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741.
Michael W. Busch Steven J. Ostro Lance A.M. Benner Jon D. Giorgini Daniel J. Scheeres Michael C. Nolan Patrick A. Taylor Walter Brisken 《Icarus》2011,212(2):649-660
We observed the near-Earth ASTEROID 2008 EV5 with the Arecibo and Goldstone planetary radars and the Very Long Baseline Array during December 2008. EV5 rotates retrograde and its overall shape is a 400 ± 50 m oblate spheroid. The most prominent surface feature is a ridge parallel to the asteroid’s equator that is broken by a concavity about 150 m in diameter. Otherwise the asteroid’s surface is notably smooth on decameter scales. EV5’s radar and optical albedos are consistent with either rocky or stony-iron composition. The equatorial ridge is similar to structure seen on the rubble-pile near-Earth asteroid (66391) 1999 KW4 and is consistent with YORP spin-up reconfiguring the asteroid in the past. We interpret the concavity as an impact crater. Shaking during the impact and later regolith redistribution may have erased smaller features, explaining the general lack of decameter-scale surface structure. 相似文献
742.
Galina P. Bulanova Michael J. Walter Chris B. Smith Simon C. Kohn Lora S. Armstrong Jon Blundy Luiz Gobbo 《Contributions to Mineralogy and Petrology》2010,160(4):489-510
We report on a suite of diamonds from the Cretaceous Collier 4 kimberlite pipe, Juina, Brazil, that are predominantly nitrogen-free
type II crystals showing complex internal growth structures. Syngenetic mineral inclusions comprise calcium- and titanium-rich
phases with perovskite stoichiometry, Ca-rich majoritic-garnet, clinopyroxene, olivine, TAPP phase, minerals with stoichiometries
of CAS and K-hollandite phases, SiO2, FeO, native iron, low-Ni sulfides, and Ca–Mg-carbonate. We divide the diamonds into three groups on the basis of the carbon
isotope compositions (δ13C) of diamond core zones. Group 1 diamonds have heavy, mantle-like δ13C (−5 to −10‰) with mineral inclusions indicating a transition zone origin from mafic protoliths. Group 2 diamonds have intermediate
δ13C (−12 to −15‰), with inclusion compositions indicating crystallization from near-primary and differentiated carbonated melts
derived from oceanic crust in the deep upper mantle or transition zone. A 206Pb/238U age of 101 ± 7 Ma on a CaTiSi-perovskite inclusion (Group 2) is close to the kimberlite emplacement time (93.1 ± 1.5 Ma).
Group 3 diamonds have extremely light δ13C (−25‰), and host inclusions have compositions akin to high-pressure–temperature phases expected to be stable in pelagic
sediments subducted to transition zone depths. Collectively, the Collier 4 diamonds and their inclusions indicate multi-stage,
polybaric growth histories in dynamically changing chemical environments. The young inclusion age, the ubiquitous chemical
and isotopic characteristics indicative of subducted materials, and the regional tectonic history, suggest a model in which
generation of sublithospheric diamonds and their inclusions, and the proto-kimberlite magmas, are related genetically, temporally
and geographically to the interaction of subducted lithosphere and a Cretaceous plume. 相似文献
743.
Deniz Bozkurt Maisa Rojas Juan Pablo Boisier Jonás Valdivieso 《Climatic change》2018,150(3-4):131-147
This study examines the projections of hydroclimatic regimes and extremes over Andean basins in central Chile (~ 30–40° S) under a low and high emission scenarios (RCP2.6 and RCP8.5, respectively). A gridded daily precipitation and temperature dataset based on observations is used to drive and validate the VIC macro-scale hydrological model in the region of interest. Historical and future simulations from 19 climate models participating in CMIP5 have been adjusted with the observational dataset and then used to make hydrological projections. By the end of the century, there is a large difference between the scenarios, with projected warming of ~ + 1.2 °C (RCP2.6), ~ +?3.5 °C (RCP8.5) and drying of ~ ? 3% (RCP2.6), ~ ? 30% (RCP8.5). Following the strong drying and warming projected in this region under the RCP8.5 scenario, the VIC model simulates decreases in annual runoff of about 40% by the end of the century. Such strong regional effect of climate change may have large implications for the water resources of this region. Even under the low emission scenario, the Andes snowpack is projected to decrease by 35–45% by mid-century. In more snowmelt-dominated areas, the projected hydrological changes under RCP8.5 go together with more loss in the snowpack (75–85%) and a temporal shift in the center timing of runoff to earlier dates (up to 5 weeks by the end of the century). The severity and frequency of extreme hydroclimatic events are also projected to increase in the future. The occurrence of extended droughts, such as the recently experienced mega-drought (2010–2015), increases from one to up to five events per 100 years under RCP8.5. Concurrently, probability density function of 3-day peak runoff indicates an increase in the frequency of flood events. The estimated return periods of 3-day peak runoff events depict more drastic changes and increase in the flood risk as higher recurrence intervals are considered by mid-century under RCP2.6 and RCP8.5, and by the end of the century under RCP8.5. 相似文献
744.
Using a One-and-a-Half Order Closure Model of the Atmospheric Boundary Layer for Surface Flux Footprint Estimation 总被引:3,自引:1,他引:3
A knowledge of the distribution of the contribution of upwind sources to measurements of vertical scalar flux densities is
important for the correct interpretation of eddy covariance data. Several approaches have been developed to estimate this
so-called footprint function. Here a new approach based on the ensemble-averaged Navier—Stokes equations is presented. Comparisons
of numerical results using this approach with results from other studies under a range of environmental conditions show that
the model predictions are robust. Moreover, the approach outlined here has the advantage of a potential wide applicability,
due to an ability to take into account the heterogeneous nature of underlying surfaces. For example, the model showed that
any variations in surface drag, such as must occur in real life heterogeneous canopies, can exert a marked influence of the
shape and extent of flux footprints. Indeed, it seems likely that under such circumstances, estimates of surface fluxes will
be weighted towards areas of highest foliage density (and therefore quite likely higher photosynthetic rates) close to the
measurement sensor.
Three-dimensional footprints during the day and night were also determined for a mixed coniferous forest in european Russia.
A marked asymmetry of the footprint in the crosswind direction was observed, this being especially pronounced for non-uniform
plant distributions involving vegetation types with different morphological and physiological properties. The model also found
that, other things being equal, the footprint peak for forest soil respiration is typically over twice the distance from the
above canopy measurement sensor compared to that for canopy photosynthesis. This result has important consequences for the
interpretation of annual ecosystem carbon balances by the eddy covariance method. 相似文献
745.
Sarah M. Hayes Scott A. White Thomas L. Thompson Raina M. Maier Jon Chorover 《Applied Geochemistry》2009
Desert mine tailings may accumulate toxic metals in the near surface centimeters because of low water through-flux rates. Along with other constraints, metal toxicity precludes natural plant colonization even over decadal time scales. Since unconsolidated particles can be subjected to transport by wind and water erosion, potentially resulting in direct human and ecosystem exposure, there is a need to know how the lability and form of metals change in the tailings weathering environment. A combination of chemical extractions, X-ray diffraction, micro-X-ray fluorescence spectroscopy, and micro-Raman spectroscopy were employed to study Pb and Zn contamination in surficial arid mine tailings from the Arizona Klondyke State Superfund Site. Initial site characterization indicated a wide range in pH (2.5–8.0) in the surficial tailings pile. Ligand-promoted (DTPA) extractions, used to assess plant-available metal pools, showed decreasing available Zn and Mn with progressive tailings acidification. Aluminum shows the inverse trend, and Pb and Fe show more complex pH dependence. Since the tailings derive from a common source and parent mineralogy, it is presumed that variations in pH and “bio-available” metal concentrations result from associated variation in particle–scale geochemistry. Four sub-samples, ranging in pH from 2.6 to 5.4, were subjected to further characterization to elucidate micro-scale controls on metal mobility. With acidification, total Pb (ranging from 5 to 13 g kg−1) was increasingly associated with Fe and S in plumbojarosite aggregates. For Zn, both total (0.4–6 g kg−1) and labile fractions decreased with decreasing pH. Zinc was found to be primarily associated with the secondary Mn phases manjiroite and chalcophanite. The results suggest that progressive tailings acidification diminishes the overall lability of the total Pb and Zn pools. 相似文献
746.