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501.
David M. Jenkins Geoffrey J. Gilleaudeau Cynthia Kawa Jaclyn M. Dibiase Maria Fokin 《Contributions to Mineralogy and Petrology》2012,164(2):229-244
Growing recognition of triple-chain silicates in nature has prompted experimental research into the conditions under which they can form and the extent of solid solution that is feasible for some key chemical substitutions. Experiments were done primarily in the range of 0.1–0.5 GPa and 200–850 °C for durations of 18–1,034 h. A wide range of bulk compositions were explored in this study that can be classified broadly into two groups: those that are Na free and involve various possible chemical substitutions into jimthompsonite (Mg10Si12O32(OH)4), and those that are Na bearing and involve chemical substitutions into the ideal end-member Na4Mg8Si12O32(OH)4. Numerous attempts to synthesize jimthompsonite or clinojimthompsonite were unsuccessful despite the type of starting material used (reagent oxides, magnesite + SiO2, talc + enstatite, or anthophyllite). Similarly, the chemical substitutions of F− for OH−, Mn2+, Ca2+, or Fe2+ for Mg2+, and 2Li+ for Mg2+ and a vacancy were unsuccessful at nucleating triple-chain silicates. Conversely, nearly pure yields of monoclinic triple-chain silicate could be made at temperatures of 440–630 °C and 0.2 GPa from the composition Na4Mg8Si12O32(OH)4, as found in previous studies, though its composition is most likely depleted in Na as evidenced by electron microprobe and FTIR analysis. Pure yields of triple-chain silicate were also obtained for the F-analog composition Na4Mg8Si12O32F4 at 550–750 °C and 0.2–0.5 GPa if a flux consisting of Na-halide salt and water in a 2:1 ratio by weight was used. In addition, limited chemical substitution could be documented for the substitutions of 2 Na+ for Na+ + H+ and of Mg2+ + vacancy for 2Na+. For the former, the Na content appears to be limited to 2.5 cations giving the ideal composition of Na2.5Mg8Si12O30.5(OH)5.5, while for the latter substitution the Na content may go as low as 1.1 cations giving the composition Na1.1Mg9.4Si12O31.9(OH)4.1 based on a fixed number of Si cations. Further investigation involving Mg for Na cation exchange may provide a pathway for the synthesis of Na-free clinojimthompsonite. Fairly extensive solid solution was also observed for triple-chain silicates made along the compositional join Na4Mg8Si12O32(OH)4–Ca2Mg8Si12O32(OH)4 where the limit of Ca substitution at 450 °C and 0.2 GPa corresponds to Na0.7Ca1.8Mg7.8Si12O31.9(OH)4.1 (with the OH content adjusted to achieve charge balance). Aside from the Na content, this composition is similar to that observed as wide-chain lamellae in host actinolite. The relative ease with which Na-rich triple chains can be made experimentally suggests that these phases might exist in nature; this study provides additional insights into the range of compositions and formation conditions at which they might occur. 相似文献
502.
ABSTRACTThe impacts of future climate change on the agricultural water supply capacities of irrigation facilities in the Geum River basin (9645.5 km2) of South Korea were investigated using an integrated modeling framework that included a water balance network model (MODSIM) and a watershed-scale hydrologic model (Soil and Water Assessment Tool, SWAT). The discharges and baseflows from upland drainage areas were estimated using SWAT, and the predicted flow was used to feed agricultural reservoirs and multipurpose dams in subwatersheds. Using a split sampling method, we calibrated the daily streamflows and dam inflows at three locations using data from 6 years, including 3 years of calibration data (2005–2007) followed by 3 years of validation data (2008–2010). In the MODSIM model, the entire basin was divided into 14 subwatersheds in which various agricultural irrigation facilities such as agricultural reservoirs, pumping stations, diversions, culverts and groundwater wells were defined as a network of hydraulic structures within each subwatershed. These hydraulic networks between subwatersheds were inter-connected to allow watershed-scale analysis and were further connected to municipal and industrial water supplies under various hydrologic conditions. Projected climate data from the HadGEM3-RA RCP 4.5 and 8.5 scenarios for the period of 2006–2099 were imported to SWAT to calculate the water yield, and the output was transferred to MODSIM in the form of time-series boundary conditions. The maximum shortage rate of agricultural water was estimated as 38.2% for the 2040s and 2080s under the RCP 4.5 scenario but was lower under the RCP 8.5 scenario (21.3% in the 2040s and 22.1% in the 2080s). Under the RCP 4.5 scenario, the projected shortage rate was higher than that during the measured baseline period (1982–2011) of 25.6% and the RCP historical period (1982–2005) of 30.1%. The future elevated drought levels are primarily attributed to the increasingly concentrated rainfall distribution throughout the year under a monsoonal climate, as projected by the IPCC climate scenarios.
EDITOR Z.W. Kundzewicz; ASSOCIATE EDITOR not assigned 相似文献
503.
Michael M. McGlue Andrew S. Cohen Geoffrey S. Ellis Andrew L. Kowler 《Basin Research》2013,25(6):638-658
Depositional models of ancient lakes in thin‐skinned retroarc foreland basins rarely benefit from appropriate Quaternary analogues. To address this, we present new stratigraphic, sedimentological and geochemical analyses of four radiocarbon‐dated sediment cores from the Pozuelos Basin (PB; northwest Argentina) that capture the evolution of this low‐accommodation Puna basin over the past ca. 43 cal kyr. Strata from the PB are interpreted as accumulations of a highly variable, underfilled lake system represented by lake‐plain/littoral, profundal, palustrine, saline lake and playa facies associations. The vertical stacking of facies is asymmetric, with transgressive and thin organic‐rich highstand deposits underlying thicker, organic‐poor regressive deposits. The major controls on depositional architecture and basin palaeogeography are tectonics and climate. Accommodation space was derived from piggyback basin‐forming flexural subsidence and Miocene‐Quaternary normal faulting associated with incorporation of the basin into the Andean hinterland. Sediment and water supply was modulated by variability in the South American summer monsoon, and perennial lake deposits correlate in time with several well‐known late Pleistocene wet periods on the Altiplano/Puna plateau. Our results shed new light on lake expansion–contraction dynamics in the PB in particular and provide a deeper understanding of Puna basin lakes in general. 相似文献
504.
Rafael Moreno‐Sanchez Geoffrey Anderson Jesus Cruz Mary Hayden 《International journal of geographical information science》2013,27(10):1135-1163
Globalization is contributing to the blurring of borders making irrelevant the distinctions between domestic and international health problems. Cross‐border and global health spatial information systems (CBHSIS) are required to address the new global health challenges. There is a need to build and document alternatives for addressing the technological, economic, and sociocultural–political challenges encountered in the creation and deployment of these systems. This paper documents the building of a prototype Web‐based multimedia GIS system for use in a public health context using Open Source Software and Open Specifications and its deployment across the US–Mexico border. These technologies offer advantages in addressing several of the challenges previously mentioned. We highlight the technological and sociocultural–political issues important in successful collaboration across borders and cultures and in the creation of interoperable CBHSIS. 相似文献
505.
Sedimentary basins represent an archive of tectonic events of the hinterland source regions. By determining the variation in sediment lagtime over time, events can be distinguished which may no longer be available as the source has been eroded. In regions characterized by rapid exhumation this is most often the case but the erosion products form a record of these events. Detrital zircon fission‐track ages from sediments of the Siwalik basin, Pakistan, originally presented by Cerveny et al. (New Perspectives in Basin Analysis, Springer‐Verlag, New York, 1988, p. 43), have been reinvestigated and reinterpreted using a revised methodological approach. Detrital age populations were determined from different stratigraphic levels and were correlated through time in order to assess the change in lag time over the stratigraphic section. This information was combined with the many new ages from the hinterland to further interpret events in the source region. The new investigation suggests that steady‐state evolution has not always existed. An overall trend of exhumation increasing by 0.1 mm Myr?1 (from 0.9 to 2.65 mm yr?1) from 18 Ma to the present is evident with a major exception of a net pulse between 11.7 and 10.9 Ma associated with an increase in sedimentation increasingly rich in hornblende. Earlier studies suggested that at this time the source of the sediments was the presently outcropping Kohistan Arc. We are able to demonstrate that this cannot be so but was rather the rapidly exhuming Nanga‐Parbat Haramosh syntaxis (> 2 mm yr?1) coevally with transpressional displacement along the Main Karakorum Thrust, whereby the overlying Kohistan Arc sequences were removed. Furthermore, comparison of our detrital thermochronological data set with another one from the same basin and one from another foreland basin to the east, in NW India suggest that the Himalayan orogenesis was probably not synchronous for the late Early–Middle Miocene. Overall, regions that undergoes today's rapid uplift may be useless to reconstruct earlier phases of exhumation as the levels that may have yielded such info were eroded and deposited into the adjacent basin(s). Such scenario is reproducible in most orogens as in the Himalaya in NW Pakistan stressing the high potential of detrital thermochronological studies to trace hinterland dynamics. Terra Nova, 18, 248–256, 2006 相似文献
506.
C. G. Tinney Chris McCarthy Hugh R. A. Jones R. Paul Butler Brad D. Carter Geoffrey W. Marcy Alan J. Penny 《Monthly notices of the Royal Astronomical Society》2002,332(3):759-763
We present the results of ultraviolet echelle spectroscopy of a sample of 59 F, G, K and M stars from the Anglo-Australian Planet Search target list. Ca ii activity indices, which are essential in the interpretation of planet detection claims, have been determined for these stars and placed on the Mount Wilson R 'HK system. 相似文献
507.
Geoffrey C. Bower 《Astrophysics and Space Science》2003,288(1-2):69-76
We describe recent results for Sgr A*, M81* and other low luminosity active galactic nuclei. We have conducted linear and circular polarimetry over a frequency range of 1.4 to 230 GHz and detected a variety of phenomena. The polarization properties of the studied sources are substantially different from higher powered AGN. In the case of Sgr A*, we are able to eliminate ADAFs and Bondi-Hoyle flows as possible models based on mm λ polarimetry. Our success with Sgr A* demonstrates that we can learn about the nature of accretion and outflow in these sources with unprecedented detail. We may also develop probes of general relativity in the strong-field limit. 相似文献
508.
509.
510.
David Kirkman † David Tytler † Nao Suzuki † Carl Melis Susan Hollywood Kory James Geoffrey So Dan Lubin † Tridivesh Jena Michael L. Norman Pascal Paschos 《Monthly notices of the Royal Astronomical Society》2005,360(4):1373-1380
We use high-quality echelle spectra of 24 quasi-stellar objects to provide a calibrated measurement of the total amount of Lyα forest absorption (DA) over the redshift range 2.2 < z < 3.2 . Our measurement of DA excludes absorption from metal lines or the Lyα lines of Lyman-limit systems and damped Lyα systems. We use artificial spectra with realistic flux calibration errors to show that we are able to place continuum levels that are accurate to better than 1 per cent. When we combine our results with our previous results between 1.6 < z < 2.2 , we find that the redshift evolution of DA is well described over 1.6 < z < 3.2 as A (1 + z )γ , where A = 0.0062 and γ= 2.75 . We detect no significant deviations from a smooth power-law evolution over the redshift range studied. We find less H i absorption than expected at z = 3 , implying that the ultraviolet background is ∼40 per cent higher than expected. Our data appears to be consistent with an H i ionization rate of Γ∼ 1.4 × 10−12 s−1 . 相似文献