Mechanisms of ringwoodite formation in shocked meteorites: Evidence from L5 chondrite Dhofar 1970          下载免费PDF全文

Erin L. Walton  Sabrina McCarthy 《Meteoritics & planetary science》2017,52(4):762-776

The formation of the high‐pressure compositional equivalents of olivine and pyroxene has been well‐documented within and surrounding shock‐induced veins in chondritic meteorites, formed by crystallization from a liquid‐ or solid‐state phase transformation. Typically polycrystalline ringwoodite grains have a narrow range of compositions that overlap with those of their olivine precursors, whereas the formation of iron‐enriched ringwoodite has been documented from only a handful of meteorites. Here, we report backscattered electron images, quantitative wavelength‐dispersive spectrometry (WDS) analyses, qualitative WDS elemental X‐ray maps, and micro‐Raman spectra that reveal the presence of Fe‐rich ringwoodite (Fa_44‐63) as fine‐grained (500 nm), polycrystalline rims on olivine (Fa_24‐25) wall rock and as clasts engulfed by shock melt in a previously unstudied L5 chondrite, Dhofar 1970. Crystallization of majorite + magnesiowüstite in the vein interior and metastable mineral assemblages within 35 μm of the vein margin attest to rapid crystallization of a superheated shock melt (>2300 K) from 20─25 GPa to ambient pressure and temperature. The texture and composition of bright polycrystalline ringwoodite rims (Fa_44‐63; MnO 0.01─0.08 wt%) surrounding dark polycrystalline olivine (Fa_8‐14; MnO 0.56─0.65 wt%) implies a solid‐state transformation mechanism in which Fe was preferentially partitioned to ringwoodite. The spatial association between ringwoodite and shock melt suggests that the rapidly fluctuating thermal regimes experienced by chondritic minerals in contact with shock melt are necessary to both drive phase transformation but also to prevent back‐transformation.  相似文献   

The influence of vegetation cover on evapotranspiration atop waste rock piles,Elk Valley,British Columbia     

Erin M. Nicholls  Gordon B. Drewitt  Stephanie Fraser  Sean K. Carey 《水文研究》2019,33(20):2594-2606

Surface mining in the Elk Valley, British Columbia, involves removing vegetation, soil, and rock to access underlying metallurgical coal. Subsequent waste rock is placed into adjacent valleys, frequently burying headwater streams. Due to their coarse texture, waste rock piles increase infiltration and percolation, increasing solute transport and concentration of geochemicals in downstream surface waters. Previous research suggests that weathering solutes are transport limited, and it is hypothesized that revegetation will enhance evapotranspiration (ET) and reduce percolation through the waste rock, potentially reducing loading. This study examined the surface‐atmosphere water and energy exchanges using the eddy covariance technique for three waste rock surfaces with different levels of reclamation: (a) an ~25‐year‐old mixed coniferous forest, (b) a grass site, and (c) bare waste rock. Measurements were taken from May to October in 2013 and 2014. Soil moisture and matric suction were measured to 1‐m depth. Sap flow at the forested site was measured to partition transpiration from total ET. In all years, ET rates were greatest at the forested site, followed by the grass cover and lowest at the bare waste rock site. Growing season ET rates at the forest were 56% higher than grass in 2013 and 35% higher in 2014. At the vegetated sites, climate was the main driver of ET, with high radiation, and warm and dry conditions enhancing fluxes. Maximum ET at these sites corresponded with peak growing season, with vegetation increasing both transpiration and rainfall interception. At the bare rock site, ET was weakly related to atmospheric conditions, and ET rates briefly increased during periods following rainfall when near‐surface soil moisture was enhanced. Transpiration comprised 29% of overall ET at the forest site from late July to early October. Results suggest that vegetation establishment can be incorporated into mine reclamation plans to enhance ET rates and limit percolation, potentially reducing downstream geochemical loads.  相似文献   

Mineralogical characterization of Baptistina Asteroid Family: Implications for K/T impactor source     

Vishnu Reddy  Jorge M. Carvano  Daniela Lazzaro  Tatiana A. Michtchenko  Michael J. Gaffey  Michael S. Kelley  Thais Mothé-Diniz  Alvaro Alvarez-Candal  Nicholas A. Moskovitz  Edward A. Cloutis  Erin L. Ryan 《Icarus》2011,216(1):184-197

Bottke et al. [Bottke, W.F., Vokrouhlicky, D., Nesvorný, D., 2007. Nature 449, 48–53] linked the catastrophic formation of Baptistina Asteroid Family (BAF) to the K/T impact event. This linkage was based on dynamical and compositional evidence, which suggested the impactor had a composition similar to CM2 carbonaceous chondrites. However, our recent study [Reddy, V., Emery, J.P., Gaffey, M.J., Bottke, W.F., Cramer, A., Kelley, M.S., 2009. Meteorit. Planet. Sci. 44, 1917–1927] suggests that the composition of (298) Baptistina is similar to LL-type ordinary chondrites rather than CM2 carbonaceous chondrites. This rules out any possibility of it being related to the source of the K/T impactor, if the impactor was of CM-type composition. Mineralogical study of asteroids in the vicinity of BAF has revealed a plethora of compositional types suggesting a complex formation and evolution environment. A detailed compositional analysis of 16 asteroids suggests several distinct surface assemblages including ordinary chondrites (Gaffey SIV subtype), primitive achondrites (Gaffey SIII subtype), basaltic achondrites (Gaffey SVII subtype and V-type), and a carbonaceous chondrite. Based on our mineralogical analysis we conclude that (298) Baptistina is similar to ordinary chondrites (LL-type) based on olivine and pyroxene mineralogy and moderate albedo. S-type and V-type in and around the vicinity of BAF we characterized show mineralogical affinity to (8) Flora and (4) Vesta and could be part of their families. Smaller BAF asteroids with lower SNR spectra showing only a ‘single’ band are compositionally similar to (298) Baptistina and L/LL chondrites. It is unclear at this point why the silicate absorption bands in spectra of asteroids with formal family definition seem suppressed relative to background population, despite having similar mineralogy.  相似文献   

A comparison of intertidal species richness and composition between Central California and Oahu,Hawaii     

Chela J. Zabin  Eric M. Danner  Erin P. Baumgartner  David Spafford  Kathy Ann Miller  John S. Pearse 《Marine Ecology》2013,34(2):131-156

The intertidal zone of tropical islands is particularly poorly known. In contrast, temperate locations such as California's Monterey Bay are fairly well studied. However, even in these locations, studies have tended to focus on a few species or locations. Here we present the results of the first broadscale surveys of invertebrate, fish and algal species richness from a tropical island, Oahu, Hawaii, and a temperate mainland coast, Central California. Data were gathered through surveys of 10 sites in the early 1970s and again in the mid‐1990s in San Mateo and Santa Cruz counties, California, and of nine sites in 2001–2005 on Oahu. Surveys were conducted in a similar manner allowing for a comparison between Oahu and Central California and, for California, a comparison between time periods 24 years apart. We report a previously undocumented richness of intertidal species in both locations: 516 for Oahu and 801 for Central California. Surprisingly, when differences in search efforts are controlled, overall (alpha) diversity appears to be similar between locations, although site level (beta) diversity is much higher in California. Species richness in California generally increased along a wave exposure gradient and distance from an urban area. Much higher numbers of both invasive and endemic species were found on Oahu. In California, more invertebrate species were found in the 1990s, likely due to an improvement in taxonomic resources since the 1970s, and species composition was different in the two surveys due to the high incidence of rare species. Although some southern species increased in number between the two time periods and some northern species decreased, we detected little evidence of change favoring southern or northern species. These results are in line with recent findings that water temperatures in the Monterey Bay have been in a cooling trend since the 1980s, in contrast to many locations elsewhere in the world.  相似文献   

Ti diffusion in quartz inclusions: implications for metamorphic time scales     

Frank S. Spear  Kyle T. Ashley  Laura E. Webb  Jay B. Thomas 《Contributions to Mineralogy and Petrology》2012,164(6):977-986

Quartz inclusions in garnet from samples collected from the staurolite zone in central New England are zoned in cathodoluminescence (CL). The CL intensity is interpreted to be a proxy for Ti concentration and the zoning attributed to Ti diffusion into the quartz grains driven by Ti exchange between quartz and enclosing garnet as a function of changing temperature. The CL zoning has been interpreted using a numerical diffusion model to constrain the time scales over which the diffusion has occurred. Temperature–time histories are sensitive to the presumed peak temperature but not to other model parameters. The total time of the metamorphic heating and cooling cycle from around 450?°C to the peak temperature (550–600?°C) back to 450?°C is surprisingly short and encompasses only 0.2–2 million years for peak temperatures of 600–550?°C. The metamorphism was accompanied by large-scale nappe and dome formation, and it is suggested that this occurred as a consequence of in-sequence thrusting resulting in a mid-crustal ductile duplex structure.  相似文献   

Modeling River Connectivity Using the Barrier Assessment Tool and Available Data on Registered Dams in the Guadalupe–San Antonio River System,Texas     

Erin D. Dascher 《The Professional geographer》2013,65(4):616-630

The rate of dam building has declined in the United States, but dam building is still considered an integral part of water supply management, particularly in arid and semiarid regions. Understanding how dams affect river connectivity is a key component of river basin management. This study analyzed the spatial and temporal patterns of fragmentation in the Guadalupe–San Antonio River System (GSARS) and how these patterns were related to dam size, age, and purpose. Using a shapefile of registered dams and the National Hydrography Dataset High Resolution (NHD-HR) as input data for the Barrier Assessment Tool (BAT), six successive connectivity models were created. The average length of functional river networks (FRNs) decreased over time but the number of FRNs over 100?km stayed relatively stable. Fragmentation in the GSARS was concentrated around urban centers, and the majority of fragmentation occurred gradually as dams separated small FRNs along tributaries and headwaters. This gradual decline in longitudinal connectivity was punctuated by sporadically large separations of river networks by particular dams based on their location and time of commissioning. The methodology presented here provides a way to analyze both the extent and the temporal evolution of fragmentation in a river system. Key Words: Barrier Assessment Tool (BAT), fragmentation, functional river networks, river connectivity.  相似文献   

Enantioselectivity of polychlorinated biphenyl atropisomers in sediment and biota from the Turtle/Brunswick River estuary, Georgia, USA     

Ross MS  Pulster EL  Ejsmont MB  Chow EA  Hessel CM  Maruya KA  Wong CS 《Marine pollution bulletin》2011,63(5-12):548-555

To investigate the potential for enantioselective transformation and accumulation, the enantiomer distributions of seven polychlorinated biphenyl (PCB) atropisomers were measured in the sediment and biota from a sub-tropical estuary heavily contaminated with Aroclor 1268, a technical mixture of highly chlorinated PCB congeners. Enantiomer fractions (EFs) of PCBs 91, 95, 136, 149, 174, 176, and 183 in marsh sediment, invertebrate, forage and predatory fish species, and bottlenose dolphins were determined. Non-racemic EFs greater than 0.75 were found in sediments for PCBs 136 and 174, likely the result of microbial dechlorination. Although enantiomer fractions in grass shrimp (Palaemonetes spp.) mirrored those of sediment, fish species had EFs that differed significantly from sediment or grass shrimp. Similarly, bottlenose dolphins were also found to contain non-racemic quantities of PCBs 91, 136, 174, 176, and 183. Non-racemic EFs in these biota were likely a result of both uptake of non-racemic proportions of PCBs from the diet and enantioselective biotransformation.  相似文献   

Future makers or future takers? A scenario analysis of climate change and the Great Barrier Reef     

Erin Bohensky  James R.A. ButlerRobert Costanza  Iris BohnetAurélie Delisle  Katharina FabriciusMargaret Gooch  Ida KubiszewskiGeorge Lukacs  Petina PertEric Wolanski 《Global Environmental Change》2011,21(3):876-893

The extent to which nations and regions can actively shape the future or must passively respond to global forces is a topic of relevance to current discourses on climate change. In Australia, climate change has been identified as the greatest threat to the ecological resilience of the Great Barrier Reef, but is exacerbated by regional and local pressures. We undertook a scenario analysis to explore how two key uncertainties may influence these threats and their impact on the Great Barrier Reef and adjacent catchments in 2100: whether (1) global development and (2) Australian development is defined and pursued primarily in terms of economic growth or broader concepts of human well-being and environmental sustainability, and in turn, how climate change is managed and mitigated. We compared the implications of four scenarios for marine and terrestrial ecosystem services and human well-being. The results suggest that while regional actions can partially offset global inaction on climate change until about mid-century, there are probable threshold levels for marine ecosystems, beyond which the Great Barrier Reef will become a fundamentally different system by 2100 if climate change is not curtailed. Management that can respond to pressures at both global and regional scales will be needed to maintain the full range of ecosystem services. Modest improvements in human well-being appear possible even while ecosystem services decline, but only where regional management is strong. The future of the region depends largely on whether national and regional decision-makers choose to be active future ‘makers’ or passive future ‘takers’ in responding to global drivers of change. We conclude by discussing potential avenues for using these scenarios further with the Great Barrier Reef region's stakeholders.  相似文献   

Water Education for Sustainability: Criteria and Recommendations     

Kelli L. Larson  Erin N. Redman 《社会与自然资源》2014,27(11):1213-1222

This article applies behavioral theories, knowledge-to-action frameworks, and educational pedagogy to the case study of water education programs in metropolitan Phoenix, AZ. First, we describe three criteria for sustainable behavior change through education. Next, we demonstrate how water education programs in Phoenix have employed inadequate approaches to reduce resource consumption relative to the criteria. Finally, we offer recommendations to enhance the effectiveness of educational initiatives directed toward transformative change for sustainability. Central insights involve incorporating diverse knowledge domains, using interactive educational techniques, and tailoring programs to specific actions and relevant targeted populations.  相似文献   

Variability patterns of the annual frequency and timing of low streamflow days across the United States and their linkage to regional and large‐scale climate     

Maryam Pournasiri Poshtiri  Indrani Pal  Upmanu Lall  Philippe Naveau  Erin Towler 《水文研究》2019,33(11):1569-1578

Low‐flow events can cause significant impacts to river ecosystems and water‐use sectors; as such, it is important to understand their variability and drivers. In this study, we characterise the variability and timing of annual total frequency of low‐streamflow days across a range of headwater streams within the continental United States. To quantify this, we use a metric that counts the annual number of low‐flow days below a given threshold, defined as the cumulative dry days occurrence (CDO). First, we identify three large clusters of stream gauge locations using a Partitioning Around Medoids (PAM) clustering algorithm. In terms of timing, results reveal that for most clusters, the majority of low‐streamflow days occur from the middle of summer until early fall, although several locations in Central and Western United States also experience low‐flow days in cold seasons. Further, we aim to identify the regional climate and larger scale drivers for these low‐streamflow days. Regionally, we find that precipitation deficits largely associate with low‐streamflow days in the Western United States, whereas within the Central and Eastern U.S. clusters, high temperature indicators are also linked to low‐streamflow days. In terms of larger scale, we examine sea surface temperature (SST) anomalies, finding that extreme dry years exhibit a high degree of co‐occurrence with different patterns of warmer SST anomalies across the Pacific and Northern Atlantic Oceans. The linkages identified with regional climate and SSTs offer promise towards regional prediction of changing conditions of low‐streamflow events.  相似文献