Ice grain size and the rheology of the martian polar deposits     

Amy C. Barr  Sarah M. Milkovich 《Icarus》2008,194(2):513-518

The history and dynamics of the martian polar deposits (MPD), the largest known water reservoirs on Mars, are of great interest, but estimates of ice grain size are required before detailed modeling can be performed. We clarify the microphysical processes that may control grain size in the MPD. If the MPD are ∼2% dust by mass, the maximum ice grain size is ∼1 mm due to grain boundary pinning by silicate microparticles. Relatively dusty layers in the MPD will have smaller grain sizes. If MPD ice has a very low impurity content and has experienced a significant amount of strain, grains may reach a steady state size of ∼1.5 to 3 mm due to dynamic recrystallization, wherein a steady state grain size is maintained due to the balance of grain growth and destruction during flow. If the near-bed ice in the MPD is warmed close to its melting point and has been extensively sheared, grain sizes at its base may be between 10 and 40 mm, by analogy with warm, dirty, near-bed ice in terrestrial ice sheets.  相似文献   

The early-type galaxies NGC 1407 and NGC 1400 – I. Spatially resolved radial kinematics and surface photometry     

Max Spolaor  Duncan A. Forbes  George K. T. Hau  Robert N. Proctor  Sarah Brough 《Monthly notices of the Royal Astronomical Society》2008,385(2):667-674

This is the first paper of a series focused on investigating the star formation and evolutionary history of the two early-type galaxies NGC 1407 and NGC 1400. They are the two brightest galaxies of the NGC 1407 (or Eridanus-A) group, one of the 60 groups studied as part of the Group Evolution Multi-wavelength Study.
Here, we present new high signal-to-noise ratio long-slit spectroscopic data obtained at the ESO 3.6-m telescope and high-resolution multiband imaging data from the Hubble Space Telescope /Advanced Camera for Surveys and wide-field imaging from Subaru Suprime-Cam. We spatially resolved integrated spectra out to ∼0.6 (NGC 1407) and ∼1.3 (NGC 1400) effective radii. The radial profiles of the kinematic parameters v _rot, σ, h ₃ and h ₄ are measured. The surface brightness profiles are fitted to different galaxy light models and the colour distributions analysed. The multiband images are modelled to derive isophotal shape parameters and residual galaxy images. The parameters from the surface brightness profile fitting are used to estimate the mass of the possible central supermassive black hole in NGC 1407. The galaxies are found to be rotationally supported and to have a flat core in the surface brightness profiles. Elliptical isophotes are observed at all radii and no fine structures are detected in the residual galaxy images. From our results, we can also discard a possible interaction between NGC 1400, NGC 1407 and the group intergalactic medium. We estimate a mass of  ∼1.03 × 10⁹ M_⊙  for the supermassive black hole in NGC 1407 galaxy.  相似文献   

<Emphasis Type="Italic">Spartina alterniflora</Emphasis> Biomass Allocation and Temperature: Implications for Salt Marsh Persistence with Sea-Level Rise     

Sarah C. Crosby  Angus Angermeyer  Jennifer M. Adler  Mark D. Bertness  Linda A. Deegan  Nathaniel Sibinga  Heather M. Leslie 《Estuaries and Coasts》2017,40(1):213-223

To predict the impacts of climate change, a better understanding is needed of the foundation species that build and maintain biogenic ecosystems. Spartina alterniflora Loisel (smooth cordgrass) is the dominant salt marsh-building plant along the US Atlantic coast. It maintains salt marsh elevation relative to sea level by the accumulation of aboveground biomass, which promotes sediment deposition and belowground biomass, which accretes as peat. Peat accumulation is particularly important in elevation maintenance at high latitudes where sediment supply tends to be limited. Latitudinal variation in S. alterniflora growth was quantified in eight salt marshes from Massachusetts to South Carolina. The hypothesis that allocation to aboveground and belowground biomass is phenotypically plastic was tested with transplant experiments among a subset of salt marshes along this gradient. Reciprocal transplants revealed that northern S. alterniflora decreased allocation to belowground biomass when grown in the south. Some northern plants also died when moved south, suggesting that northern S. alterniflora may be stressed by future warming. Southern plants that were moved north showed phenotypic plasticity in biomass allocation, but no mortality. Belowground biomass also decomposed more quickly in southern marshes. Our results suggest that warming will lead northern S. alterniflora to decrease belowground allocation and that belowground biomass will decompose more quickly, thus decreasing peat accumulation. Gradual temperature increases may allow for adaptation and acclimation, but our results suggest that warming will lower the ability of salt marshes to withstand sea-level rise.  相似文献   

Midlittoral polychaete communities in the Eastern Mediterranean Sea: new information from the implementation of the Natural Geography in Shore Areas (NaGISA) protocol and comparisons at local and regional scales          下载免费PDF全文

Georgios Chatzigeorgiou  Kleoniki Keklikoglou  Sarah Faulwetter  Fabio Badalamenti  Miltiadis‐Spyridon Kitsos  Christos Arvanitidis 《Marine Ecology》2017,38(1)

The aim of this study was to attempt to fill current knowledge gaps on midlittoral Mediterranean biodiversity at local and regional scales, by using benthic polychaetes as a model taxon. Two different data sets were analysed: (i) a quantitative data set from the two Natural Geography in Shore Areas (NaGISA) study sites in Crete and (ii) a qualitative data set from multiple sites across the Mediterranean. At the local scale, the results provide evidence that (i) discrete species communities are formed in midlittoral Mediterranean habitats, which vary by geographical location and year, depending on the scale of observation; (ii) macrophyte coverage and Chl‐a are the only environmental variables associated, albeit weakly, with the above pattern; (iii) although naturally disturbed, the Cretan NaGISA sites do not seem to experience any anthropogenic stress; (iv) environmental heterogeneity and history seem to be much less important in shaping the polychaete communities than inter‐specific interactions; however, it is not possible to specify at this stage whether local or regional processes or even their interactions may shape the polychaete communities. At the regional scale, the results indicate that (i) the only factor that seems to be involved in the regional pattern is the identity of the study providing the data sets, which implies variability and bias in how research projects are carried out, from the sampling design through to data collection and analysis; (ii) the Cretan NaGISA sites may be considered as representative of the habitat in the Mediterranean, under certain conditions.  相似文献   

Formation of atoll garnets in the UHP eclogites of the Tso Morari Complex,Ladakh, Himalaya   总被引：1，自引：0，他引：1  

Mallika K Jonnalagadda  Nitin R Karmalkar  Raymond A Duraiswami  Shivani Harshe  Sarah Gain  William L Griffin 《Journal of Earth System Science》2017,126(8):107

The eclogites of the Tso Morari Complex, Ladakh, NW Himalayas preserve both garnets with spectacular atoll textures, as well as whole porphyroblastic garnets. Whole garnets are euhedral, idiomorphic and enclose inclusions of amphibole, phengite and zoisite within the cores, and omphacite and quartz/coesite towards the rims. Detailed electron microprobe analyses and back-scattered electron images show well-preserved prograde zoning in the whole garnets with an increase in Mg and decrease in Ca and Mn contents from the core to the rim. The atoll garnets commonly consist of euhedral ring over island/peninsular core containing inclusions of phengite, omphacite and rarely amphibole between the core and ring. Compositional profiles across the studied atoll grains show elemental variations with higher concentrations of Ca and Mn with low Mg at the peninsula/island cores; contrary to this low Ca, Mn and high Mg is observed at the outer rings. Temperature estimates yield higher values at the Mg-rich atoll garnet outer rings compared to the atoll cores. Atoll garnet formation was favoured by infiltration of fluid formed due to breakdown of hydrous phases, and/or the release of structurally bounded OH from nominally anhydrous minerals at the onset of exhumation. Infiltration of fluids along pre-existing fracture pathways and along mineral inclusion boundaries triggered breakdown of the original garnet cores and released elements which were subsequently incorporated into the newly-grown garnet rings. This breakdown of garnet cores and inward re-growth at the outer ring produced the atoll structure. Calibrated geo-thermobarometers and mineral equilibria reflect that the Tso Morari eclogites attain peak pressures prior to peak temperatures representing a clockwise path of evolution.  相似文献   

Collecting <Emphasis Type="Italic">Ophiocordyceps sinensis</Emphasis>: an emerging livelihood strategy in the Garhwal,Indian Himalaya     

Laura Caplins  Sarah J. Halvorson 《山地科学学报》2017,14(2):390-402

In the Garhwal of Uttarakhand, India, the Bhotiya, an ethnically and culturally distinct tribal group, were historically engaged in seasonal migration (i.e. transhumance) to take advantage of scarce mountain resources and trade relations with Tibet. This livelihood practice has all but disappeared. Households are adapting to these changing circumstances by engaging in the collection and sale of the valuable alpine medicinal fungus Ophiocordyceps sinensis, widely known as Cordyceps. The collection of this fungus has exploded, emerging as a lucrative yet high-risk livelihood strategy for many Bhotiya communities. The Bhotiyas’ historic herding and trade-based interactions and knowledge of these alpine environments where Cordyceps are found uniquely positions them to access this valuable biological resource. Elsewhere in the Himalayan region, some households are earning as much as two-thirds of their income from the collection of Cordyceps; in China Cordyceps is now listed as an endangered species due to intense over-exploitation in the Tibetan Plateau. This paper seeks to fill the void in the scientific literature on the social, ecological and economic aspects of the emerging Cordyceps trade in the Garhwal. Our study investigates the socio-spatial dimensions of Cordyceps collection in the high alpine meadows. We document how a fusion of local knowledge and practice with alpine mountain systems has served to reinvigorate the economic integrity of mountain communities at a time of rapid socio-economic change and to reimagine a new relationship between alpine resources and community well-being. The article offers suggestions to address the sustainability of both Cordyceps collection and livelihood activities which hinge on this fungus population. We find a need for (1) community-based conservation measures that are rooted in (2) secure resource access rights for local communities to continue sustainable collection and sale of Cordyceps and (3) participatory-and science-based processes for determining appropriate local collection numbers.  相似文献   

Wind-driven monthly variations in transport and the flow field in the Faroe–Shetland Channel     

Toby J. Sherwin  Sarah L. Hughes  William R. Turrell  Bogi Hansen  & Svein Østerhus 《Polar research》2008,27(1):7-22

  相似文献   

Stable inversions for complete moment tensors   总被引：5，自引：0，他引：5  

Sarah E. Minson  Douglas S. Dreger 《Geophysical Journal International》2008,174(2):585-592

The seismic moment tensors for certain types of sources, such as volcanic earthquakes and nuclear explosions are expected to contain an isotropic component. Some earlier efforts to calculate the isotropic component of these sources are flawed due to an error in the method of Jost & Herrmann. We corrected the method after Herrmann & Hutchensen and found great improvement in the recovery of non-double-couple moment tensors that include an isotropic component. Tests with synthetic data demonstrate the stability of the corrected linear inversion method, and we recalculate the moment tensor solutions reported in Dreger et al. for Long Valley caldera events and Dreger & Woods for Nevada Test Site nuclear explosions. We confirm the findings of Dreger et al. that the Long Valley volcanic sources contain large statistically significant isotropic components. The nuclear explosions have strikingly anomalous source mechanisms, which contain very large isotropic components, making it evident that these events are not tectonic in origin. This indicates that moment tensor inversions could be an important tool for nuclear monitoring.  相似文献   

Isotopic mapping of groundwater perchlorate plumes   总被引：1，自引：0，他引：1  

Sturchio NC  Hoaglund JR  Marroquin RJ  Beloso AD  Heraty LJ  Bortz SE  Patterson TL 《Ground water》2012,50(1):94-102

Analyses of stable isotope ratios of chlorine and oxygen in perchlorate can, in some cases, be used for mapping and source identification of groundwater perchlorate plumes. This is demonstrated here for large, intersecting perchlorate plumes in groundwater from a region having extensive groundwater perchlorate contamination and a large population dependent on groundwater resources. The region contains both synthetic perchlorate derived from rocket fuel manufacturing and testing activities and agricultural perchlorate derived predominantly from imported Chilean (Atacama) nitrate fertilizer, along with a likely component of indigenous natural background perchlorate from local wet and dry atmospheric deposition. Most samples within each plume reflect either a predominantly synthetic or a predominantly agricultural perchlorate source and there is apparently a minor contribution from the indigenous natural background perchlorate. The existence of isotopically distinct perchlorate plumes in this area is consistent with other lines of evidence, including groundwater levels and flow paths as well as the historical land use and areal distribution of potential perchlorate sources.  相似文献   

A Phenomenological Model for Particle Retention in Single,Saturated Fractures     

Sandrina Rodrigues  Sarah Dickson 《Ground water》2014,52(2):277-283

Fractured aquifers are some of the most poorly characterized subsurface environments despite posing one of the highest risks to the protection of potable groundwater. This research was designed to improve the understanding of the factors affecting particle transport through fractures by developing a phenomenological model based on laboratory‐scale transport data. The model presented in this research employed data from over 70 particle tracer tests conducted in single, saturated, variable‐aperture fractures that were obtained from the natural environment and fractured in the laboratory or cast from epoxy in the laboratory. The particles employed were Escherichia coli RS2‐GFP and microspheres. The tracer experiments were conducted in natural (dolomitic limestone and granite) as well as epoxy replicas of the natural fractures. The multiple linear regression analysis revealed that the most important factors influencing particle retention in fractures are the ratio of the ionic strength of solution to collector charge, the ratio of particle to collector charge, and the ratio of advective to diffusive forces as described by the Peclet number. The model was able to reasonably (R² = 0.64) predict the fraction of particles retained; however, it is evident that some factors not accounted for in the model also contributed to retention. This research presents a novel approach to understanding particle transport in fractures, and illustrates the relative importance of various factors affecting the transport mechanisms. The utility of this model lies in the increased understanding of particle transport in fractures, which is extremely useful for directing future research.  相似文献