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301.
302.
Mark Woodworth Miguel Goi Eric Tappa Kathy Tedesco Robert Thunell Yrene Astor Ramon Varela Jose Rafael Diaz-Ramos Frank Müller-Karger 《Deep Sea Research Part I: Oceanographic Research Papers》2004,51(12):2249
This study examined the relationship between carbon isotopic composition of sinking organic matter (OM) and the biological, physical and chemical properties of the surface ocean in the Cariaco Basin. The 13C/12C ratio of OM (δ13Corg) in sinking particles was determined on sediment trap samples from four depths collected from 1996 to 1999 as part of the CArbon Retention In A Colored Ocean time series. Water column properties, including temperature, productivity, chlorophyll and concentration of dissolved CO2, were concurrently measured on monthly cruises. The δ13Corg varied from a high of –17.7‰ to a low of –22.6‰ during the study period. The variation of the δ13Corg throughout seasonal cycles was directly proportional to the strength of upwelling and was negatively correlated with temperature (r2=0.64). During the 1996–1997 upwelling event, the strongest during the study period, the δ13Corg increased by 4.4‰ whereas during the 1998–1999 upwelling event, the weakest during the study period, the δ13Corg only increased by 3.3‰. Contrary to most previous studies, we observed a negative relationship (r2=0.53) between [CO2 aq] and the estimated isotopic fractionation factor (εp). However, there was no correlation between εp and the calculated growth rates indicating that there was non-diffusive uptake of carbon into phytoplankton cells. It thus appears that [CO2 aq] does not control the δ13Corg in the water column of the study site. The best explanation for the isotopic enrichment observed is a carbon concentrating mechanism (CCM) in phytoplankton. The existence of a CCM in phytoplankton has major implications for the interpretation of the δ13Corg in the Cariaco Basin. 相似文献
303.
Current observational constraints on anisotropies in the CMB (Cosmic Microwave Background) and on the clustering of galaxies have considerably narrowed the range of acceptable models for galaxy formation. Observations of these anisotropies on intermediate and large angular scales provide the best constraint on the amplitude of very long wavelength density fluctuations. We present results from the Tenerife CMB fluctuation experiment (a Jodrell Bank-IAC collaboration) at intermediate angular scales (6 deg) which taken together with recently reported upper limits to fluctuations on small (arc min) angular scales constrain severely the spectral index of initial fluctuations in baryon dominated model universes. Constraints on adiabatic fluctuations in hot and cold dark matter models are also briefly discussed. 相似文献
304.
Silvana Arreghini Laura de Cabo Rafael Seoane Nicolás Tomazin Roberto Serafini Alicia Fabrizio de Iorio 《GeoJournal》2007,70(4):281-288
The Reconquista River is one of the most polluted rivers in Latin America. This paper aims at identifying the dynamics of
water quality in an area with low or “background” concentrations of pollutants within the Reconquista River system in order
to better define levels of pollution in the main system. In order to describe the dynamics of water quality in the background
area, we propose a methodology based on flow estimation with the instantaneous unit hydrograph model and on measurements of
physical and chemical water variables under different hydrological conditions. Because of high dissolved oxygen and low ammonium
and o-phosphate concentrations, the Arroyo Durazno, a tributary stream of the Reconquista River, is defined as a background area. When a storm event begins, the concentration
of nitrates and the electrical conductivity diminish. An increase in dissolved organic carbon suggests an important input
of carbon from hillslope runoff. The proportion of fulvic and humic acids also increases. On the receding limb of the hydrograph,
nitrate concentration was lower than during maximum flow and organic carbon concentration remained high. This behavior, known
as the “flushing effect”, suggests that the soluble material accumulated in the drainage area during dry periods is transported
to the stream by leaching or “lixiviation” and surface runoff, thus raising solute concentrations during the first few hours
of the storm. Water quality changes rapidly, even in background areas, due to its dependence on the flow. The methodology
followed in this paper can also be applied to other basins with similar characteristics. Due to the difficulty in defining
baseline areas for surface waters, a knowledge of background water quality and its dynamics is essential for understanding
pollution trends and anthropogenic impacts on rivers. 相似文献
305.
The electricity generation capacity in the Limay River basin is approximately 26% of the total electrical power generation
in Argentina. Assessing the potential effects of climate change on the hydrological regime of this basin is an important issue
for water resources management. This study explores the presence of trends in streamflow series, evaluates climate sensitivity
and studies the effects on the flow regime of predicted changes in precipitation in the basin. In order to identify and quantify
changes in observed streamflow series, the Mann–Kendall test, with a modification for autocorrelated data, and an estimator
of the magnitude of the trend are applied. In order to evaluate the sensitivity of streamflow to changes in climate, the concept
of elasticity is used. Precipitation elasticity of streamflow is used to quantify the sensitivity of streamflow to changes
in precipitation and is estimated using a power law model and a linear statistical model in two sub-basins, Aluminé and Nahuel
Huapi. The effects on flow regime of the predicted changes in precipitation under different scenarios are studied. Climatic
results for different scenarios of growth in greenhouse gases from some General Circulation Models are used as inputs into
the proposed models. The analysis identifies decreasing trends in mean and minimum annual flows and in the low flow season.
The estimates of the precipitation elasticity imply that changes in precipitation produce similar changes in streamflow and
the climatic results for different scenarios show that the variations are moderate. 相似文献
306.
Nemesio M. Pérez Pedro A. Hernández Eleazar Padrón Rafael Cartagena Rodolfo Olmos Francisco Barahona Gladys Melián Pedro Salazar Dina L. López 《Pure and Applied Geophysics》2006,163(4):883-896
On January 16, 2002, short-term unrest occurred at San Miguel volcano. A gas-and-steamash plume rose a few hundred meters
above the summit crater. An anomalous microseismicity pattern, about 75 events between 7:30 and 10:30 hours, was also observed.
Continuous monitoring of CO2 efflux on the volcano started on November 24, 2001, in the attempt to provide a multidisciplinary approach for its volcanic
surveillance. The background mean of the diffuse CO2 emission is about 16 g m-2 d-1, but a 17- fold increase, up to 270 g m-2 d-1, was detected on January 7, nine days before the January 2002 short-term unrest at San Miguel volcano. These observed anomalous
changes on diffuse CO2 degassing could be related to either a sharp increase of CO2 pressure within the volcanic-hydrothermal system or degassing from an uprising fresh gas-rich magma within the shallow plumbing
system of the volcano since meteorological fluctuations cannot explain this observed increase of diffuse CO2 emission. 相似文献
307.
We explore the link between channel‐bed texture and river basin concavity in equilibrium catchments using a numerical landscape evolution model. Theory from homogeneous sediment transport predicts that river basin concavity directly increases with bed sediment size. If the effective grain size on a river bed governs its concavity, then natural phenomena such as grain‐size sorting and channel armouring should be linked to concavity. We examine this hypothesis by allowing the bed sediment texture to evolve in a transport‐limited regime using a two grain‐size mixture of sand and gravel. Downstream ?ning through selective particle erosion is produced in equilibrium. As the channel‐bed texture adjusts downstream so does the local slope. Our model predicts that it is not the texture of the original sediment mixture that governs basin concavity. Rather, concavity is linked to the texture of the sorted surface layer. Two different textural regimes are produced in the experiments: a transitional regime where the mobility of sand and gravel changes with channel‐bed texture, and a sand‐dominated region where the mobility of sand and gravel is constant. The concavity of these regions varies depending on the median gravel‐ or sand‐grain size, erosion rate, and precipitation rate. The results highlight the importance of adjustments in both surface texture and slope in natural rivers in response to changes in ?uvial and sediment inputs throughout a drainage network. This adjustment can only be captured numerically using multiple grain sizes or empirical downstream ?ning rules. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
308.
In this paper we study the effect of operating time of AgI ground acetonic generatorsbefore hailing on the hail characteristics (size and hardness), and hailfall crop damages.The study was carried out in a large area of northern Spain over an area of 12,000 km2.Data are available from 1,984 hailstorms that occurred from May to September in theperiod 1982–1987. By using several Stepwise Logistic Regression Models, a statisticalsignificant (p < 0.05) decrease shows up in hailfall crop damage due to hail prevention.This effect is due to the influence of ground seeding on the hardness and mainly on thesize of hailstones. 相似文献
309.
Relation between direct wave Q and coda Q: a numerical approach 总被引:1,自引:0,他引:1
310.
Laura Bennati Anthony Finizola James A. Walker Dina L. Lopez I. Camilo Higuera-Diaz Claudia Schütze Francisco Barahona Rafael Cartagena Vladimir Conde Renan Funes Cristobal Rios 《Journal of Volcanology and Geothermal Research》2011,199(3-4):216-229
The region encompassing Santa María, Cerro Quemado, and Zunil volcanoes, close to Quetzaltenango, the second largest city of Guatemala, is volcanically and tectonically complex. In addition, the huge Xela caldera, about 20 km in diameter, crosses this area and links up to the important Zunil fault zone located between the three volcanoes. Two highly active geothermal sites, named Zunil-I and Zunil-II, are also located between these three volcanic edifices at the southeastern boundary of Xela caldera. In order to determine the permeability variations and the main structural discontinuities within this complex volcano-tectonic setting, self-potential and soil CO2 flux measurements have been coupled, with a step of 20 m, along a 16.880 km-long profile crossing the entire area. Two shallow hydrothermal systems, with maximum lateral extensions of 1.5 km in diameter, are indicated by positive self-potential/elevation gradients below Santa María and Cerro Quemado volcanoes. Such small hydrothermal systems cannot explain the intense geothermal manifestations at Zunil-I and Zunil-II. Another minor hydrothermal system is indicated by self-potential measurements on the flank of Santa María along the edge of the Xela caldera. CO2 flux measurements display slight variations inside the caldera and decreasing values crossing outside the caldera boundary. We hypothesize the presence of a magmatic body, inside the southeastern border of Xela caldera, to explain the deeper and more intense hydrothermal system manifested by the Zunil-I and the Zunil-II geothermal fields. This magmatic system may be independent from Santa María and Cerro Quemado volcanoes. Alternatively, the hypothesized Xela magmatic system could have a common magmatic origin with the Cerro Quemado dome complex, consistent with previous findings on regional gas emissions. Sectors bordering the Cerro Quemado dome complex also have high amplitude minima-short wavelength anomalies in self-potential, interpreted as preferential rain water infiltration along faults of major permeability, probably related with the most recent stages of Cerro Quemado dome growth. 相似文献