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491.
Josep M. TRIGO‐RODRÍGUEZ Jordi LLORCA Alan E. RUBIN Jeffrey N. GROSSMAN Derek W. G. SEARS Mateo NARANJO Stacy BRETZIUS Mar TAPIA Marino H. GUARÍN SEPÚLVEDA 《Meteoritics & planetary science》2009,44(2):211-220
Abstract— The fall of the Cali meteorite took place on 6 July 2007 at 16 h 32 ± 1 min local time (21 h 32 ± 1 min UTC). A daylight fireball was witnessed by hundreds of people in the Cauca Valley in Colombia from which 10 meteorite samples with a total mass of 478 g were recovered near 3°24.3′N, 76°30.6′W. The fireball trajectory and radiant have been reconstructed with moderate accuracy. From the computed radiant and from considering various plausible velocities, we obtained a range of orbital solutions that suggest that the Cali progenitor meteoroid probably originated in the main asteroid belt. Based on petrography, mineral chemistry, magnetic susceptibility, thermoluminescence, and bulk chemistry, the Cali meteorite is classified as an H/L4 ordinary chondrite breccia. 相似文献
492.
Neal S. Gupta Hong Yang Qin Leng Derek E.G. Briggs George D. Cody Roger E. Summons 《Organic Geochemistry》2009,40(7):802-809
Analysis of modern Metasequoia leaves revealed the presence of the structural polyester cutin, guaiacyl lignin units and polysaccharides. Analysis of environmentally decayed Metasequoia leaves revealed that guaiacyl lignin units and cellulose were degraded more than vinyl phenol (the last being the primary pyrolysis product of cutin and plant cuticles) suggesting that cutin is more stable than lignin and cellulose during degradation, contrary to some previous studies. This observation is supported by electron microscopy showing changes in the cellular structure and cuticle of modern, decayed and fossil Metasequoia leaves. Metasequoia fossils from the Eocene of Republic (Washington State) showed a significant aliphatic component, but biopolymeric lignin and polysaccharides were not detected. Fossils from the Eocene of Axel Heiberg revealed the presence of lignin and an aliphatic polymer up to C29 with cellulose, and fossils from the Miocene Clarkia deposit (Idaho) revealed lignin and an aliphatic polymer up to C27 without any polysaccharides. Modern Metasequoia needles heated experimentally in confined conditions generated a macromolecular composition with an aliphatic polymer up to C32 and additional phenolic compounds similar to those present in the fossils. Experimental heating of cutin is known to generate an aliphatic polymer with carbon chain length units <C20. Thus, the n-alkyl component with chain length units >C20 in the heated Metasequoia needles is a product of incorporation of longer chain plant waxes, indicated by the odd/even predominance of the >C27 n-alkanes. The resistant nature of cutin compared to lignin and polysaccharides explains the presence of an n-alkyl component (<C20) in fossil leaves even when polysaccharides are absent and lignin has decayed; cutin and its diagenetically altered products contribute significantly to the presence of aliphatic components in terrestrially derived sedimentary organic matter. 相似文献
493.
Vance T. Holliday Bruce B. Huckell Robert H. Weber Marcus J. Hamilton William T. Reitze James H. Mayer 《Geoarchaeology》2009,24(3):348-370
The Mockingbird Gap site is one of the largest Clovis sites in the western United States, yet it remains poorly known after it was tested in 1966–1968. Surface collecting and mapping of the site revealed a dense accumulation of Clovis lithic debris stretching along Chupadera Draw, which drains into the Jornada del Muerto basin. We conducted archaeological testing and geoarchaeological coring to assess the stratigraphic integrity of the site and gain clues to the paleoenvironmental conditions during the Clovis occupation. The 1966–1968 excavations were in stratified Holocene eolian sand and thus that assemblage was from a disturbed content. An intact Clovis occupation was found elsewhere in the site, embedded in the upper few centimeters of a well‐developed buried Bt horizon formed in eolian sand, representing the regional Clovis landscape. Coring in Chupadera Draw revealed ∼11 m of fill spanning the past ∼11,000 14C years. The stratified deposits provide evidence of flowing and standing water on the floor of the draw during Clovis times, a likely inducement to settlement. © 2009 Wiley Periodicals, Inc. 相似文献
494.
Derek D. Stretch James W. Rottman S.Karan Venayagamoorthy Keiko K. Nomura Chris R. Rehmann 《Dynamics of Atmospheres and Oceans》2010
Mixing efficiency in stratified flows is a measure of the proportion of turbulent kinetic energy that goes into increasing the potential energy of the fluid by irreversible mixing. In this research direct numerical simulations (DNS) and rapid distortion theory (RDT) calculations of transient turbulent mixing events are carried out in order to study this aspect of mixing. In particular, DNS and RDT of decaying, homogeneous, stably-stratified turbulence are used to determine the mixing efficiency as a function of the initial turbulence Richardson number Rit0=(NL0/u0)2, where N is the buoyancy frequency and L0 and u0 are initial length and velocity scales of the turbulence. The results show that the mixing efficiency increases with increasing Rit0 for small Rit0, but for larger Rit0 the mixing efficiency becomes approximately constant. These results are compared with data from towed grid experiments. There is qualitative agreement between the DNS results and the available experimental data, but significant quantitative discrepancies. The grid turbulence experiments suggest a maximum mixing efficiency (at large Rit0) of about 6%, while the DNS and RDT results give about 30%. We consider two possible reasons for this discrepancy: Prandtl number effects and non-matching initial conditions. We conclude that the main source of the disagreement probably is due to inaccuracy in determining the initial turbulence energy input in the case of the grid turbulence experiments. 相似文献
495.
The eurypterids of the Silurian Bertie Group of North America are some of the most striking fossils known, with their dark organic cuticle outlined against a background of light coloured dolomite. Eurypterus is the most abundant genus but the pterygotids with their long grasping claws have fired our imagination as giant extinct predators. Large complete specimens are rarely found—the specimens are dominated by moulted exoskeletons that often disarticulate—but a giant that emerged from the rock more than 50 years ago is being prepared to go on display. 相似文献
496.
Rivers as municipal infrastructure: Demand for environmental services in informal settlements along an Indonesian river 总被引:1,自引:0,他引:1
In many developing world cities, where municipal infrastructure lags urban growth, lower-income communities may compensate by relying on local waterways to meet basic needs for water, sanitation, and recreational space. Access to these environmental services is possible because residents settle in floodplains, but thus entails elevated exposure to several water-related hazards, especially flooding. We examine this complex relationship in the neighborhoods of Bukit Duri and Kampung Melayu on the Ciliwung River in Jakarta, Indonesia. Based on a spatially referenced household survey, we analyze and map the patterns of use of six environmental services provided by the river: direct sanitary use, recreation, harvesting plants, groundwater use, solid waste disposal, and sewage disposal. Using spatial interpolation and regression methods, we identify the most probable areas where services are being used and analyze possible influences on this behavior. We find that proximity to the river significantly influences households’ behavior toward the river, as do infrastructure-related variables and neighborhing households’ behavior, while household demographic factors appear less significant. These results indicate that many households rely on multiple environmental services, and that residents most reliant on these services are also at greater risk of water-related hazards, service disruption (e.g., a decline in water quality), and potentially, eviction. This pattern of floodplain development is prevalent in many low-income countries, and a better understanding of how informal settlements rely on environmental services can be used to assess their vulnerabilities and inform more sustainable courses of development. 相似文献
497.
The St Lucia estuarine lake in South Africa forms part of a World Heritage Site and is an important local source of biodiversity. Like many estuarine systems worldwide, St Lucia has experienced significant anthropogenic impacts over the past century. Abstractions have decreased fresh water inflows from the lake catchments by about 20%. Furthermore the Mfolozi river, which previously shared a common inlet with St Lucia and contributed additional fresh water during droughts, was diverted from the system in 1952 because of its high silt loads. The separated St Lucia mouth was subsequently kept artificially open until the onset of a dry period in 2002 when the mouth was left to close naturally. These changes and the current drought have placed the system under severe stress with unprecedented hypersaline conditions coupled with desiccation of large portions of the lake. Long-term simulations of the water and salt balance were used to estimate the occurrence and persistence of water levels and salinities for different management scenarios. The risks of desiccation and hyper-salinity were assessed for each case. The results show that the configuration of the Mfolozi/St Lucia inlets plays a key role in the physicochemical environment of the system. Without the Mfolozi link desiccation (of about 50% of the lake area) would occur for 32% of the time for an average duration of 15 months. Artificially maintaining an open mouth would decrease the chance of desiccation but salinities would exceed 65 about 17% of the time. Restoring the Mfolozi link would reduce the occurrence of both desiccation and hypersaline conditions and a mostly open mouth state would occur naturally. Integrating these modeled scenarios with observed biological responses due to changes in salinity and water depth suggests that large long-term changes in the biological structure can be expected in the different management scenarios. 相似文献
498.
Uranium mill tailings were anaerobically incubated in the presence of H2 with Alteromonas putrefaciens, a bacterium known to couple the oxidation of H2 and organic compounds to the reduction of Fe(III) oxides. There was a direct correlation between the extent of Fe(III) reduction and the accumulation of dissolved226Ra. In sterile tailings in which Fe(III) was not reduced, there was negligible leaching of226Ra. The behavior of Ba was similar to that of Ra in inoculated and sterile systems. These results demonstrate that under anaerobic conditions, microbial reduction of Fe(III) may result in the release of dissolved226Ra from uranium mill tailings. 相似文献
499.
Episodic terrace cutting cannot be a complex response to the steady isostatic rebound forecast by an elastic flexure model because three terraces match epiphreatic passage levels in nearby caves. There is also some doubt about how fluvial incision rates should be extrapolated to catchment-wide rates. At the local scale of the Lachlan River highlands, active tectonic compression may be more important than flexure, which is more important at a regional scale. 相似文献
500.
Single grain argon laser probe dating of phengites from the blueschist to greenschist transition on Sifnos (Cyclades,Greece) 总被引:4,自引:1,他引:4
Jan R. Wijbrans Manfred Schliestedt Derek York 《Contributions to Mineralogy and Petrology》1990,104(5):582-593
Measurements of 40Ar/39Ar age spectra on single white mica crystals of Tertiary age is a new means of obtaining detailed geochronological information of Alpine blueschists. Internal consistency of the data set, as well as excellent agreement with previous conventional K–Ar results, demonstrates that this new technique can be used with confidence to obtain information on the thermal evolution of young metamorphic belts.As mineral grains can be dated individually with this technique, problems related to multiple generations of mica occurring side in the rock can be addressed, potentially yielding much information which cannot be obtained easily by other dating techniques. Ages decrease from 41.7±0.3 Ma in the fresh blueschists and eclogites in northern Sifnos to ca 30 Ma in the more severely overprinted greenschists of central Sifnos. It is argued that this decreasing trend appears gradual, rather than stepwise. This gradual character of the age trend is taken to indicate that it may be caused by differential uplift and cooling, rather than by tectonic juxtaposition of different rock units. In addition, the rocks of central Sifnos contain mica with a plateau age of 18.9±0.3 Ma. This latter age is younger than previous estimates for the age of greenschist overprinting, and more in line with the age of high temperature metamorphism and granite emplacement elsewhere in the Cyclades. 相似文献