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  • 2010-2014  (8)
  • 1935-1939
  • 2013  (8)
  • 1
    Publication Date: 2013-07-10
    Description: Centrioles are evolutionary conserved organelles that give rise to cilia and flagella as well as centrosomes. Centrioles display a characteristic ninefold symmetry imposed by the spindle assembly abnormal protein 6 (SAS-6) family. SAS-6 from Chlamydomonas reinhardtii and Danio rerio was shown to form ninefold symmetric, ring-shaped oligomers in vitro that...
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 2
    Publication Date: 2013-06-12
    Description: Bacterial toxins have evolved successful strategies for coopting host proteins to access the cytosol of host cells. Anthrax lethal factor (LF) enters the cytosol through pores in the endosomal membrane formed by anthrax protective antigen. Although in vitro models using planar lipid bilayers have shown that translocation can occur in...
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 3
    Publication Date: 2013-06-04
    Description: There is a need to better characterize discrete fractures in contaminated hard rock aquifers to determine the fate of remediation injections away from boreholes and also to evaluate hydraulic fracturing performance. A synthetic cross-borehole electrical resistivity study was conducted assuming a discrete fracture model of an existing contaminated site with known fracture locations. Four boreholes and two discrete fracture zones, assumed to be the dominant electrical and hydraulically conductive pathways, were explicitly modeled within an unstructured tetrahedral mesh. We first evaluated different regularization constraints starting with an uninformed smoothness-constrained inversion, to which a priori information was incrementally added. We found major improvements when (1) smoothness regularization constraints were relaxed (or disconnected) along boreholes and fractures, (2) a homogeneous conductivity was assumed along boreholes, and (3) borehole conductivity constraints that could be determined from a specific conductance log were applied. We also evaluated the effect of including borehole packers on fracture zone model recovery. We found that the fracture zone conductivities with the inclusion of packers were comparable to similar trials excluding the use of packers regardless of electrical potential changes. The misplacement of fracture regularization disconnects (FRDs) can easily be misinterpreted as actual fracture locations. Conductivities within these misplaced disconnects were near the starting model value, and removing smoothing between boreholes and assumed fracture locations helped in identifying incorrectly located FRDs. We found that structural constraints used after careful evaluation of a priori information are critical to improve imaging of fracture electrical conductivities, locations, and orientations.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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  • 4
    Publication Date: 2013-04-19
    Description: Alluvial riverbed elevation responds to the balance between sediment supply and transport capacity, which is largely dependent on climate and its translation into fluvial discharge. We examine these relations using U.S. Geological Survey streamflow and channel measurements in conjunction with basin characteristics for 915 reference ("least disturbed") measurement stations across the conterminous United States for the period A.D. 1950–2011. We find that (1) 68% of stations have bed elevation change ( BEC ) trends (p 〈 0.05) with median values of +0.5 cm/yr for aggradation and –0.6 cm/yr for degradation, with no obvious relation to drainage basin structure, physiography, or lithology; (2) BEC correlates with drainage basin area; (3) high-flow variability ( Q 90 / Q 50 , where Q is discharge and 90 and 50 are annual flow percentiles) translates directly into the magnitude, though not the direction, of BEC , after accounting for the scale dependence; (4) Q 90 / Q 50 declines systematically from dry to wet climates, producing disproportionately high rates of BEC in drier regions; and (5) marked increases in precipitation and streamflow occurred disproportionately at dry sites, while streamflow declined disproportionately at wet sites. Climatic shifts in streamflow have the potential to increase/decrease sediment flux and thus affect riverbed elevation by altering flood frequency. These unforeseen responses of bed elevation to climate and climate change have important implications for sediment budgets, longitudinal profiles, ecology, and river management.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 5
    Publication Date: 2013-06-29
    Description: There is an increasing need to characterize fractured rock systems and to monitor the movement of fluids in these systems. Fractured rock aquifers are increasingly exploited for water resources, and are subject to contamination from industrial activities at the Earth's surface. Deep rock repositories of hazardous waste must be carefully characterized in terms of fracture transport characteristics. More recently, there has been a surge in technologies designed to increase permeability of shale reservoirs by creating fractures to promote fluid removal. However, fractured rock systems present unique challenges for characterization and monitoring technologies. Fracturing typically generates highly heterogeneous and anisotropic systems, making the evaluation of the distribution of physical properties from sparse subsurface measurements particularly problematic. Geophysical imaging technologies are increasingly applied in an effort to overcome the limitations of sparsely located direct observations of subsurface properties. However, the application of geophysical technologies to fractured rock systems presents challenges for imaging as appropriate regularization model constraints for complex, heterogeneous systems are hard to define without additional subsurface information.
    Print ISSN: 1070-485X
    Electronic ISSN: 1938-3789
    Topics: Geosciences
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  • 6
    Publication Date: 2013-11-16
    Description: Recent studies have shown that induced polarization (IP) coupled with electrical resistivity surveys can be used for in situ lithological and hydrologic discrimination of the subsurface; yet the driving factors behind the effects of water content dynamics on IP are relatively understudied. We sought to improve the understanding of the relationship of IP on variations in water saturation degree for an undisturbed agricultural soil. Our experiment consisted of collecting IP measurements concurrently with hydraulic data during multistep outflow experiments. We determined the hydraulic properties of the undisturbed soil samples and correlated saturation degree with IP data. Due to an increase in pore fluid conductivity in the column pore water with decreasing saturation degree, we found that imaginary conductivity ('') may offer distinct advantages for determining water content over real conductivity (') measurements. Although '' exhibits a weaker dependence on saturation degree compared to ', the relative insensitivity of '' to pore fluid conductivity results in a simpler dependence on saturation change in the presence of varying salinities. As changes in pore fluid conductivity are likely to occur in the field simultaneously with water content variations, we suggest that although IP has mostly been used to discriminate lithology, time lapse IP measurements may additionally provide a robust indicator of changes in saturation degree.
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 7
    Publication Date: 2013-07-24
    Description: Northern Peatlands cover more than 350 million ha and are an important source of methane (CH4) and other biogenic gases contributing to climate change. Free phase gas (FPG) accumulation and episodic release has recently been recognized as an important mechanism for biogenic gas flux from peatlands. It is likely that gas production and groundwater flow are interconnected in peatlands: groundwater flow influences gas production by regulating geochemical conditions and nutrient supply available for methanogenesis while FPG influences groundwater flow through a reduction in peat permeability and by creating excess pore water pressures. Water samples collected from three well sites at Caribou Bog, Maine, show substantial dissolved CH4 (5–16 mg L−1) in peat waters below 2 m depth and an increase in concentrations with depth. This suggests substantial production and storage of CH4 in deep peat that may be episodically released as FPG. Two minute increment pressure transducer data reveal approximately 5 cm fluctuations in hydraulic head from both deep and shallow peat that are believed to be indicative of FPG release. FPG release persists up to 24 h during decreasing atmospheric pressure and a rising water table. Preferential flow is seen towards an area of relatively lower hydraulic head associated with the esker and pool system. Increased CH4 concentrations are also found at the depth of the esker crest suggesting that the high permeability esker is acting as a conduit for groundwater flow, driving a downward transport of labile carbon, resulting in higher rates of CH4 production.
    Print ISSN: 1812-2108
    Electronic ISSN: 1812-2116
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 8
    Publication Date: 2013-09-01
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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