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401.
The surface mass balance for Greenland and Antarctica has been calculated using model data from an AMIP-type experiment for the period 1979?C2001 using the ECHAM5 spectral transform model at different triangular truncations. There is a significant reduction in the calculated ablation for the highest model resolution, T319 with an equivalent grid distance of ca 40?km. As a consequence the T319 model has a positive surface mass balance for both ice sheets during the period. For Greenland, the models at lower resolution, T106 and T63, on the other hand, have a much stronger ablation leading to a negative surface mass balance. Calculations have also been undertaken for a climate change experiment using the IPCC scenario A1B, with a T213 resolution (corresponding to a grid distance of some 60?km) and comparing two 30-year periods from the end of the twentieth century and the end of the twenty-first century, respectively. For Greenland there is change of 495?km3/year, going from a positive to a negative surface mass balance corresponding to a sea level rise of 1.4?mm/year. For Antarctica there is an increase in the positive surface mass balance of 285?km3/year corresponding to a sea level fall by 0.8?mm/year. The surface mass balance changes of the two ice sheets lead to a sea level rise of 7?cm at the end of this century compared to end of the twentieth century. Other possible mass losses such as due to changes in the calving of icebergs are not considered. It appears that such changes must increase significantly, and several times more than the surface mass balance changes, if the ice sheets are to make a major contribution to sea level rise this century. The model calculations indicate large inter-annual variations in all relevant parameters making it impossible to identify robust trends from the examined periods at the end of the twentieth century. The calculated inter-annual variations are similar in magnitude to observations. The 30-year trend in SMB at the end of the twenty-first century is significant. The increase in precipitation on the ice sheets follows closely the Clausius-Clapeyron relation and is the main reason for the increase in the surface mass balance of Antarctica. On Greenland precipitation in the form of snow is gradually starting to decrease and cannot compensate for the increase in ablation. Another factor is the proportionally higher temperature increase on Greenland leading to a larger ablation. It follows that a modest increase in temperature will not be sufficient to compensate for the increase in accumulation, but this will change when temperature increases go beyond any critical limit. Calculations show that such a limit for Greenland might well be passed during this century. For Antarctica this will take much longer and probably well into following centuries. 相似文献
402.
Carlton Allen Astromaterials Curator Judith Allton Genesis Sample Curator Gary Lofgren Lunar Sample Curator Kevin Righter Antarctic Meteorite Curator Michael Zolensky Cosmic Dust Stardust Sample Curator 《Chemie der Erde / Geochemistry》2011,(1):1-20
The NASA Johnson Space Center Astromaterials Acquisition and Curation Office has the unique responsibility to curate NASA's extraterrestrial samples – from past and forthcoming missions – into the indefinite future. Presently curation includes documentation, preservation, preparation, and distribution of samples from the Moon, asteroids, comets, the solar wind, and the planet Mars. Each of these sample sets has a unique history and comes from a unique environment. The curation laboratories and procedures developed over forty years have proven both necessary and sufficient to serve the evolving needs of a worldwide research community. A new generation of sample return missions is being planned and proposed to destinations across the solar system. Curation must evolve to meet the increased challenges of these new samples. 相似文献
403.
Terrence Blackburn Samuel A. Bowring Blair Schoene Kevin Mahan Francis Dudas 《Contributions to Mineralogy and Petrology》2011,162(3):479-500
A new approach to U-Pb accessory mineral thermochronology allows high-resolution time-temperature histories to be extracted
from lower crustal xenoliths. The combination of the U-Pb system’s dual decay scheme with the effects of temperature dependent
Pb-diffusion can yield a time sensitive record of Pb production/diffusion within accessory phases. The difference in half-life
for parent isotopes 238U and 235U results in the time-variable production of Pb isotopes 206Pb and 207Pb, while Pb diffusion can result in large variations in the time-scales at which Pb retention occurs between grains of different
sizes. The combined effects of variable production rates between the two systems and diffusion result in data topologies on
a concordia diagram that permit distinction between slow cooling and reheating t-T paths. In slowly cooled systems, the difference
in time for Pb retention for grains of variable size yields a measure of partial retention zone (PRZ) residence time, and
provides a robust measure of cooling rate through the PRZ. In Montana, three lower crustal xenoliths, each from a different
depth, yield U-Pb rutile data that record a prolonged (>1 Ga) and slow cooling history towards a steady state geothermal gradient
following the amalgamation of the terrain onto North America. The shallowest samples record the initial recovery of a conductive
geothermal gradient and cool through the mineral PRZ at rates of <0.25°C/Ma over ~500 Ma. Deeper xenoliths record cooling
at younger times over similar time scales and rates. This multi-depth thermal history provides a long-term record of lithosphere
cooling and stabilization. 相似文献
404.
Silicon isotope fractionation during magmatic differentiation 总被引:3,自引:0,他引:3
Paul S. Savage R. Bastian Georg Kevin W. Burton Alex N. Halliday 《Geochimica et cosmochimica acta》2011,75(20):6124-6139
The Si isotopic composition of Earth’s mantle is thought to be homogeneous (δ30Si = −0.29 ± 0.08‰, 2 s.d.) and not greatly affected by partial melting and recycling. Previous analyses of evolved igneous material indicate that such rocks are isotopically heavy relative to the mantle. To understand this variation, it is necessary to investigate the degree of Si isotopic fractionation that takes place during magmatic differentiation. Here we report Si isotopic compositions of lavas from Hekla volcano, Iceland, which has formed in a region devoid of old, geochemically diverse crust. We show that Si isotopic composition varies linearly as a function of silica content, with more differentiated rocks possessing heavier isotopic compositions. Data for samples from the Afar Rift Zone, as well as various igneous USGS standards are collinear with the Hekla trend, providing evidence of a fundamental relationship between magmatic differentiation and Si isotopes. The effect of fractionation has been tested by studying cumulates from the Skaergaard Complex, which show that olivine and pyroxene are isotopically light, and plagioclase heavy, relative to the Si isotopic composition of the Earth’s mantle. Therefore, Si isotopes can be utilised to model the competing effects of mafic and felsic mineral fractionation in evolving silicate liquids and cumulates.At an average SiO2 content of ∼60 wt.%, the predicted δ30Si value of the continental crust that should result from magmatic fractionation alone is −0.23 ± 0.05‰ (2 s.e.), barely heavier than the mantle. This is, at most, a maximum estimate, as this does not take into account weathered material whose formation drives the products toward lighter δ30Si values. Mass balance calculations suggest that removal of continental crust of this composition from the upper mantle will not affect the Si isotopic composition of the mantle. 相似文献
405.
406.
407.
Kevin Hall 《Environmental Earth Sciences》2011,62(2):289-297
A number of modern buildings employing claddings of granites or marbles have experienced bowing of the rock panels together
with weathering of the material. Theoretical and field-based data analysis and laboratory experimentation have assumed that
heat exchange resulting from incoming solar radiation is at the material surface. However, a number of recent experiments
have clearly shown that some lithologies, including both marble and granite, comprise a number of light-transmissive minerals
that significantly change the thermal responses in the outer few millimetres of the rock. Further, this translucence will
create mineral-to-mineral stresses where light-transmissive minerals are in contact with opaque ones. The whole is further
exacerbated by differences in thermal coefficients of expansion and conductivity which themselves may depend on the mineral
axis; surface modifiers such as paints further complicate the situation. The degree of light penetration, based on field measurements,
can be significant and can facilitate rapid changes in temperature (ΔT/Δt > 2°C min−1) at depth within the rock thereby increasing the sub-surface stresses. The amount of light penetration for any given mineral
will be dependent upon material slope, the latitude, season, and albedo. Albedo is identified as a complex variable, changing
as a function of the angle of the sun to the particular surface; polished surfaces, as often with cladding, will further influence
this. Data analysis suggests that, in the Northern hemisphere, south of the polar circle, the summer may not be the time of
the largest heat loading on the southern aspect and that larger loadings, coupled with lower air temperatures, occur early
and late in the year. This seasonal impact has great potential for thermal stresses on the southern aspect. The presence of
light-transmissive minerals also allows sub-surface biotic colonization and results in weathering. Overall, where light transmissive
minerals/lithologies occur the thermal responses are highly complex and in need of more rigorous consideration. 相似文献
408.
409.
Kevin Hamilton 《大气与海洋》2013,51(2):188-192
Abstract The initial development of the westerly acceleration phase of the tropical quasi‐biennial oscillation in late 1979 was examined using FGGE analyses at the 20‐ and 10‐mb levels. The analysed winds were found to undergo strong equatorially‐centred westerly accelerations. These accelerations are narrower in meridional extent than those expected to result from the interaction of the Wallace‐Kousky Kelvin wave with the mean flow. 相似文献
410.
Kevin E. McHugh 《The Professional geographer》1989,41(4):429-439
The US Hispanic population has grown rapidly over the last two decades and remains geographically concentrated in nine states. Redistribution away from core states through internal migration has been largely offset by heavy immigration to traditional areas of Hispanic concentration. Geographical patterns of Hispanic miration show broad similarities to overall patterns of population redistribution in the United States. New York and California serve as key spatial redistributors or pivots in the Hispanic migration system. 相似文献