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  • 1
    Language: English
    In: Earth and Planetary Science Letters, March 1, 2014, Vol.389, p.43(9)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.epsl.2013.12.009 Byline: Richard Greenberg, Peter B. Sak Abstract: The surface of Europa displays numerous generations of intersecting arrays of linear ridges. At some locations along these ridges, older ridges on adjacent terrain appear to extend up the flank of a more recent ridge. It has thus been suggested that the ridges may have formed by upturning of that adjacent terrain. However, the newer ridges generally appear to be material deposited over the older terrain. Here we consider how the morphology of the overprinted topography may have been inherited by the more recent ridges. An analogous process occurs along some sediment-starved convergent plate boundaries on Earth, where the poorly consolidated material of a frontal prism of an overriding plate is pushed over preexisting ridges and seamounts on the downgoing plate. The overriding plate inherits the morphology of the downgoing plate even though the actual extension of that topography has been underthrust and buried. A well-studied example lies offshore of Costa Rica where the Caribbean plate overrides the Cocos plate. Experiments show other mechanisms as well: mass-wasting down a flank can result in extensions of adjacent ridges thanks to the geometry imposed by a constant angle of repose; in addition, more pronounced extensions of the older ridges result if the new ridge grows as it is bulldozed from behind (i.e., from the central groove of a double ridge on Europa). The shapes of the ridge extensions are distinctly different in these two cases. If tidal pumping extrudes material to the surface at the center of a double ridge, it might drive the latter mechanism. The ridge extensions observed on the flanks of more recent ridges may provide a crucial diagnostic of dominant ridge-building mechanisms when and if additional images are obtained at high resolution from future exploration. In additional to their morphology, the distribution of ridge extensions at only isolated locales may also provide constraints on ridge formation processes and their diversity. Author Affiliation: (a) Lunar and Planetary Laboratory, University of Arizona, 1629 East University Blvd., Tucson, AZ 85721, United States (b) Department of Earth Sciences, Dickinson College, Carlisle, PA 17013, United States Article History: Received 11 January 2013; Revised 26 November 2013; Accepted 9 December 2013 Article Note: (miscellaneous) Editor: C. Sotin
    Keywords: Plate Boundaries ; Seamounts
    ISSN: 0012-821X
    Source: Cengage Learning, Inc.
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  • 2
    Language: English
    In: Environmental Pollution, August 2014, Vol.191, pp.266-267
    Keywords: Engineering ; Environmental Sciences ; Anatomy & Physiology
    ISSN: 0269-7491
    E-ISSN: 1873-6424
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  • 3
    Language: English
    In: Environmental Pollution, August 2014, Vol.191, pp.262-263
    Keywords: Engineering ; Environmental Sciences ; Anatomy & Physiology
    ISSN: 0269-7491
    E-ISSN: 1873-6424
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  • 4
    Language: English
    In: Environmental Pollution, Feb, 2014, Vol.185, p.59(10)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.envpol.2013.10.015 Byline: Amy E. Witter, Minh H. Nguyen, Sunil Baidar, Peter B. Sak Abstract: We used land-use analysis, PAH concentrations and assemblages, and multivariate statistics to identify sediment PAH sources in a small ([approximately equal to]1303 km.sup.2) urbanizing watershed located in South-Central, Pennsylvania, USA. A geographic information system (GIS) was employed to quantify land-use features that may serve as PAH sources. Urban PAH concentrations were three times higher than rural levels, and were significantly and highly correlated with combined residential/commercial/industrial land use. Principal components analysis (PCA) was used to group sediments with similar PAH assemblages, and correlation analysis compared PAH sediment assemblages to common PAH sources. The strongest correlations were observed between rural sediments (n = 7) and coke-oven emissions sources (r = 0.69-0.78, n = 5), and between urban sediments (n = 22) and coal-tar-based sealcoat dust (r = 0.94, n = 47) suggesting that coal-tar-based sealcoat is an important urban PAH source in this watershed linked to residential and commercial/industrial land use. Author Affiliation: (a) Department of Chemistry, Dickinson College, PO Box 1773, Carlisle, PA 17013, USA (b) Department of Earth Sciences, Dickinson College, PO Box 1773, Carlisle, PA 17013, USA Article History: Received 4 July 2013; Revised 8 October 2013; Accepted 11 October 2013
    Keywords: River Sediments -- Analysis
    ISSN: 0269-7491
    Source: Cengage Learning, Inc.
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  • 5
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(23), pp.7644-7667
    Description: Saprolite formation rates influence many important geological and environmental issues ranging from agricultural productivity to landscape evolution. Here we investigate the chemical and physical transformations that occur during weathering by studying small-scale “saprolites” in the form of weathering rinds, which form on rock in soil or saprolite and grow in thickness without physical disturbance with time. We compare detailed observations of weathered basalt clasts from a chronosequence of alluvial terraces in Costa Rica to diffusion-reaction simulations of rind formation using the fully coupled reactive transport model CrunchFlow. The four characteristic features of the weathered basalts which were specifically used as criteria for model comparisons include (1) the mineralogy of weathering products, (2) weathering rind thickness, (3) the coincidence of plagioclase and augite reaction fronts, and (4) the thickness of the zones of mineral reaction, i.e. reaction fronts. Four model scenarios were completed with varying levels of complexity and degrees of success in matching the observations. To fit the model to all four criteria, however, it was necessary to (1) treat diffusivity using a threshold in which it increased once porosity exceeded a critical value of 9%, and (2) treat mineral surface area as a fitting factor. This latter approach was presumably necessary because the mineral-water surface area of the connected (accessible) porosity in the Costa Rica samples is much less than the total porosity ( ). The model-fit surface area, here termed reacting surface area, was much smaller than the BET-measured surface area determined for powdered basaltic material. In the parent basalt, reacting surface area and diffusivity are low due to low pore connectivity, and early weathering is therefore transport controlled. However, as pore connectivity increases as a result of weathering, the reacting surface area and diffusivity also increase and weathering becomes controlled by mineral reaction kinetics. The transition point between transport and kinetic control appears to be related to a critical porosity (9%) at which pore connectivity is high enough to allow rapid transport. Based on these simulations, we argue that the rate of weathering front advance is controlled by the rate at which porosity is created in the weathering interface, and that this porosity increases because of mineral dissolution following a rate that is largely surface-reaction controlled.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 6
    Language: English
    In: Geology, Sept, 2013, Vol.41(9), p.995(4)
    Description: Integrated Ocean Drilling Program (IODP) Expedition 334 to southern Costa Rica, Central America, documented unprecedented subduction erosion in an area of active seismic slip. Widespread subduction erosion of the upper plate initiated when the Cocos Ridge, an overthickened aseismic ridge, arrived at the Middle America Trench. Subduction erosion was coeval with the rapid formation of deposition centers on the forearc of the upper plate. The completely recovered shelf sequence constrains a short (〈2 m.y.) interval of extreme subsidence (~1200 m) with a rapid pulse occurring during the first ~0.3 m.y. This event removed an estimated 1.2 x [10.sup.6] [km.sup.3] of forearc material at a rate of ~1690 [km.sup.3]/m.y./km of trench during a time of rapid (~1035 m/m.y.) shelf sediment accumulation. At this erosive margin, a sediment-starved trench persisted, in spite of abundant sediment supply, because subduction erosion led to the creation of forearc basins. Similar rapid pulses of subduction erosion may punctuate the evolution of many margins, contributing disproportionately to crustal recycling at subduction zones with implications for the evolution of continental crust and mountain belts, and recycling of continental material into the mantle. doi: 10.1130/G34355.1
    Keywords: Uplift (Geology) -- Research ; Land Subsidences -- Research ; Erosion (Geology) -- Research
    ISSN: 0091-7613
    E-ISSN: 19432682
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  • 7
    Language: English
    In: Earth and Planetary Science Letters, May, 2013, Vol.369-370, p.1(12)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.epsl.2013.04.007 Byline: Scott R. Miller, Peter B. Sak, Eric Kirby, Paul R. Bierman Abstract: The persistence of topography within ancient orogens remains one of the outstanding questions in landscape evolution. In the eastern North American Appalachians, this question is manifest in the outstanding problem of whether topographic relief is in a quasi-equilibrium state, decaying slowly over many millennia, or whether relief has increased during the late Cenozoic. Here we present quantitative geomorphic data from the nonglaciated portion of the Susquehanna River drainage basin that provide insight into these end-member models. Analysis of channel profiles draining upland catchments in the northern Valley and Ridge, Appalachian Plateau, Blue Ridge, and Piedmont provinces reveals that a large number of streams have well defined knickpoints clustered at 300-600m elevation but not systematically associated with transitions from weak to resistant substrate. Cosmogenic.sup.10Be inventories of modern stream sediment indicate that erosion rates are spatially variable, ranging from ~5-30m/Myr above knickpoints to ~50-100m/Myr below knickpoints. Overall, channel gradients, normalized for drainage area, scale linearly with catchment-averaged erosion rates. Collectively, regionally consistent spatial relationships among erosion rate, channel steepness, and knickpoints reveal an ongoing wave of transient channel adjustment to a change in relative base level. Reconstructions of relict channel profiles above knickpoints suggest that higher rates of incision are associated with ~100-150m of relative base level fall that accompanied epierogenic rock uplift rather than a change to a more erosive climate or drainage reorganization. Channel response timescales imply that the onset of relative base level change predates ~3.5Ma and may have begun as early as ~15Ma. We suggest that adjustment of the channel network was likely driven by changes in mantle dynamics along the eastern seaboard of North America during the Neogene. Author Affiliation: (a) Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, USA (b) Department of Earth Sciences, Dickinson College, Carlisle, PA 17013, USA (c) Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA (d) Institute for Earth and Environmental Sciences, University of Potsdam, Potsdam, Germany (e) Department of Geology, University of Vermont, Burlington, VT 05405, USA Article History: Received 15 November 2012; Revised 6 April 2013; Accepted 6 April 2013 Article Note: (miscellaneous) Editor: T.M. Harrison
    Keywords: Landscape Evolution -- Analysis ; Rivers -- Analysis ; Sediments (Geology) -- Analysis
    ISSN: 0012-821X
    Source: Cengage Learning, Inc.
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  • 8
    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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  • 9
    Language: English
    In: Environmental Pollution, February 2014, Vol.185, pp.59-68
    Description: We used land-use analysis, PAH concentrations and assemblages, and multivariate statistics to identify sediment PAH sources in a small (∼1303 km ) urbanizing watershed located in South-Central, Pennsylvania, USA. A geographic information system (GIS) was employed to quantify land-use features that may serve as PAH sources. Urban PAH concentrations were three times higher than rural levels, and were significantly and highly correlated with combined residential/commercial/industrial land use. Principal components analysis (PCA) was used to group sediments with similar PAH assemblages, and correlation analysis compared PAH sediment assemblages to common PAH sources. The strongest correlations were observed between rural sediments (  = 7) and coke-oven emissions sources (  = 0.69–0.78,  = 5), and between urban sediments (  = 22) and coal-tar-based sealcoat dust (  = 0.94,  = 47) suggesting that coal-tar-based sealcoat is an important urban PAH source in this watershed linked to residential and commercial/industrial land use. Coal-tar-based sealcoated pavement is a major PAH source to urban freshwater stream sediments in south-central Pennsylvania, USA.
    Keywords: Sediment ; Coal-Tar-Based Sealcoat ; Polycyclic Aromatic Hydrocarbons ; Urban Sprawl ; Streams ; Land Use ; Engineering ; Environmental Sciences ; Anatomy & Physiology
    ISSN: 0269-7491
    E-ISSN: 1873-6424
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  • 10
    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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