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  • Articles  (82)
  • Geosciences  (82)
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  • 1
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    Redox interactions of technetium with iron-bearing minerals (2011)
    Mineralogical Society of Great Britain and Ireland
    Details
    Publication Date: 2011-08-30
    Description: Iron minerals influence the environmental redox behaviour and mobility of metals including the long-lived radionuclide technetium. Technetium is highly mobile in its oxidized form pertechnetate (Tc(VII)[IMG]f1.gif" ALT="Formula" BORDER="0"〉), however, when it is reduced to Tc(IV) it immobilizes readily via precipitation or sorption. In low concentration tracer experiments, and in higher concentration XAS experiments, pertechnetate was added to samples of biogenic and abiotically synthesized Fe(II)-bearing minerals (bio-magnetite, bio-vivianite, bio-siderite and an abiotically precipitated Fe(II) gel). Each mineral scavenged different quantities of Tc(VII) from solution with essentially complete removal in Fe(II)-gel and bio-magnetite systems and with 84{+/-}4% removal onto bio-siderite and 68{+/-}5% removal onto bio-vivianite over 45 days. In select, higher concentration, Tc XAS experiments, XANES spectra showed reductive precipitation to Tc(IV) in all samples. Furthermore, EXAFS spectra for bio-siderite, bio-vivianite and Fe(II)-gel showed that Tc(IV) was present as short range ordered hydrous Tc(IV)O2-like phases in the minerals and for some systems suggested possible incorporation in an octahedral coordination environment. Low concentration reoxidation experiments with air-, and in the case of the Fe(II) gel, nitrate-oxidation of the Tc(IV)-labelled samples resulted in only partial remobilization of Tc. Upon exposure to air, the Tc bound to the Fe-minerals was resistant to oxidative remobilization with a maximum of [~]15% Tc remobilized in the bio-vivianite system after 45 days of air exposure. Nitrate mediated oxidation of Fe(II)-gel inoculated with a stable consortium of nitrate-reducing, Fe(II)-oxidizing bacteria showed only 3.8{+/-}0.4% remobilization of reduced Tc(IV), again highlighting the recalcitrance of Tc(IV) to oxidative remobilization in Fe-bearing systems. The resultant XANES spectra of the reoxidized minerals showed Tc(IV)-like spectra in the reoxidized Fe-phases. Overall, this study highlights the role that Fe-bearing biogenic mineral phases have in controlling reductive scavenging of Tc(VII) to hydrous TcO2-like phases onto a range of Fe(II)-bearing minerals. In addition, it suggests that on reoxidation of these phases, Fe-bound Tc(IV) may be octahedrally coordinated and is largely recalcitrant to reoxidation over medium-term timescales. This has implications when considering remediation approaches and in predictions of the long-term fate of Tc in the nuclear legacy.
    Print ISSN: 0026-461X
    Electronic ISSN: 1471-8022
    Topics: Geosciences
    Published by Mineralogical Society of Great Britain and Ireland
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  • 2
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    Global climatic drivers of leaf size (2017)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publication Date: 2017-09-01
    Description: Leaf size varies by over a 100,000-fold among species worldwide. Although 19th-century plant geographers noted that the wet tropics harbor plants with exceptionally large leaves, the latitudinal gradient of leaf size has not been well quantified nor the key climatic drivers convincingly identified. Here, we characterize worldwide patterns in leaf size. Large-leaved species predominate in wet, hot, sunny environments; small-leaved species typify hot, sunny environments only in arid conditions; small leaves are also found in high latitudes and elevations. By modeling the balance of leaf energy inputs and outputs, we show that daytime and nighttime leaf-to-air temperature differences are key to geographic gradients in leaf size. This knowledge can enrich "next-generation" vegetation models in which leaf temperature and water use during photosynthesis play key roles.
    Keywords: Ecology
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 3
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    Uranium uptake onto Magnox sludge minerals studied using EXAFS (2013)
    Mineralogical Society of Great Britain and Ireland
    Details
    Publication Date: 2013-01-30
    Description: Around the world large quantities of sludge wastes derived from nuclear energy production are currently kept in storage facilities. In the UK, the British government has marked sludge removal as a top priority as these facilities are nearing the end of their operational lifetimes. Therefore chemical understanding of uranium uptake in Mg-rich sludge is critical for successful remediation strategies. Previous studies have explored uranium uptake by the calcium carbonate minerals, calcite and aragonite, under conditions applicable to both natural and anthropogenically perturbed systems. However, studies of the uptake by Mg-rich minerals such as brucite [Mg(OH) 2 ], nesquehonite [MgCO 3 ·3H 2 O] and hydromagnesite [Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O], have not been previously conducted. Such experiments will improve our understanding of the mobility of uranium and other actinides in natural lithologies as well as provide key information applicable to nuclear waste repository strategies involving Mg-rich phases. Experiments with mineral powders were used to determine the partition coefficients ( K d ) and coordination of $${\mathrm{UO}}_{2}^{2+}$$ during adsorption and co-precipitation with brucite, nesquehonite and hydromagnesite. The K d values for the selected Mg-rich minerals were comparable or greater than those published for calcium carbonates. Extended X-ray absorption fine structure analysis results showed that the structure of the uranyl-triscarbonato [UO 2 (CO 3 ) 3 ] species was maintained after surface attachment and that uptake of uranyl ions took place mainly via mineral surface reactions.
    Print ISSN: 0026-461X
    Electronic ISSN: 1471-8022
    Topics: Geosciences
    Published by Mineralogical Society of Great Britain and Ireland
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  • 4
    Electronic Resource
    Electronic Resource
    Conditions of contact metamorphism, Papoose Flat Pluton, eastern California, USA: implications for cooling and strain histories (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of metamorphic geology 13 (1995), S. 0 
    Details
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Abstract Petrological study of highly strained carbonate and pelitic rocks within the contact aureole surrounding the western part of the Papoose Flat pluton yields thermal profiles (plots of metamorphic temperature versus distance) across the aureole that show temperature gradients which are relatively flat and narrow (〈100m). The gradients occur close to the contact and indicate a slight decrease in temperature from 500–550°C at the pluton/wall rock contact to 450–500°C at the outer margin of the aureole. One thermal profile across low-strain metasedimentary rocks located in the southern part of the aureole shows that thermal effects from emplacement extend no further than 600 m from the contact. Coexistence of andalusite and cordierite in pelitic rocks of the aureole constrain pressures to 〈4 kbar. Thermal modelling using an analytical solution of the conductive heat flow equation for a rectangular-shaped pluton reproduces the observed thermal maxima and profile shape. Conductive rather than convective cooling also is supported by isotopic and field evidence for limited fluid flow along the strongly deformed margin of the pluton. Simple thermal models coupled with observed high-temperature deformation features and a measured 90% attenuation of stratigraphic units in the plastically deformed western part of the pluton's aureole indicate that strain rates may have been of the order of 10-12s-1. Evidence for episodic heating, such as two distinct generations of andalusite growth in pelites from the aureole, alternatively may indicate a longer heating event and, therefore, slower strain rates. Thermal models also indicate that parts of the pluton still may have been above the solidus during deformation of the pluton margin and aureole.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1525-1314.1995.tb00247.x
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  • 5
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    Mid-crustal deformation of the Annapurna-Dhaulagiri Himalaya, central Nepal: An atypical example of channel flow during the Himalayan orogeny (2016)
    Geological Society of America (GSA)
    In: Geosphere
    Details
    Publication Date: 2016-05-29
    Description: The channel-flow model for the Greater Himalayan Sequence (GHS) of the Himalayan orogen involves a partially molten, rheologically weak, mid-crustal layer "flowing" southward relative to the upper and lower crust during late Oligocene–Miocene. Flow was driven by topographic overburden, underthrusting, and focused erosion. We present new structural and thermobarometric analyses from the GHS in the Annapurna-Dhaulagiri Himalaya, central Nepal; these data suggest that during exhumation, the GHS cooled, strengthened, and transformed from a weak "active channel" to a strong "channel plug" at greater depths than elsewhere in the Himalaya. After strengthening, continued convergence resulted in localized top-southwest (top-SW) shortening on the South Tibetan detachment system (STDS). The GHS in the Annapurna-Dhaulagiri Himalaya displays several geological features that distinguish it from other Himalayan regions. These include reduced volumes of leucogranite and migmatite, no evidence for partial melting within the sillimanite stability field, reduced structural thickness, and late-stage top-southwest shortening in the STDS. New and previously published structural and thermobarometric constraints suggest that the channel-flow model can be applied to mid-Eocene–early Miocene mid-crustal evolution of the GHS in the Annapurna-Dhaulagiri Himalaya. However, pressure-temperature-time (PTt) constraints indicate that following peak conditions, the GHS in this region did not undergo rapid isothermal exhumation and widespread sillimanite-grade decompression melting, as commonly recorded elsewhere in the Himalaya. Instead, lower-than-typical structural thickness and melt volumes suggest that the upper part of the GHS (Upper Greater Himalayan Sequence [UGHS]—the proposed channel) had a greater viscosity than in other Himalayan regions. We suggest that viscosity-limited, subdued channel flow prevented exhumation on an isothermal trajectory and forced the UGHS to exhume slowly. These findings are distinct from other regions in the Himalaya. As such, we describe the mid-crustal evolution of the GHS in the Annapurna-Dhaulagiri Himalaya as an atypical example of channel flow during the Himalayan orogeny.
    Electronic ISSN: 1553-040X
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 6
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    On balanced exploitation of marine ecosystems: results from dynamic size spectra (2012)
    Oxford University Press
    Details
    Publication Date: 2012-04-10
    Description: Law, R., Plank, M. J., and Kolding, J. 2012. On balanced exploitation of marine ecosystems: results from dynamic size spectra. – ICES Journal of Marine Science, 69: 602–614. Fisheries are often managed to protect small young fish and to harvest big old fish. This can be wasteful, leading to large parts of catches being discarded. A recent suggestion is that it could be better to distribute fishing more widely across species and body sizes, balancing it more closely to the natural productivity of different organisms. Here, we test effects of such fishing against more traditional methods using a model of a single fish species with a dynamic size spectrum together with a fixed spectrum of plankton. This has the feature that productivity is determined by the bookkeeping of biomass in the model, and decreases as fish grow larger. The results show that harvesting smaller fish (which have higher productivity) allows a greater sustainable biomass yield than harvesting larger fish (which have lower productivity); the greater spawning-stock biomass that comes from protecting large fish contributes to this. Balanced exploitation brings fishing mortality more in line with this natural variation in productivity. In addition, the resilience of the ecosystem to perturbations can be improved, and disruption to the size distribution of organisms in the ecosystem reduced. We argue that there are potentially real benefits to be gained by moving towards more balanced exploitation of marine ecosystems, unconventional though this is.
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press
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  • 7
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    Ti resetting in quartz during dynamic recrystallization: Mechanisms and significance (2014)
    Mineralogical Society of America
    Details
    Publication Date: 2014-10-02
    Description: The ubiquity of quartz in continental crust, and the involvement of SiO 2 in multiple metamorphic processes such as reactions, fluid flux, and solution-transfer processes, makes quartz an obvious choice for reconstructing prograde metamorphic conditions in various rock types. Recent studies have shown the usefulness of analyzing Ti distribution in quartz to constrain pressure-temperature-(relative) time-deformation ( P-T-t-D ) in metamorphosed tectonites. New high-precision single-crystal X-ray diffraction volume constraints on Ti-doped and chemically pure quartz provide further evidence for substitution of Ti 4+ for Si 4+ in the tetrahedral site in quartz, with resultant lattice strain on the structure. Recent applications of the Ti-in-quartz thermobarometer to dynamically recrystallized quartz have identified recrystallized subgrains that contain lower Ti concentrations ([Ti]) than their host porphyroclasts. In addition, [Ti] are lower than expected for the temperatures of recrystallization. Atomistic simulations that estimate energetic perturbations resulting from Ti incorporation into the quartz lattice indicate that significant increases in strain energy occur only at very high [Ti]; the strain-energy increase is negligible for [Ti] typical of quartz grown under mid-crustal conditions. This suggests that lattice strain rarely provides an appreciable driving force for Ti loss from quartz; instead, it appears that subgrain boundaries and dislocation arrays migrating through recrystallizing quartz crystals can promote localized re-equilibration, thermodynamically regulated by the composition of the intergranular medium (typically undersaturated in Ti). It therefore appears that analyses from dynamically recrystallized quartz cannot be meaningfully interpreted until methods are developed that can account quantitatively for the reduction of Ti resulting from crystal plastic flow.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
    Published by Mineralogical Society of America
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  • 8
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    Structural and thermal evolution of the South Tibetan Detachment shear zone in the Mt Everest region, from the 1933 sample collection of L. R. Wager (2018)
    Geological Society of London
    Details
    Publication Date: 2018
    Description: 〈p〉Microstructural and petrological data from 〉60 samples, collected by L.R. Wager in 1933, have been used alongside existing data to investigate temperature gradients and deformational style in four profiles across the South Tibetan Detachment shear zone, over a north–south distance of 35 km in the Mt Everest area, east-central Himalaya. The ductile shear zone, defined on petrographic criteria, extends for 〈i〉c.〈/i〉 900 m beneath the brittle Qomolangma Detachment (QD). New thermobarometry from the north flank of Mt Everest reveals a gradient from 440°C at the QD down to samples recording peak conditions around 650°C, 5.5 kbar. The upper limit of leucogranite sheets forms an approximately isothermal surface at 600–650°C within the developing shear zone. The recrystallized grain size of quartz shows a systematic increase down-section in four transects. Profiles of deformation temperature reveal gradients of up to 200°C km〈sup〉–1〈/sup〉 whose formation and preservation required a combination of processes: a shear zone active for a short period (≤18–15.5 Ma) at high strain rates, with a component of vertical shortening, and a contribution of latent heat from emplacement of sheeted granites. The likely horizontal displacement was 〉40 km, with up to 10 km of vertical exhumation.〈/p〉 〈p〉〈b〉Supplementary material:〈/b〉 List of Wager's examined specimens, mineral compositions used for thermobarometry and results of 〈scp〉thermocalc〈/scp〉 average 〈i〉P〈/i〉–〈i〉T〈/i〉 and 〈i〉T〈/i〉 calculations are available at 〈a href="https://doi.org/10.6084/m9.figshare.c.4157594"〉https://doi.org/10.6084/m9.figshare.c.4157594〈/a〉〈/p〉
    Print ISSN: 0375-6440
    Electronic ISSN: 2041-4927
    Topics: Geosciences
    Published by Geological Society of London
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  • 9
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    Structural and thermal evolution of the South Tibetan Detachment shear zone in the Mt Everest region, from the 1933 sample collection of L. R. Wager (2018)
    Geological Society (of London)
    Details
    Publication Date: 2018-07-14
    Description: Microstructural and petrological data from 〉60 samples, collected by L.R. Wager in 1933, have been used alongside existing data to investigate temperature gradients and deformational style in four profiles across the South Tibetan Detachment shear zone, over a north–south distance of 35 km in the Mt Everest area, east-central Himalaya. The ductile shear zone, defined on petrographic criteria, extends for c. 900 m beneath the brittle Qomolangma Detachment (QD). New thermobarometry from the north flank of Mt Everest reveals a gradient from 440°C at the QD down to samples recording peak conditions around 650°C, 5.5 kbar. The upper limit of leucogranite sheets forms an approximately isothermal surface at 600–650°C within the developing shear zone. The recrystallized grain size of quartz shows a systematic increase down-section in four transects. Profiles of deformation temperature reveal gradients of up to 200°C km –1 whose formation and preservation required a combination of processes: a shear zone active for a short period (≤18–15.5 Ma) at high strain rates, with a component of vertical shortening, and a contribution of latent heat from emplacement of sheeted granites. The likely horizontal displacement was 〉40 km, with up to 10 km of vertical exhumation. Supplementary material: List of Wager's examined specimens, mineral compositions used for thermobarometry and results of thermocalc average P – T and T calculations are available at https://doi.org/10.6084/m9.figshare.c.4157594
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
    Published by Geological Society (of London)
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  • 10
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    Europe's insufficient pollutant remediation (2017)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publication Date: 2017-04-15
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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