvature measured at the scale of the clast, while mineral reaction rates documented by reaction front thickness are strongly affected by curvature at the scale of phenocrysts. Similarly, the weathering advance rate (m s ) for the entire Bras David watershed can be extrapolated from the clast weathering rate if roughness at the watershed scale equals values of approximately 400–800.
    Keywords: Chemical Weathering ; Weathering Rinds ; Basalt Weathering ; Guadeloupe ; Geology
    ISSN: 0009-2541
    E-ISSN: 1872-6836
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  • 7
    Language: English
    In: Earth Surface Dynamics, Dec 18, 2017, Vol.5(4), p.841
    Description: The critical zone (CZ), the dynamic living skin of the Earth, extends from the top of the vegetative canopy through the soil and down to fresh bedrock and the bottom of the groundwater. All humans live in and depend on the CZ. This zone has three co-evolving surfaces: the top of the vegetative canopy, the ground surface, and a deep subsurface below which Earth's materials are unweathered. The network of nine CZ observatories supported by the US National Science Foundation has made advances in three broad areas of CZ research relating to the co-evolving surfaces. First, monitoring has revealed how natural and anthropogenic inputs at the vegetation canopy and ground surface cause subsurface responses in water, regolith structure, minerals, and biotic activity to considerable depths. This response, in turn, impacts aboveground biota and climate. Second, drilling and geophysical imaging now reveal how the deep subsurface of the CZ varies across landscapes, which in turn influences aboveground ecosystems. Third, several new mechanistic models now provide quantitative predictions of the spatial structure of the subsurface of the CZ.Many countries fund critical zone observatories (CZOs) to measure the fluxes of solutes, water, energy, gases, and sediments in the CZ and some relate these observations to the histories of those fluxes recorded in landforms, biota, soils, sediments, and rocks. Each US observatory has succeeded in (i) synthesizing research across disciplines into convergent approaches; (ii) providing long-term measurements to compare across sites; (iii) testing and developing models; (iv) collecting and measuring baseline data for comparison to catastrophic events; (v) stimulating new process-based hypotheses; (vi) catalyzing development of new techniques and instrumentation; (vii) informing the public about the CZ; (viii) mentoring students and teaching about emerging multidisciplinary CZ science; and (ix) discovering new insights about the CZ. Many of these activities can only be accomplished with observatories. Here we review the CZO enterprise in the United States and identify how such observatories could operate in the future as a network designed to generate critical scientific insights. Specifically, we recognize the need for the network to study network-level questions, expand the environments under investigation, accommodate both hypothesis testing and monitoring, and involve more stakeholders. We propose a driving question for future CZ science and a hubs-and-campaigns model to address that question and target the CZ as one unit. Only with such integrative efforts will we learn to steward the life-sustaining critical zone now and into the future.
    Keywords: Terrestrial Planets – Analysis ; Vegetation Management – Analysis
    ISSN: 2196-6311
    E-ISSN: 2196632X
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  • 8
    Language: English
    In: Biogeosciences, August 15, 2018, Vol.15(15), p.4815
    Description: pLong-term environmental research networks are one approach to advancing local, regional, and global environmental science and education. A remarkable number and wide variety of environmental research networks operate around the world today. These are diverse in funding, infrastructure, motivating questions, scientific strengths, and the sciences that birthed and maintain the networks. Some networks have individual sites that were selected because they had produced invaluable long-term data, while other networks have new sites selected to span ecological gradients. However, all long-term environmental networks share two challenges. Networks must keep pace with scientific advances and interact with both the scientific community and society at large. If networks fall short of successfully addressing these challenges, they risk becoming irrelevant. The objective of this paper is to assert that the biogeosciences offer environmental research networks a number of opportunities to expand scientific impact and public engagement. We explore some of these opportunities with four networks: the International Long-Term Ecological Research Network programs (ILTERs), critical zone observatories (CZOs), Earth and ecological observatory networks (EONs), and the FLUXNET program of eddy flux sites. While these networks were founded and expanded by interdisciplinary scientists, the preponderance of expertise and funding has gravitated activities of ILTERs and EONs toward ecology and biology, CZOs toward the Earth sciences and geology, and FLUXNET toward ecophysiology and micrometeorology. Our point is not to homogenize networks, nor to diminish disciplinary science. Rather, we argue that by more fully incorporating the integration of biology and geology in long-term environmental research networks, scientists can better leverage network assets, keep pace with the ever-changing science of the environment, and engage with larger scientific and public audiences.
    Keywords: Environmental Research – Innovations ; Environmental Research – Forecasts and Trends
    ISSN: 1726-4170
    E-ISSN: 17264189
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