27,895 research outputs found
Discussion
This is an edited transcript of remarks made by John C. Burton in response to Thomas S. Currier\u27s paper Mandating Disclosure in Municipal Securities Issues: Proposed New York Legislation. The remarks focus on the importance of the federal government in mandating disclosure
The effects of inorganic nitrogen and phosphorus enrichment on herbaceous species growth of the Kimages Creek wetland (VA)
Dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) infiltrate waterways through fertilizer application, urban stormwater runoff, and sewer infrastructure leaks. As surrounding waterbodies experience increased DIN and DIP inputs, wetlands can experience corresponding nutrient enrichment. Vegetation uses DIN and DIP for structural growth, color, and seed production. Changes in DIN and DIP availability can influence species distribution due to differences in photosynthetic rates, root morphology and structure, and tissue type.
DIP and DIN inputs are projected to increase 15-30% and 30-60% in the next fifty yearsÂč. It is of interest to examine plant growth characteristics within this nutrient enrichment projection as well as nutrient enrichment from a potential 100-year projection to analyze future species composition responses within a freshwater tidal marsh
Quasi-static granular flow of ice mélange
We use Landsat 8 imagery to generate ice mĂ©lange velocity fields at Greenlandâs three most
productive outlet glaciers: Jakobshavn IsbrĂŠ, Helheim Glacier, and Kangerdlugssuaq Glacier. Winter velocity
fields are generally steady and highly uniform. Summer velocity fields, on the other hand, tend to be much
more variable and can be uniform, compressional, or extensional. We rarely observe compressional flow
at Jakobshavn IsbrĂŠ or extensional flow at Helheim Glacier, while both are observed at Kangerdlugssuaq
Glacier. Transverse velocity profiles from all three locations are suggestive of viscoplastic flow, in which
deformation occurs primarily in shear zones along the fjord walls. We analyze the transverse profiles in
the context of quasi-static flow using continuum rheologies for granular materials and find that the force
per unit width that ice mélange exerts on glacier termini increases exponentially with the ice mélange
length-to-width ratio and the effective coefficient of friction. Our estimates of ice mélange resistance are
consistent with other independent estimates and suggest that ice mélange may be capable of inhibiting
iceberg calving events, especially during winter. Moreover, our results provide geophysical-scale support for
constitutive relationships for granular materials and suggest a potential avenue for modeling ice mélange
dynamics with continuum models.From acknowledgments:
Funding for this project was provided by the U.S. National Science Foundation (DMR-1506446 and DMR-1506307).
Digital elevation models were provided by the Polar Geospatial Center under the U.S. National Science Foundation (OPP-1043681, OPP-1559691, and OPP-1542736)Ye
Quasi-static granular flow of ice mélange
We use Landsat 8 imagery to generate ice mĂ©lange velocity fields at Greenlandâs three most
productive outlet glaciers: Jakobshavn IsbrĂŠ, Helheim Glacier, and Kangerdlugssuaq Glacier. Winter velocity
fields are generally steady and highly uniform. Summer velocity fields, on the other hand, tend to be much
more variable and can be uniform, compressional, or extensional. We rarely observe compressional flow
at Jakobshavn IsbrĂŠ or extensional flow at Helheim Glacier, while both are observed at Kangerdlugssuaq
Glacier. Transverse velocity profiles from all three locations are suggestive of viscoplastic flow, in which
deformation occurs primarily in shear zones along the fjord walls. We analyze the transverse profiles in
the context of quasi-static flow using continuum rheologies for granular materials and find that the force
per unit width that ice mélange exerts on glacier termini increases exponentially with the ice mélange
length-to-width ratio and the effective coefficient of friction. Our estimates of ice mélange resistance are
consistent with other independent estimates and suggest that ice mélange may be capable of inhibiting
iceberg calving events, especially during winter. Moreover, our results provide geophysical-scale support for
constitutive relationships for granular materials and suggest a potential avenue for modeling ice mélange
dynamics with continuum models.From acknowledgments:
Funding for this project was provided by the U.S. National Science Foundation (DMR-1506446 and DMR-1506307).
Digital elevation models were provided by the Polar Geospatial Center under the U.S. National Science Foundation (OPP-1043681, OPP-1559691, and OPP-1542736)Ye
Quantifying flow and stress in ice mĂ©lange, the worldâs largest granular material.
Tidewater glacier fjords are often filled with a collection of calved icebergs, brash ice, and sea ice. For glaciers with high calving rates, this âm Ìelangeâ of ice can be jam-packed, so that the flow of ice fragments is mostly determined by granular interactions. In the jammed state, ice m Ìelange has been hypothesized to influence iceberg calving and capsize, dispersion and attenuation of ocean waves, injection of freshwater into fjords, and fjord circulation. However, detailed measurements of ice m Ìelange are lacking due to difficulties in instrumenting remote, ice-choked fjords. Here we characterize the flow and associated stress in icem Ìelange, using a combination of terrestrial radar data, laboratory experiments, and numerical simulations. We find that, during periods of terminus quiescence, ice m Ìelange experiences laminar flow over timescales of hours to days. The uniform flow fields are bounded by shear margins along fjord walls where force chains between granular icebergs terminate. In addition, the average force per unit width that is transmitted to the glacier terminus, which can exceed 107N/m, increases exponentially with them Ìelange length-to-width ratio. These âbuttressingâ forces are sufficiently high to inhibit the initiation of large-scale calving events, supporting the notion that ice m Ìelange can be viewed as a weak granular ice shelf that transmits stresses from fjord walls back to glacier termini.Ye
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Widespread evidence for heterogeneous accretion of the terrestrial planets and planetisimals
The abundance and relative proportion of highly siderophile elements (HSEs) in Earthâs mantle deviate from those predicted by low-pressure equilibrium partitioning between metal and silicate during formation of the core. For many elements, high-pressure equilibration in a deep molten silicate layer (or âmagma oceanâ) may account for this discrepancy [1], but some highly siderophile element abundances demand the late addition, a âlate veneerâ, of extraterrestrial material (i.e. heterogeneous accretion) after core formation was complete [2]. Siderophile elements in smaller asteroidal bodies will not be affected by high-pressure metal-silicate equilibration and so, with highly efficient core formation [3] and if a âlate veneerâ is absent, significant differences in the proportions of HSEs can be anticipated. Here we present new HSE abundance and 187Os/188Os isotope data for basaltic meteorites, the HEDs (howardites, eucrites and diogenites thought to sample the asteroid 4 Vesta), anomalous eucrites (considered to be from distinct Vesta-like parent bodies) angrites and aubrites (from unidentified parent bodies) and SNCs (thought to be from Mars). Our data, taken with those for lunar rocks [4], demonstrate that these igneous meteorites all formed from mantle sources that possessed chondritic (i.e. primitive solar system) elemental and isotope compositions, indicating that late accretion is not unique to Earth, but is a common feature of differentiated planets and asteroidal bodies. Variations in the total HSE abundance suggest that the proportion of âlate veneerâ added is a simple consequence of the size of each body (cross-section and/or gravitational-attraction), and may account for the volatile element budget, and the oxidationstate of Earth, Mars, the Moon and Vesta
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