960 research outputs found
A new model of cosmogenic production of radiocarbon 14C in the atmosphere
We present the results of full new calculation of radiocarbon 14C production
in the Earth atmosphere, using a numerical Monte-Carlo model. We provide, for
the first time, a tabulated 14C yield function for the energy of primary cosmic
ray particles ranging from 0.1 to 1000 GeV/nucleon. We have calculated the
global production rate of 14C, which is 1.64 and 1.88 atoms/cm2/s for the
modern time and for the pre-industrial epoch, respectively. This is close to
the values obtained from the carbon cycle reservoir inventory. We argue that
earlier models overestimated the global 14C production rate because of outdated
spectra of cosmic ray heavier nuclei. The mean contribution of solar energetic
particles to the global 14C is calculated as about 0.25% for the modern epoch.
Our model provides a new tool to calculate the 14C production in the Earth's
atmosphere, which can be applied, e.g., to reconstructions of solar activity in
the past.Comment: Published in EPSL, 337, 114, 201
NATURAL AND ENVIRONMENTAL VULNERABILITY OF THE JAURU RIVER BASIN-MATO GROSSO, BRAZIL
The disorderly use of land is currently one of the main causes of environmental problems. From this perspective, the objective of this study was to determine the natural and environmental vulnerability of the Jauru river basin in the Brazilian state of Mato Grosso. This goal was made from the perspective that the results generated contribute to environmental planning and management, considering that the Jauru river is tributary of the Paraguay river, responsible for the flooding of the Pantanal plain and, that possible impacts caused by the basin may compromise the dynamic equilibrium of the Pantanal's flood pulse. The model of assessment of natural and environmental vulnerability was adopted, with the identification of the following environmental characteristics: geology, slope, agricultural aptitude and vegetation cover and land use. Data were related through Geographic Information Systems to generate natural and environmental vulnerability maps. It was verified, through the data obtained, that in both vulnerabilities studied there was a predominance of the middle class, mainly resulting from the presence of fragile soils. Also, the predominance of the classes of agricultural aptitude found in the basin, as well as the suppression of the natural vegetation to expand the pasture areas. It was concluded that the areas covered by natural vegetation and/or pastures, in the Jauru river basin, present average natural vulnerability and state of medium to high environmental vulnerability, evidencing that livestock, the main activity regional economic development, has been developed in disagreement with environmental aptitude, contributing to the elevation of the state of environmental vulnerability in the basin
Disorder induced Dirac-point physics in epitaxial graphene from temperature-dependent magneto-transport measurements
We report a study of disorder effects on epitaxial graphene in the vicinity
of the Dirac point by magneto-transport. Hall effect measurements show that the
carrier density increases quadratically with temperature, in good agreement
with theoretical predictions which take into account intrinsic thermal
excitation combined with electron-hole puddles induced by charged impurities.
We deduce disorder strengths in the range 10.2 31.2 meV, depending on
the sample treatment. We investigate the scattering mechanisms and estimate the
impurity density to be cm for our samples.
An asymmetry in the electron/hole scattering is observed and is consistent with
theoretical calculations for graphene on SiC substrates. We also show that the
minimum conductivity increases with increasing disorder potential, in good
agreement with quantum-mechanical numerical calculations.Comment: 6 pages, 3 figure
Use of the D-R Model to Define Trends in the Emergence of Ceftazidime-Resistant Escherichia coli in China
OBJECTIVE: To assess the efficacy of the D-R model for defining trends in the appearance of Ceftazidime-resistant Escherichia coli. METHODS: Actual data related to the manifestation of Ceftazidime-resistant E. coli spanning years 1996-2009 were collected from the China National Knowledge Internet. These data originated from 430 publications encompassing 1004 citations of resistance. The GM(1,1) and the novel D-R models were used to fit current data and from this, predict trends in the appearance of the drug-resistant phenotype. The results were evaluated by Relative Standard Error (RSE), Mean Absolute Deviation (MAD) and Mean Absolute Error (MAE). RESULTS: Results from the D-R model showed a rapid increase in the appearance of Ceftazidime-resistant E. coli in this region of the world. These results were considered accurate based upon the minor values calculated for RSE, MAD and MAE, and were equivalent to or better than those generated by the GM(1,1) model. CONCLUSION: The D-R model which was originally created to define trends in the transmission of swine viral diseases can be adapted to evaluating trends in the appearance of Ceftazidime-resistant E. coli. Using only a limited amount of data to initiate the study, our predictions closely mirrored the changes in drug resistance rates which showed a steady increase through 2005, a decrease between 2005 and 2008, and a dramatic inflection point and abrupt increase beginning in 2008. This is consistent with a resistance profile where changes in drug intervention temporarily delayed the upward trend in the appearance of the resistant phenotype; however, resistance quickly resumed its upward momentum in 2008 and this change was better predicted using the D-R model. Additional work is needed to determine if this pattern of "increase-control-increase" is indicative of Ceftazidime-resistant E. coli or can be generally ascribed to bacteria acquiring resistance to drugs in the absence of alternative intervention
Talc-dominated seafloor deposits reveal a new class of hydrothermal system
The Von Damm Vent Field (VDVF) is located on the flanks of the Mid-Cayman Spreading Centre, 13?km west of the axial rift, within a gabbro and peridotite basement. Unlike any other active vent field, hydrothermal precipitates at the VDVF comprise 85–90% by volume of the magnesium silicate mineral, talc. Hydrothermal fluids vent from a 3-m high, 1-m diameter chimney and other orifices at up to 215?°C with low metal concentrations, intermediate pH (5.8) and high concentrations (667?mmol?kg?1) of chloride relative to seawater. Here we show that the VDVF vent fluid is generated by interaction of seawater with a mafic and ultramafic basement which precipitates talc on mixing with seawater. The heat flux at the VDVF is measured at 487±101?MW, comparable to the most powerful magma-driven hydrothermal systems known, and may represent a significant mode of off-axis oceanic crustal cooling not previously recognized or accounted for in global models
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