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Most (～90%) of the estimated original volume of outgassed water on Mars cannot be satisfactorily accounted for by exospheric escape or storage in the atmosphere, as frost, or in the permanent north polar ice cap. The balance may be stored as ground ice in the Martian cryosphere, a zone of permanently frozen ground that is protected from the atmosphere by a debris cover. Ground ice can exist throughout the entire cryosphere, but it need not fill it. If the ground ice does fill the cryosphere, then excess water can exist in a confined aquifer. The theoretical distribution of ground ice can be tested by identification of forms on the Martian surface that may be related to the presence of subsurface ice. The observed features that are most likely to reflect ground ice are thermokarst-like pits and debris flows. Landforms with ambivalent origins include polygonally patterned ground, lobate ejecta blankets, craters with central pits, and curvilinear features. The most persuasive morphologic evidence for ground ice is thermokarst pits and debris flows; the thermokarst pits are primarily located in the volcanic regions of Tharsis and Elysium. The association of ice-related features with these volcanic areas suggests that these forms are not directly latitude dependent. Activation by orbital variations could produce periodic, multiple episodes of melting that are dependent upon latitude. The presence of ice-related features in both hemispheres and the equatorial region of Mars indicates that ground ice may be—or have been—present over the entire planet, as predicted by the cryosphere model. 相似文献
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In 1994, 1995, and 1996, seagrasses in 46 of the 89 coastal embayments and portions of seven open-water near-shore areas in
Massachusetts were mapped with a combination of aerial photography, digital imagery, and ground truth verification. In the
open-water areas, 9,477.31 ha of seagrass were identified, slightly more than twice the 4,846.2 ha detected in the 46 coastal
embayments. A subset of the 46 embayments, including all regions of the state were remapped in 2000, 2001, and 2002 and again
in 2006 and 2007. We detected a wide range of changes from increases as high as 29% y−1 in Boston Harbor to declines as large as −33% y−1 in Salem Harbor. One embayment, Waquoit Bay, lost all of its seagrass during the mapping period. For the 12-year change analysis
representing all geographic regions of the state, only three embayments exhibited increases in seagrass coverage while 30
of the original 46 embayments showed some indication of decline. For the decadal period, rates of decline in the individual
embayments ranged from −0.06% y−1 to as high as −14.81% y−1. The median rate of decline by region ranged from −2.21% y−1 to −3.51% y−1 and was slightly less than the recently reported global rate of decline for seagrasses (−3.7% y−1). Accounting for the gains in three of the embayments, 755.16 ha (20.6%) of seagrass area originally detected was lost during
the mapping interval. The results affirm that previously reported losses in a few embayments were symptomatic of more widespread
seagrass declines in Massachusetts. State and Federal programs designed to improve environmental quality for conservation
and restoration of seagrasses in Massachusetts should continue to be a priority for coastal managers. 相似文献
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Since archaeological sites are often located in or near fluvial environments, a thorough understanding of corresponding formation processes is a critical component of archaeological research. Within this context, we address the following question: how does flake morphology condition transport potential? Specifically, do flakes behave as the idealized spheres often used to characterize geological materials and if so, how might this process modify a debitage assemblage? To address this important issue, we present the results of a fluvial transport experiment designed to evaluate the relationships between flake size, shape, weight, curvature, and transport potential. We begin with an overview of fluvial mechanics and previous archaeological fluvial transport studies and use these studies to design an experiment intended to fill gaps in our knowledge of the relationships between flake morphology and transport potential. Our experimental results suggest that all things being equal, flake weight is the most important variable in both settling velocity and transport distance, although flake shape conditions transport as well. Finally, our study demonstrates the potential structure of fluvially transported assemblages as lag deposits and suggests that fluvially transported assemblages may retain key behavioral information. 相似文献
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Modeling decadal timescale interactions between surface water and ground water in the central Everglades, Florida, USA 总被引：1，自引：0，他引：1
Surface-water and ground-water flow are coupled in the central Everglades, although the remoteness of this system has hindered many previous attempts to quantify interactions between surface water and ground water. We modeled flow through a 43,000 ha basin in the central Everglades called Water Conservation Area 2A. The purpose of the model was to quantify recharge and discharge in the basin's vast interior areas. The presence and distribution of tritium in ground water was the principal constraint on the modeling, based on measurements in 25 research wells ranging in depth from 2 to 37 m. In addition to average characteristics of surface-water flow, the model parameters included depth of the layer of ‘interactive’ ground water that is actively exchanged with surface water, average residence time of interactive ground water, and the associated recharge and discharge fluxes across the wetland ground surface. Results indicated that only a relatively thin (8 m) layer of the 60 m deep surfical aquifer actively exchanges surface water and ground water on a decadal timescale. The calculated storage depth of interactive ground water was 3.1 m after adjustment for the porosity of peat and sandy limestone. Modeling of the tritium data yielded an average residence time of 90 years in interactive ground water, with associated recharge and discharge fluxes equal to 0.01 cm d−1. 3H/3He isotopic ratio measurements (which correct for effects of vertical mixing in the aquifer with deeper, tritium-dead water) were available from several wells, and these indicated an average residence time of 25 years, suggesting that residence time was overestimated using tritium measurements alone. Indeed, both residence time and storage depth would be expected to be overestimated due to vertical mixing. The estimate of recharge and discharge (0.01 cm d−1) that resulted from tritium modeling therefore is still considered reliable, because the ratio of residence time and storage depth (used to calculated recharge and discharge) is much less sensitive to vertical mixing compared with residence time alone. We conclude that a small but potentially significant component of flow through the Everglades is recharged to the aquifer and stored there for years to decades before discharged back to surface water. Long-term storage of water and solutes in the ground-water system beneath the wetlands has implications for restoration of Everglades water quality. 相似文献
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Development of structural modeling techniques for evaluating HDPE plastic net pens used in marine aquaculture 总被引：3，自引：0，他引：3
Finite-element modeling (FEM) techniques are developed to determine the structural capabilities of net pen flotation structures made of high-density polyethylene (HDPE). The modeling approach uses shell elements and localized failure criteria to predict critical loading conditions. Finite element modeling simulations were performed using values for the modulus of elasticity for weathered HDPE determined from a series of tensile tests. Poisson's ratio and yield stress used with the approach were obtained from standard manufacturers values. To investigate the method, a series of experiments were performed in the laboratory by testing circular sections of HDPE pipe to localized failure (“kinking”). The same test was replicated with the FEM using localized failure techniques. Laboratory and model results were within 16%. A FEM was then built to represent the complex geometry of a net pen flotation structure deployed at an operational fish farm located in Eastport, ME, USA. Simulations were performed using attachment line tension values measured at the site. The goal was to assess flotation pipe stress levels for typical operational conditions. Simulations were also performed to investigate the maximum capabilities of the net pen structure with different attachment line configurations. Quantifying the operational limits will become more important as these systems are considered for more exposed, energetic environments. 相似文献
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The uncertainty in a transducer/tensiometer system was assessed with temperature and pressure calibrations. A reference transducer/tensiometer pair was used to factor out temperature related deviations from two monitoring pairs. The reference pair removed most of the deviations, resulting in a high estimate of precision. In contrast to earlier reports of high accuracy, these estimates of accuracy were considerably reduced by a time correlated residual pattern. The calibrations suggested that the electronic components may be responsible for these residual errors and illustrated the need for experimentation which isolates the error among groups of components. The complexity of transducer/tensiometer networks, and the differing response of each component to thermal loading, demonstrated the necessity of using a reference system, which when properly designed can yield reliable pressure readings for soil water. 相似文献
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