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41.
Serena Viti Wendy Brown Martin McCoustra Helen Fraser Nigel Mason Robert Massey 《Astronomy& Geophysics》2004,45(6):6.22-6.24
The team behind a successful project to broaden the understanding of astrochemistry – Serena Viti, Wendy Brown, Martin McCoustra, Helen Fraser, Nigel Mason and Robert Massey – recount how they went about it and what benefits resulted. 相似文献
42.
A multi‐proxy approach was used to examine the geomorphic dynamics and environmental history of an upper deltaic ?oodplain tract in the Sacramento–San Joaquin Delta, California. Three long cores were collected from the McCormack–Williamson Tract (MWT) and these cores were analyzed for bulk density, loss‐on‐ignition, ?ne (clay and silt) content, Al concentration, magnetic susceptibility, pollen, and charcoal. Radiocarbon dates obtained for the cores were converted into calendar years and an age–depth model was constructed. Long‐term vertical accretion and sedimentation rates were estimated from the age–depth model. Cross‐core relations show that coarse sediment generally accumulates more rapidly and has greater magnetic susceptibility compared to ?ne sediment. Percentage ?ne and LOI data show a strong linear relationship that indicates ?ooding is the primary mechanism for the deposition of particular organic matter on the ?oodplain and that landscape wash load has contributed a highly consistent fraction of persistent organic matter averaging 5·5 per cent to the site. Down‐core grain size pro?les show two hydrological domains in the cores, namely millennial ?ne–coarse ?uctuations superimposed on general up‐core ?ning. Coarse sediment is viewed as channel or near‐channel overbank deposits, whereas ?ne deposits are considered to be distal overbank ?ood deposits. The coarse–?ne ?uctuations are indicative of changing depositional settings as channels migrated laterally across MWT, whereas the upward ?ning trend re?ects a combination of self‐limiting overbank deposition as ?oodplain elevation increased and decreasing competence as sea‐level rise reduced ?ood‐pulse energy slopes. MWT has been cross‐cut and incised numerous times in the past, only to have the channels abandoned and subsequently ?lled by ?ne sediment. The channels around MWT attained their modern con?guration about 4000 years ago. MWT likely came under tidal in?uence at about 2500 cal BP. Wetlands have recently developed on MWT, but they are inorganic sediment dominated. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
43.
P. Ma K. Aggrey C. Tonzola C. Schnabel P. de Nicola J.T. Wasson L. Brown R. Middleton 《Geochimica et cosmochimica acta》2004,68(19):3883-3896
By using accelerator mass spectrometry we have measured the 10Be concentrations of 86 Australasian tektites. Corrected to the time of tektite production ∼0.8 My ago, the 10Be concentrations (106 atom/g) range from 59 for a layered tektite from Huai Sai, Thailand, to 280 for an australite from New South Wales, Australia. The average value is 143 ± 50. When tektites are sorted by country, their average measured 10Be concentrations increase slowly with increasing distance from Southeast Asia, the probable location of the tektite producing event, from 59 ± 9 for 6 layered tektites from Laos to 136 ± 20 for 20 splash-form tektites from Australia. The lowest 10Be concentrations for tektites fall on or within a contour centered off the shore of Vietnam, south of the Gulf of Tonkin (107°E; 17°N), but also encompassing two other locations in the area of northeastern Thailand previously proposed for the site of a single tektite-producing impact. The 10Be concentrations of layered tektites show only a weak anticorrelation (R ∼ −0.3) with the numbers of relict crystalline inclusions.Loosely consolidated, fine-grained terrestrial sediments or recently consolidated sedimentary rocks are the most likely precursor materials. Dilution of sediments with other kinds of rock raises problems in mixing and is not supported by petrographic data. Sedimentary columns that have the right range of 10Be concentrations occur off the coasts of places where sedimentation rates are high relative to those in the deep sea. A single impact into such a region, 15 to 300 m thick, could have propelled near-surface, high-10Be material farthest—to Australia—while keeping the deeper-lying, low-10Be layers closer to home. We do not rule out, however, other proposed mechanisms for tektite formation. 相似文献
44.
Evidence for Neotethys rooted within the Vardar suture zone from the Voras Massif, northernmost Greece 总被引:2,自引:0,他引:2
Three conflicting models are currently proposed for the location and tectonic setting of the Eurasian continental margin and adjacent Tethys ocean in the Balkan region during Mesozoic–Early Tertiary time. Model 1 places the Eurasian margin within the Rhodope zone relatively close to the Moesian platform. A Tethyan oceanic basin was located to the south bordering a large “Serbo-Pelagonian” microcontinent. Model 2 correlates an integral “Serbo-Pelagonian” continental unit with the Eurasian margin and locates the Tethys further southwest. Model 3 envisages the Pelagonian zone and the Serbo-Macedonian zone as conjugate continental units separated by a Tethyan ocean that was sutured in Early Tertiary time to create the Vardar zone of northern Greece and former Yugoslavia. These published alternatives are tested in this paper based on a study of the tectono-stratigraphy of a completely exposed transect located in the Voras Mountains of northernmost Greece. The outcrop extends across the Vardar zone, from the Pelagonian zone in the west to the Serbo-Macedonian zone in the east.Within the Voras Massif, six east-dipping imbricate thrust sheets are recognised. Of these, Units 1–4 correlate with the regional Pelagonian zone in the west (and related Almopias sub-zone). By contrast, Units 5–6 show a contrasting tectono-stratigraphy and correlate with the Paikon Massif and the Serbo-Macedonian zone to the east. These units form a stack of thrust sheets, with Unit 1 at the base and Unit 6 at the top. Unstacking these thrust sheets places ophiolitic units between the Pelagonian zone and the Serbo-Macedonian zone, as in Model 3. Additional implications are, first, that the Paikon Massif cannot be seen as a window of Pelagonian basement, as in Model 1, and, secondly, Jurassic andesitic volcanics of the Paikon Massif locally preserve a gneissose continental basement, ruling out a recently suggested origin as an intra-oceanic arc.We envisage that the Almopias (Vardar) ocean rifted in Triassic time, followed by seafloor spreading. The Almopias ocean was consumed beneath the Serbo-Macedonian margin in Jurassic time, generating subduction-related arc volcanism in the Paikon Massif and related units. Ophiolites were emplaced onto the Pelagonian margin in the west and covered by Late Jurassic (pre-Kimmeridgian) conglomerates. Other ophiolitic rocks formed within the Vardar zone (Ano Garefi ophiolite, Unit 4) in latest Jurassic–Early Cretaceous time and were not deformed until Early Tertiary time. The Vardar zone finally sutured in the Early Tertiary creating the present imbricate thrust structure of the Voras Mountains. 相似文献
45.
We have determined the production rate of in situ-produced 10Be in calcite through measurements of 10Be and 36Cl concentrations in the same calcite samples and of 10Be concentrations in depth profiles of flint from the same erosional surface. This yields a 10Be production rate in calcite of 37.9 ± 6.0 at/g/yr at sea level and high latitude, approximately sixfold higher than production in the coexisting flint. We propose that this higher rate of production may be due to high production cross sections for C spallation by cosmic rays with energies below 50 MeV. These results also open the possibility of dating burial events in carbonate-rich environments by differential radioactive decay of 10Be and 36Cl. 相似文献
46.
Uranyl adsorption onto montmorillonite: Evaluation of binding sites and carbonate complexation 总被引:2,自引:0,他引:2
The fate and transport of uranium in contaminated soils and sediments may be affected by adsorption onto the surface of minerals such as montmorillonite. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO22+) onto Wyoming montmorillonite. At low pH (∼4) and low ionic strength (10−3 M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange. At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites. Linear-combination fitting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites. Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH)6] octahedral sites over [Al(O,OH)6] sites. When bound to edge sites, U(VI) occurs as uranyl-carbonato ternary surface complexes in systems equilibrated with atmospheric CO2. Polymeric surface complexes were not observed under any of the conditions studied. Current surface complexation models of uranyl sorption on clay minerals may need to be reevaluated to account for the possible increased importance of cation exchange reactions at low ionic strengths, the presence of reactive octahedral iron surface sites, and the formation of uranyl-carbonato ternary surface complexes. Considering the adsorption mechanisms observed in this study, future studies of U(VI) transport in the environment should consider how uranium retardation will be affected by changes in key solution parameters, such as pH, ionic strength, exchangeable cation composition, and the presence or absence of CO2. 相似文献
47.
Fluid generation and evolution during exhumation of deeply subducted UHP continental crust: Petrogenesis of composite granite–quartz veins in the Sulu belt,China 下载免费PDF全文
S.‐J. Wang L. Wang M. Brown P. M. Piccoli T. E. Johnson P. Feng H. Deng K. Kitajima Y. Huang 《Journal of Metamorphic Geology》2017,35(6):601-629
Composite granite–quartz veins occur in retrogressed ultrahigh pressure (UHP) eclogite enclosed in gneiss at General's Hill in the central Sulu belt, eastern China. The granite in the veins has a high‐pressure (HP) mineral assemblage of dominantly quartz+phengite+allanite/epidote+garnet that yields pressures of 2.5–2.1 GPa (Si‐in‐phengite barometry) and temperatures of 850–780°C (Ti‐in‐zircon thermometry) at 2.5 GPa (~20°C lower at 2.1 GPa). Zircon overgrowths on inherited cores and new grains of zircon from both components of the composite veins crystallized at c. 221 Ma. This age overlaps the timing of HP retrograde recrystallization dated at 225–215 Ma from multiple localities in the Sulu belt, consistent with the HP conditions retrieved from the granite. The εHf(t) values of new zircon from both components of the composite veins and the Sr–Nd isotope compositions of the granite consistently lie between values for gneiss and eclogite, whereas δ18O values of new zircon are similar in the veins and the crustal rocks. These data are consistent with zircon growth from a blended fluid generated internally within the gneiss and the eclogite, without any ingress of fluid from an external source. However, at the peak metamorphic pressure, which could have reached 7 GPa, the rocks were likely fluid absent. During initial exhumation under UHP conditions, exsolution of H2O from nominally anhydrous minerals generated a grain boundary supercritical fluid in both gneiss and eclogite. As exhumation progressed, the volume of fluid increased allowing it to migrate by diffusing porous flow from grain boundaries into channels and drain from the dominant gneiss through the subordinate eclogite. This produced a blended fluid intermediate in its isotope composition between the two end‐members, as recorded by the composite veins. During exhumation from UHP (coesite) eclogite to HP (quartz) eclogite facies conditions, the supercritical fluid evolved by dissolution of the silicate mineral matrix, becoming increasingly solute‐rich, more ‘granitic’ and more viscous until it became trapped. As crystallization began by diffusive loss of H2O to the host eclogite concomitant with ongoing exhumation of the crust, the trapped supercritical fluid intersected the solvus for the granite–H2O system, allowing phase separation and formation of the composite granite–quartz veins. Subsequently, during the transition from HP eclogite to amphibolite facies conditions, minor phengite breakdown melting is recorded in both the granite and the gneiss by K‐feldspar+plagioclase+biotite aggregates located around phengite and by K‐feldspar veinlets along grain boundaries. Phase equilibria modelling of the granite indicates that this late‐stage melting records P–T conditions towards the end of the exhumation, with the subsolidus assemblage yielding 0.7–1.1 GPa at <670°C. Thus, the composite granite–quartz veins represent a rare example of a natural system recording how the fluid phase evolved during exhumation of continental crust. The successive availability of different fluid phases attending retrograde metamorphism from UHP eclogite to amphibolite facies conditions will affect the transport of trace elements through the continental crust and the role of these fluids as metasomatic agents interacting with the mantle wedge in the subduction channel. 相似文献
48.
49.
Arctic river basins are amongst the most vulnerable to climate change. However, there is currently limited knowledge of the hydrological processes that govern flow dynamics in Arctic river basins. We address this research gap using natural hydrochemical and isotopic tracers to identify water sources that contributed to runoff in river basins spanning a gradient of glacierization (0–61%) in Svalbard during summer 2010 and 2011. Spatially distinct hydrological processes operating over diurnal, weekly and seasonal timescales were characterized by river hydrochemistry and isotopic composition. Two conceptual water sources (‘meltwater’ and ‘groundwater’) were identified and used as a basis for end‐member mixing analyses to assess seasonal and year‐to‐year variability in water source dynamics. In glacier‐fed rivers, meltwater dominated flows at all sites (typically >80%) with the highest contributions observed at the beginning of each study period in early July when snow cover was most extensive. Rivers in non‐glacierized basins were sourced initially from snowmelt but became increasingly dependent on groundwater inputs (up to 100% of total flow volume) by late summer. These hydrological changes were attributed to the depletion of snowpacks and enhanced soil water storage capacity as the active layer expanded throughout each melt season. These findings provide insight into the processes that underpin water source dynamics in Arctic river systems and potential future changes in Arctic hydrology that might be expected under a changing climate. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
50.