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排序方式: 共有188条查询结果,搜索用时 265 毫秒
161.
162.
Yu. N. Korenkov Carl W. Cable Vernon F. Cormier Vassily Levin Jacek A. Majorowicz Yehuda Bock Waclaw M. Zuberek Teng-fong Wong William L. Power Gary A. Robbins Piotr Raczka Krzysztof E. Haman Katarzyna Chałasinska-Macukow Ryszard Balcer Henryk Piwkowski 《Pure and Applied Geophysics》1991,137(3):309-336
163.
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
164.
Joseph Arvai Gavin Bridge Nives Dolsak Robert Franzese Tomas Koontz April Luginbuhl Paul Robbins Kenneth Richards Katrina Smith Korfmacher Brent Sohngen James Tansey Alexander Thompson 《Climatic change》2006,78(1):217-225
To date the Intergovernmental Panel on Climate Change (IPCC) has concerned itself with gathering a state of the art review of the science of climate change. While significant progress has been made in enhancing our integrated understanding of the climate system and the dynamics of the social systems that produce an array of potential greenhouse gases, it is also clear from the panel's reports how far the science community is from being able to present a dynamic and synoptic view of the climate system as a whole. Clear evidence of these complexities and uncertainties inherent in the climate system is evident in efforts aimed at designing robust policy interventions. In this paper, we argue that the adaptive management framework in ecosystem management may be a useful model for guiding how the IPCC can continue to be relevant both as a scientific establishment and as a policy-relevant scientific endeavor. 相似文献
165.
Michael Hofreiter Julio L. Betancourt Alicia Pelliza Sbriller Vera Markgraf H. Gregory McDonald 《Quaternary Research》2003,59(3):364-378
Advancements in ancient DNA analyses now permit comparative molecular and morphological studies of extinct animal dung commonly preserved in caves of semiarid regions. These new techniques are showcased using a unique dung deposit preserved in a late glacial vizcacha (Lagidium sp.) midden from a limestone cave in southwestern Argentina (38.5° S). Phylogenetic analyses of the mitochondrial DNA show that the dung originated from a small ground sloth species not yet represented by skeletal material in the region, and not closely related to any of the four previously sequenced extinct and extant sloth species. Analyses of pollen and plant cuticles, as well as analyses of the chloroplast DNA, show that the Cuchillo Curá ground sloth browsed on many of the same herb, grass, and shrub genera common at the site today, and that its habitat was treeless Patagonian scrub-steppe. We envision a day when molecular analyses are used routinely to supplement morphological identifications and possibly to provide a time-lapse view of molecular diversification. 相似文献
166.
Srivastava Nandita Gonzalez Walter D. Gonzalez Alicia L.C. Masuda Satoshi 《Solar physics》1998,183(2):419-434
Intense geomagnetic storms with DST index -100 nT were recorded on 9 March and 11 March 1993 associated with solar activity on 6 March and 9-10 March, respectively. In this paper, we discuss the characteristic features of the solar origins of the two events that gave rise to coronal and interplanetary disturbances and as a consequence produced strong geomagnetic activity at the Earth. The source of the activity in one case is attributed to a major 3M7.0 flare that occurred on 6 March 1993 and in the other case, to two large filament disruptions on the disk during 9-10 March, 1993. Both these sources were found to be located near changing or varying low-latitude coronal holes. They were also located close to the heliospheric currents sheets. Distinct X-ray activity was observed for both the events as observed by the Yohkoh SXT telescope. The detailed evolution and a comparison of these events on the basis of Yohkoh soft X-ray observations are presented here. 相似文献
167.
The distribution of 210Pb, 137Cs, and Ambrosia (ragweed) pollen in two sediment cores from Lake Ontario and in three cores from Lake Erie provides independent estimates of sediment accumulation rates. Geochronology with 210Pb is based on radioactive decay of the isotope following burial in sediments. The method can reveal with precision changes in sedimentation occurring over the past 100 yr or so. Geochronologies with 137Cs and Ambrosia are based on the occurrence of a horizon corresponding, respectively, to the onset of nuclear testing 25 yr ago and to regional forest clearance in the middle 1800s. These methods provide estimates of long-term average sediment accumulation rates. In all but one core, the distributions of 137Cs and 210Pb indicate no physical mixing of near-surface sediments. In two cores, including one from central Lake Erie collected by diver, all three estimates of sedimentation rates are in excellent agreement. In two other cores, rates based on 210Pb are significantly higher than those inferred from Ambrosia pollen profiles. Lower average rates appear to result from occasional massive losses of sediments. Such events, apparent in the distribution of 210Pb but not in pollen records, correlate with the occurrence of major storm surges on the lakes during this century. In one core from western Lake Erie, exponential distributions of both 210Pb and Ambrosia appear to be artifacts which may result from extensive biological or physical reworking of sediments in shallow water (11 m). Previous indications of increased sedimentation in Lake Erie since about 1935 based on Castanea (chestnut) pollen data are not substantiated. 相似文献
168.
This paper describes the use of 210Pb and 137Cs radioactivity measurements to determine the rates of sedimentation in the Great Lakes. Cores from eight locations in Lake Michigan were chosen for examination to cover as wide as possible a range of sedimentation rates and representative sedimentary environments. The surficial 210Pb activity in the sediments varies between 7 and 23 pCi/g dry wt and its profile in each core shows the expected exponential decrease with depth consistent with the assumption of uniform sedimentation rate over the last hundred years and secular equilibrium between supported 210Pb and 226Ra (0.5-1.0 pCi/g dry wt). Companion measurements of 137Cs indicate that the coring technique satisfactorily recovered the uppermost levels of the deposit and that the mobility of both radionuclides within the sediment is probably small.Based on the limited number of cores analyzed to date, it appears that modern sedimentation rates are not very different from average rates for the last 7000 yr. The excess 210Pb appears to originate primarily from atmospheric fallout, but a further inventory of the 210Pb distribution over the lake bottom must be made to properly assess the significance of other sources. The spatial distributions of both 137Cs and 210Pb at certain stations suggest that the mode of transport of these radionuclides are comparable and involve attachment to settling particles. A mathematical model is developed which accounts for the observed limited mobility of both 210Pb and 137Cs in several of the cores in terms of post-depositional redistribution by physical or biological mixing processes. 相似文献
169.
170.