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191.
N. Gehrels C. J. Crannell D. J. Forrest R. P. Lin L. E. Orwig R. Starr 《Solar physics》1988,118(1-2):233-268
Basic principles of operation and characteristics of scintillation and semi-conductor detectors used for solar hard X-ray and gamma-ray spectrometers are presented. Scintillation materials such as NaI offer high stopping power for incident gamma rays, modest energy resolution, and relatively simple operation. They are, to date, the most often used detector in solar gamma-ray spectroscopy. The scintillator BGO has higher stopping power than NaI, but poorer energy resolution. The primary advantage of semi-conductor materials such as Ge is their high-energy resolution. Monte-Carlo simulations of the response of NaI and Ge detectors to model solar flare inputs show the benefit of high resolution for studying spectral lines. No semi-conductor material besides Ge is currently available with adequate combined size and purity to make general-use hard X-ray and gamma-ray detectors for solar studies. 相似文献
192.
193.
Melissa J. Nelson Horton E. Newsom David S. Draper 《Geochimica et cosmochimica acta》2005,69(10):2701-2711
The martian soil is a fine-grained regolith that is chemically basaltic in character with evidence for both gains and losses of volatile and mobile elements compared to martian basalt compositions. These chemical fractionations provide clues to geochemical processes on the surface of Mars. Geochemical processes contributing to the soil proposed in the past include the chemical and mechanical breakdown of rocks under surface conditions, the addition of volcanic aerosols containing S and Cl compounds, and the alteration of basaltic glass to palagonite. Our studies of terrestrial analogs suggest that hydrothermal alteration processes involving impact craters and volcanism could also contribute to the major element trends observed in martian soil. Data from Viking, Pathfinder, and the current MER missions consistently show that relative to basaltic martian meteorite compositions, the major element compositions of the soils are (1) depleted in the fluid-mobile element calcium, (2) generally similar or somewhat enriched in iron oxide and magnesium but MgO depleted compared to Gusev rocks, (3) locally variable in potassium, (4) possibly poorer in aluminum, and (6) very enriched in chlorine and sulfur. The major element trends, aside from the Cl and S enrichment, could be explained by the formation or addition of palagonite according to McSween and Keil (2000), but the missing CaO remains a problem. The chlorine and sulfur are probably derived from other processes such as volcanic aerosols and hydrothermal fluids. McSween and Keil (2000) also argued that hydrothermal alteration of basalts produce alteration trends that are inconsistent with the Mars soil, but this study concludes otherwise. We have used quantitative mass balance mixing models to investigate possible models involving mixtures of basaltic compositions with different types of alteration materials, including palagonite. We show that the Mars soil composition can be matched with a combination of unweathered basaltic martian meteorites with basaltic FeO-rich, CaO-poor alteration products. Palagonite is a possible, but not a necessary component of successful model mixtures. The hydrothermal alteration materials that form successful model mixtures are formed in low temperature, low water/rock ratio environments, and they can reproduce the required geochemical trends because they are poorer in CaO but not in FeO compared to their respective protoliths. These results argue that material altered by hydrothermal processes could be a plausible component of the soil, and that removal of CaO from the soil into some undiscovered reservoir after its formation is not required. The current soil on Mars, therefore, did not have to undergo an episode of in situ aqueous alteration but could represent a sink for materials that experienced aqueous processes in a different setting before erosion to form the soil. The soil can also represent a sink for mobile elements (e.g., S, Cl, and Br) derived from other sources such as volcanic aerosols and hydrothermal fluids. 相似文献
194.
Duane G. Horton 《Contributions to Mineralogy and Petrology》1985,91(2):171-179
Ordered illite/smectite is the most abundant and widespread product of argillic alteration associated with the Oligocene, epithermal, Ag-Pb-Zn-Cu-Au Amethyst vein system, southwest Colorado. Hydrothermal illite/smectites exhibit all Reichweite from 1 to 3 and span the composition range from about 25% to about 3% illite.— The composition and Reichweite of illite/smectite vary smoothly with distance from the Amethyst vein. With increasing distance from the vein, percent illite and the Reichweite decrease. When composition-temperature relationships of illite/smectite from modern geothermal systems are applied to the fossil Amethyst system, isotherms describing the thermal regime at the time of argillization can be estimated. Temperatures near 240° C appear to have existed near the Amethyst vein; these temperatures agree with homogenization temperatures obtained from fluid inclusions in vein minerals. The most distal illite/smectites, sampled about 260 m from the vein, indicate temperatures near 110° C. Estimated thermal gradients are on the order of 0.4 to 1° C per meter. — Although illite/smectite composition and structure vary systematically with distance from the Amethyst vein, there are no systematic trends associated with the numerous, smaller veins and veinlets in the hanging wall of the system. This indicates that temperatures of both wallrock and the fluids in all but the major Amethyst vein were nearly the same during clay formation. Apparently, the hydrothermal system had reached a fairly steady, mature, thermal state at the time of argillization. 相似文献
195.
Kate Forrest Brian Tarroja Felicia Chiang Amir AghaKouchak Scott Samuelsen 《Climatic change》2018,147(3-4):395-409
Climate change is already affecting species and their distributions. Distributional range changes have occurred and are projected to intensify for many widespread plants and animals, creating associated risks to many ecosystems. Here, we estimate the climate change-related risks to the species in globally significant biodiversity conservation areas over a range of climate scenarios, assessing their value as climate refugia. In particular, we quantify the aggregated benefit of countries’ emission reduction pledges (Intended Nationally Determined Contributions and Nationally Determined Contributions under the Paris Agreement), and also of further constraining global warming to 2 °C above pre-industrial levels, against an unmitigated scenario of 4.5 °C warming. We also quantify the contribution that can be made by using smart spatial conservation planning to facilitate some levels of autonomous (i.e. natural) adaptation to climate change by dispersal. We find that without mitigation, on average 33% of each conservation area can act as climate refugium (or 18% if species are unable to disperse), whereas if warming is constrained to 2 °C, the average area of climate refuges doubles to 67% of each conservation area (or, without dispersal, more than doubles to 56% of each area). If the country pledges are fulfilled, an intermediate estimate of 47–52% (or 31–38%, without dispersal) is obtained. We conclude that the Nationally Determined Contributions alone have important but limited benefits for biodiversity conservation, with larger benefits accruing if warming is constrained to 2 °C. Greater benefits would result if warming was constrained to well below 2 °C as set out in the Paris Agreement. 相似文献
196.
The role of organizations in migration has received less attention than warranted; individual choice has typically been emphasized. As an in‐depth illustration, we consider refugee resettlement in the United States, post–World War II, wherein intermediary organizations play(ed) a major role. Central to this system are voluntary agencies (VOLAGs) and community organizations, but secondary migration also is critical. Attention is given to all refugees between 2000 and 2010, and in greater detail to Somalis. The latter provides deeper understanding through state refugee coordinators and case studies of Columbus, Ohio, Minneapolis, Minnesota, and Lewiston, Maine. Aside from process, it is evident that the geography of the foreign‐born settlement has been altered. While refugee resettlement and subsequent migration is the example, we broaden that to argue that migration studies have neglected the derived nature of movement via intermediary organizations; directed migrations and/or similar interventions have played a significant, if not dominant, role in population redistribution; and organization‐led migration should be considered in terms of general aspects, not simply as discrete case studies. 相似文献
197.
A depositional model for offshore deposits of the lower Blue Gate Member,Mancos Shale,Uinta Basin,Utah, USA
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Lauren P. Birgenheier Brendan Horton Andrew D. McCauley Cari L. Johnson Angela Kennedy 《Sedimentology》2017,64(5):1402-1438
Depositional models that use heterogeneity in mud‐dominated successions to distinguish and diagnose environments within the offshore realm are still in their infancy, despite significant recent advances in understanding the complex and dynamic processes of mud deposition. Six cored intervals of the main body of the Mancos Shale, the lower Blue Gate Member, Uinta Basin, were examined sedimentologically, stratigraphically and geochemically in order to evaluate facies heterogeneity and depositional mechanisms. Unique sedimentological and geochemical features are used to identify three offshore environments of deposition: the prodelta, the mudbelt and the sediment‐starved shelf. Prodelta deposits consist of interlaminated siltstone and sandstone and exhibit variable and stressed trace fossil assemblages, and indicators of high sedimentation rates. The prodelta was dominated by river‐fed hyperpycnal flow. Mudbelt deposits consist of interlaminated siltstone and sandstone and are characterized by higher bioturbation indices and more diverse trace fossil assemblages. Ripples, scours, truncations and normally graded laminations are abundant in prodelta and mudbelt deposits indicating dynamic current conditions. Mudbelt sediment dispersal was achieved by both combined flow above storm wave base and current‐enhanced and wave‐enhanced sediment gravity flows below storm wave base. Sediment‐starved shelf deposits are dominantly siltstone to claystone with the highest calcite and organic content. Bioturbation is limited to absent. Sediment‐starved shelf deposits were the result of a combination of shelfal currents and hypopycnal settling of sediment. Despite representing the smallest volume, sediment‐starved shelf deposits are the most prospective for shale hydrocarbon resource development, due to elevated organic and carbonate content. Sediment‐starved shelf deposits are found in either retrogradational to aggradational parasequence sets or early distal aggradational to progradational parasequence sets, bounding the maximum flooding surface. An improved framework classification of offshore mudstone depositional processes based on diagnostic sedimentary criteria advances our predictive ability in complex and dynamic mud‐dominated environments and informs resource prospectivity. 相似文献
198.
Alice A. Horton Claus Svendsen Richard J. Williams David J. Spurgeon Elma Lahive 《Marine pollution bulletin》2017,114(1):218-226
Sewage effluent input and population were chosen as predictors of microplastic presence in sediments at four sites in the River Thames basin (UK). Large microplastic particles (1 mm–4 mm) were extracted using a stepwise approach to include visual extraction, flotation and identification using Raman spectroscopy. Microplastics were found at all four sites. One site had significantly higher numbers of microplastics than other sites, average 66 particles 100 g? 1, 91% of which were fragments. This site was downstream of a storm drain outfall receiving urban runoff; many of the fragments at this site were determined to be derived of thermoplastic road-surface marking paints. At the remaining three sites, fibres were the dominant particle type. The most common polymers identified included polypropylene, polyester and polyarylsulphone. This study describes two major new findings: presence of microplastic particles in a UK freshwater system and identification of road marking paints as a source of microplastics.
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This study is the first to quantify microplastics of any size in river sediments in the UK and links their presence to terrestrial sources including sewage and road marking paints. 相似文献199.
In order to reconstruct former sea level we have developed a foraminifera-based transfer function using three models based on a modern dataset of 59 samples and 23 species obtained from four Basque marshes in Northern Spain. The relationship between observed and foraminifera-predicted elevation illustrated the strong performance of the transfer function (r2jack ranges from 0.74 to 0.81). These results indicated that precise reconstructions of former sea levels are possible (error ranges from 0.11 to 0.19 m). The transfer function was used to calibrate the foraminiferal assemblages collected from a 50 cm salt marsh core. We placed the foraminifera-based reconstructions into a temporal framework using 137Cs, Pb concentrations, and 210Pb-derived sediment accumulation rates. The resulting relative sea-level curve is in good agreement with regional tide-gauge data. Both instrumental data and microfossil records suggest a rate of relative sea-level rise of approximately 2 mm yr− 1 for the 20th century. 相似文献