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1.
Abstract— The hypothesis that the soluble fraction of the organic compounds present in carbonaceous chondrite meteorites was formed during aqueous alteration of the parent body was tested with mass‐transfer, reaction‐path calculations. In these calculations, we start with likely compositions of the original parent body and asteroidal fluids that are far from thermodynamic equilibrium, and metastable and stable equilibrium constraints are imposed as the total Gibbs free energy of the parent body environment is minimized. The results of these calculations suggest that the classes of soluble organic compounds present in carbonaceous chondrite meteorites could have formed during relatively low temperature aqueous alteration of the meteorite parent body or bodies. The main controls on the potential for synthesis and transformation of organic compounds were the oxidation state of the rock/fluid system, the bulk composition of that system, and the temperatures that were achieved during the alteration event or events. It also appears that the alteration mineral assemblages were influenced by the presence of soluble organic compounds and reaction among them. 相似文献
2.
Janel E. Grebel Joseph J. Pignatello Weihua Song William J. Cooper William A. Mitch 《Marine Chemistry》2009,115(1-2):134-144
Understanding absorbance photobleaching of marine dissolved organic matter (DOM) is important because DOM chromophores impact oceanic primary productivity by affecting the depth of the photic zone, absorb UV radiation and affect ocean color used in remote sensing. However, the fundamental mechanisms which account for this bleaching are largely unknown. Controlled laboratory studies demonstrated that the presence of seawater concentrations of chloride and bromide ions enhanced absorbance photobleaching reaction rates by ~ 40%, regardless of DOM source or the presence or absence of carbonate ions. In contrast, halide ions generally did not affect fluorescence bleaching rates. Variations in ionic strength did not alter the enhancement in absorbance photobleaching by halide ions. Accordingly, the enhancement in absorbance photobleaching was specific to halide ions, rather than a generalized salinity effect. We confirmed the formation of hydroxyl radical (HO•) in illuminated samples, and its significant scavenging in the presence of halide salts. Gamma-radiolysis experiments and associated modeling indicated that a small component (~ 12%) of the photobleaching enhancement by halides was consistent with the hypothesis that halide scavenging of HO• will form reactive halogen radicals that target electron-rich chromophores within DOM more selectively than HO•. The mechanism responsible for the major component of absorbance photobleaching rate enhancement by halides remains unresolved. 相似文献
3.
Mitch Schulte 《Aquatic Geochemistry》2010,16(4):621-637
Thermodynamic properties for aqueous alkyl sulfides have been compiled and/or estimated through established methods. These
properties are used to investigate reactions among various sulfur compounds in a variety of geological environments, ranging
from sea floor hydrothermal systems to organic-rich sludge. Using thermodynamic data and the revised Helgeson-Kirkham-Flowers
(HKF) equations of state, along with geochemical constraints imposed by the environment, it is possible to estimate the abiotic
production of this class of organic sulfur compounds. For example, in hydrothermal systems in which H2 and H2S concentrations are buffered by the pyrite–pyrrhotite–magnetite (PPM) mineral assemblage, calculated equilibrium activities
of dimethyl sulfide (DMS) are as high as 10−3 through formation reactions in which the environment contains millimolal concentrations of CO2. Higher activities are obtained when DMS formation from CO is considered and when more reducing mineral assemblages are present. 相似文献
4.
Mitch Rose 《Geoforum》2002,33(4):455-467
In the endeavour to reveal the politics behind landscape production, cultural geographers often neglect the most fundamental question of landscape, namely the question of how: how does the landscape work as both a visual and material space? How does it ‘stick' in the mind and in the world? By relying on concepts such as ideology, hegemony and naturalized discourse, cultural geographers have parried the question by assuming a structural connection between the landscape's appearance in the world and people's everyday consciousness. The goal of this paper is to provide an alternative account of the landscape's existence. I argue that the landscape comes to appear in the world as it is put to task. This means that the landscape's existence is not founded on its capacity to inscribe or normalize consciousness through its appearance in the world but on the landscape's capacity to be called forth through practice. The argument is elaborated through the work of George Bataille whose concept of the labyrinth provides the theoretical groundwork for an alternative understanding of what the landscape is as well as how it can be studied. 相似文献
5.
We synthesized existing data on chemical contaminants in Long Island Sound (LIS) from published reports and unpublished databases.
We found several cases of systematic differences between data sources, which complicated the tasks of understanding the health
of LIS and of identifying trends over time. Of the three media examined—water, sediment, and biota—sediment (especially in
western LIS) most often exhibited pollutant concentrations that were high relative to guidelines and to other estuaries. These
high sediment concentrations did not appear to be efficiently transmitted to biota. With the exception of Cd, median pollutant
levels in embayment sediments were not higher than in open-water sediments, but the highest levels found in embayments were
much higher than at open-water sites, especially for Ag and Hg. Trends over time in contaminant levels were mixed. We identify
the most problematic contaminants in LIS and recommend adding Ag to the LIS Study’s List of Contaminants of Concern. 相似文献
6.
Dating alluvial fan surfaces in Owens Valley,California, using weathering fractures in boulders 
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下载免费PDF全文 Mitch D'Arcy Duna C. Roda Boluda Alexander C. Whittaker Alfredo Carpineti 《地球表面变化过程与地形》2015,40(4):487-501
A wide range of sedimentological and geomorphological field research depends on the availability of accurate and detailed depositional age models. Although exposure dating techniques such as cosmogenic nuclide and luminescence dating are now widely available, they remain expensive and time‐consuming, and this frequently limits the density of age constraints and the resolutions of age models for many study areas. We present a simple and effective, field‐based approach for extending and correlating existing age models to un‐dated surfaces. In Owens Valley, California, we make use of detailed beryllium‐10 (10Be) chronologies reported for four different alluvial fan systems, to precisely calibrate the rate at which weathering fractures are enlarged in granitic surface boulders. We show that these fractures have widened at a time‐integrated rate of 1.05 ± 0.03 mm ka?1 for at least 140 ka at this location, and this relationship can be represented by a linear regression that makes them ideal chronometers for surface dating. Our analysis offers a new approach to refining the uncertainties of both surface erosion rate and cosmogenic age estimates at this location. Ultimately, we integrate our observations to devise a robust age calibration for clast fracture widths in Owens Valley, and we demonstrate its application by estimating the ages of 27 additional local fan surfaces. We present an updated and extended stratigraphy for eight Sierra Nevada fan systems in total, with exceptional age control. This novel approach to dating sedimentary surfaces is inexpensive and easily applied in the field, and has the potential to significantly increase the temporal and spatial density of age constraints available for a particular study area. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
7.
There has been a notable increase in the use of geographic information systems (GIS) in studies of environmental (in)justice in the last two decades. Whilst the potential of such techniques is increasingly being recognised, there remain some key research challenges facing researchers interested in wider notions of environmental justice (EJ). One avenue of research concerns the estimation of population denominator and the influence of the estimation model on the conclusions emanating from such studies. Whilst the potential of so-called dasymetric mapping techniques have been explored in other substantive areas such as crime and health geography, their use in EJ applications remains under-explored. This paper represents a preliminary attempt to redress this gap in the literature through a comparison of the deprivation profiles of residents living in the vicinity of landfill sites in Wales using both ‘traditional’ and innovative methods of population estimation that provide a more realistic representation of actual population distribution. The results as we demonstrate suggest that more emphasis needs to be placed on the methods by which population is estimated if the results of GIS-based environmental justice studies are to be more widely applied. 相似文献
8.
研究河口沙洲及其植被的演变规律对维护河势稳定、保护河口地区生态环境具有重大意义。然而在较大时空尺度上，针对河口沙洲演变与其植被变化之间的相互作用关系尚讨论不足。因此，本文通过对文献资料中九段沙区域河势演变、涉水工程、植被面积等数据的收集和总结，结合海图及遥感影像分析，归纳分析了近三十年九段沙沙体及植被的演变特征及其相关作用关系。结果显示，近三十年九段沙沙体扩张明显，特别是0m以上沙体面积增长迅速，总增长达到176.5%；而5m等深线以内沙体面积扩张相对缓慢。九段沙区域河口汊道分流比、分沙比、悬沙浓度等水文、泥沙要素变化显著。九段沙植被面积由1990年的9.1 km2扩张到了2015年的65.68 km2，其中不同植被类群的变化特征有所不同。筛选得到的与0m等深线以内沙体面积变化具有显著相关性的环境因子包括互花米草群落面积和芦苇群落面积。未筛选得到与5m 等深线以内沙体面积变化有显著相关性的环境因子。九段沙沙体的演变受到了水文、泥沙条件及河势变化的重要影响。同时，沙体的发育提供了适宜植物生长的高程及环境条件进而促进了植被面积的扩张。沙洲演变与植被演变之间的相互作用关系在不同植被和不同高程间有所差异。未来需要进行长期有效监测以系统综合分析水文过程、沙洲演变与植被变化之间的相互作用关系。 相似文献
9.
Planning information pertaining to the potential visual impacts of proposed construction developments is particularly important in the case of wind farm planning, given the high levels of concern amongst members of the public regarding the perceived negative visual impacts of wind turbines on the landscape. Previous research has highlighted the shortcomings associated with traditional visualization techniques used to assess these impacts, and also the means by which such information is then disseminated to the wider public during the consultation stages of the wind farm planning process. This research is concerned with examining the potential of Web‐based mapping and digital landscape visualization techniques for addressing some of these shortcomings. This article reports the findings of a Web‐based survey study designed to evaluate the potential of online GIS‐based approaches for improving the effectiveness and dissemination of wind farm visualizations and enhancing public participation in the wind farm planning process. Results from the survey study add to the research literature by demonstrating how innovative Web‐based approaches have real potential for augmenting existing methods of information provision and public participation in the planning process. The findings of this study are also potentially transferrable to other landscape planning scenarios. 相似文献
10.