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排序方式: 共有137条查询结果,搜索用时 15 毫秒
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Abstract— Hydrogen isotope ratios of organic compounds in carbonaceous chondrites provide critical information about their origins and evolutionary history. However, because many of these compounds are obtained by aqueous extraction, the degree of hydrogen‐deuterium (H/D) exchange that occurs during the process needs to be quantitatively evaluated. This study uses compound‐specific hydrogen isotopic analysis to quantify the H/D exchange during aqueous extraction. Three common meteoritic dicarboxylic acids (succinic, glutaric, and 2‐methyl glutaric acids) were refluxed under conditions simulating the extraction process. Changes in δD values of the dicarboxylic acids were measured following the reflux experiments. A pseudo‐first order rate law was used to model the H/D exchange rates which were then used to calculate the isotope exchange resulting from aqueous extraction. The degree of H/D exchange varies as a result of differences in molecular structure, the alkalinity of the extraction solution and presence/absence of meteorite powder. However, our model indicates that succinic, glutaric, and 2‐methyl glutaric acids with a δD of 1800%***o would experience isotope changes of 38o, 10o, and 6o, respectively during the extraction process. Therefore, the overall change in δD values of the dicarboxylic acids during the aqueous extraction process is negligible. We also demonstrate that H/D exchange occurs on the chiral α‐carbon in 2‐methyl glutaric acid. The results suggest that the racemic mixture of 2‐methyl glutaric acid in the Tagish Lake meteorite could result from post‐synthesis aqueous alteration. The approach employed in this study can also be used to quantify H/D exchange for other important meteoritic compounds such as amino acids. 相似文献
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Arif Mohaimin Sadri Satish V. Ukkusuri Seungyoon Lee Rosalee Clawson Daniel Aldrich Megan Sapp Nelson Justin Seipel Daniel Kelly 《Natural Hazards》2018,90(3):1377-1406
The factors that explain the speed of recovery after disaster remain contested. While many have argued that physical infrastructure, social capital, and disaster damage influence the arc of recovery, empirical studies that test these various factors within a unified modeling framework are few. We conducted a mail survey to collect data on household recovery in four small towns in southern Indiana that were hit by deadly tornadoes in March 2012. The recovery effort is ongoing; while many of the homes, businesses, and community facilities were rebuilt in 2013, some are still under construction. We investigate how households in these communities are recovering from damage that they experienced and the role of social capital, personal networks, and assistance from emergency responders on the overall recovery experience. We used an ordered probit modeling framework to test the combined as well as relative effects of (a) damage to physical infrastructures (houses, vehicles, etc.); (b) recovery assistance from emergency responders (FEMA) as well as friends and neighbors; (c) personal network characteristics (size, network density, proximity, length of relationship); (d) social capital (civic engagement, contact with neighbors, trust); and (e) household characteristics. Results show that while households with higher levels of damage experienced slower recovery, those with recovery assistance from neighbors, stronger personal networks, and higher levels of social capital experienced faster recovery. The insights gained in this study will enable emergency managers and disaster response personnel to implement targeted strategies in facilitating post-disaster recovery and community resilience. 相似文献
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Megan Elwood Madden Shannon Ulrich Phillip Szymcek Scott McCallum Tommy Phelps 《Marine and Petroleum Geology》2009
In order for methane to be economically produced from the seafloor, prediction and detection of massive hydrate deposits will be necessary. In many cases, hydrate samples recovered from seafloor sediments appear as veins or nodules, suggesting that there are strong geologic controls on where hydrate is likely to accumulate. Experiments have been conducted examining massive hydrate accumulation from methane gas bubbles within natural and synthetic sediments in a large volume pressure vessel through temperature and pressure data, as well as visual observations. Observations of hydrate growth suggest that accumulation of gas bubbles within void spaces and at sediment interfaces likely results in the formation of massive hydrate deposits. Methane hydrate was first observed as a thin film forming at the gas/water interface of methane bubbles trapped within sediment void spaces. As bubbles accumulated, massive hydrate growth occurred. These experiments suggest that in systems containing free methane gas, bubble pathways and accumulation points likely control the location and habit of massive hydrate deposits. 相似文献
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Megandhren Govender Adhir Maharaj Darren Lortan David Day 《Astrophysics and Space Science》2018,363(8):165
In a recent paper by Das et al. (Astrophys. Space Sci. 361:99, 2016) it was shown that the sign of the anisotropy parameter plays a pivotal role in determining the time of formation of the horizon of a collapsing radiating star. Spurred on by this observation we investigate the impact of the (an)isotropy featured in the (Das et al. in Astrophys. Space Sci. 361:99, 2016) collapsing model on the temperature profiles of the evolving system. We show that the temperature within the stellar interior is increased as the anisotropy in the pressure grows. Relaxational effects due to heat dissipation within the core further enhances the temperature at each interior point of the stellar distribution. 相似文献
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Tailings composition effects on shear strength behavior of co-mixed mine waste rock and tailings 总被引:1,自引:0,他引:1
The objective of this study was to evaluate the effect of mine tailings composition on shear behavior and shear strength of co-mixed mine waste rock and tailings (WR&T). Crushed gravel was used as a synthetic waste rock and mixed with four types of tailings: (1) fine-grained garnet, (2) coarse-grained garnet, (3) copper, and (4) soda ash. Co-mixed WR&T specimens were prepared to target mixture ratios of mass of waste rock to mass of tailings (R) such that tailings “just filled” interparticle void space of the waste rock (i.e., optimum mixture ratio, R opt). Triaxial compression tests were conducted on waste rock, tailings, and mixed waste at effective confining stresses (\(\sigma_{\text{c}}^{{\prime }}\)) ranging from 5 to 40 kPa to represent stresses anticipated in final earthen covers for waste containment facilities. Waste rock and co-mixed WR&T specimens were 150 mm in diameter by 300 mm tall, whereas tailings specimens were 38 mm in diameter by 76 mm tall. Shear strength was quantified using effective stress friction angles (?′) from undrained tests: ?′ for waste rock was 37°, ?′ for tailings ranged from 34° to 41°, and ?′ for WR&T mixtures ranged from 38° to 40°. Thus, shear strength of co-mixed WR&T was comparable to waste rock regardless of tailings composition. Shear behavior of WR&T mixtures was a function of R and tailings composition. Tailings influenced shear behavior for R < R opt and when tailings predominantly were silt. Shear behavior was influenced by waste rock for R ≥ R opt and when tailings predominantly were sand or included clay particles. 相似文献
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Quantifying sedimentation patterns of small landslide-dammed lakes in the central Oregon Coast Range
Logan R. Wetherell Lisa L. Ely Joshua J. Roering Megan K. Walsh Molly J. Burchfield Keifer E. Nace Meaghan M. Wetherell William T. Struble 《地球表面变化过程与地形》2021,46(7):1375-1392
Understanding sedimentation patterns in small coastal watersheds due to landscape perturbations is critical for connecting hillslope and fluvial processes, in addition to managing aquatic habitats for anadromous fish and other aquatic species in the Oregon Coast Range (OCR). Changes in sedimentation patterns spanning the last 250 years are preserved in two landslide-dammed lakes in small watersheds (< 10 km2) underlain by the Tyee Formation in the central OCR. Dendrochronology of drowned Douglas-fir stumps in both lakes provided precise timing of the damming and formation of the lakes, with Klickitat Lake forming in winter ad 1751/52 and Wasson Lake in winter ad 1819/20. Perturbations from wildfires, logging and road development, and previously underappreciated snow events affect sedimentation rates in the lakes to different degrees, and are identified in the sediment record using cesium-137 (137Cs), high-resolution charcoal stratigraphy, local fire records, and aerial photography. Each lake has variable sedimentation accumulation rates (0.05–4.4 cm yr−1) and mass accumulation rates (0.02–1.42 g cm−2 yr−1). Sedimentation rates remained low from the landslide-damming events until the mid-19th century, when they increased following stand-replacing wildfires. Aside from a sediment remobilization triggered by human modification of the landslide dam at Klickitat Lake around 1960, the largest peaks in mass accumulation rates in the mid-20th century at both lakes in the early 1950s precede major road construction and logging activity in the watersheds. Subsequent sedimentation rates are lower, but variable, and possible effects of logging and road development might be exacerbated by abnormal precipitation and heavy snow events. A comparison of previous studies of landslide-dammed lakes in larger watershed of the OCR are consistent with our findings of increased sedimentation in the mid-20th century, as well as higher sedimentation rates in the debris-flow dominated southern Tyee Formation than in the lower-relief northern Tyee Formation. 相似文献