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841.
A magnetospheric model including convection and corotation electric fields is developed. Drift shells and pitch-angle evolution are obtained through perturbation methods. The asymmetry of the shell is maximum in a plane different from the noon-midnight meridian. As a consequence the dip in fluxes is not maximum in this meridian and the amount of shift is shown to be directly proportional to the convection electric field intensity. It is inversely proportional to the energy or the pitch-angle of the particles. 相似文献
842.
Future extreme events in European climate: an exploration of regional climate model projections 总被引:7,自引:0,他引:7
Martin Beniston David B. Stephenson Ole B. Christensen Christopher A. T. Ferro Christoph Frei Stéphane Goyette Kirsten Halsnaes Tom Holt Kirsti Jylhä Brigitte Koffi Jean Palutikof Regina Schöll Tido Semmler Katja Woth 《Climatic change》2007,81(1):71-83
This paper compares how well satellite versus weather station measurements of climate predict agricultural performance in
Brazil, India, and the United States. Although weather stations give accurate measures of ground conditions, they entail sporadic
observations that require interpolation where observations are missing. In contrast, satellites have trouble measuring some
ground phenomenon such as precipitation but they provide complete spatial coverage of various parameters over a landscape.
The satellite temperature measurements slightly outperform the interpolated ground station data but the precipitation ground
measurements generally outperform the satellite surface wetness index. In India, the surface wetness index outperforms station
precipitation but this may be reflecting irrigation, not climate. The results suggest that satellites provide promising measures
of temperature but that ground station data may still be preferred for measuring precipitation in rural settings. 相似文献
843.
Zhenhong Du Yuhua Gu Chuanrong Zhang Feng Zhang Renyi Liu Jean Sequeira 《International Journal of Digital Earth》2018,11(6):609-634
Spatiotemporal clustering is one of the most advanced research topics in geospatial data mining. It has been challenging to discover cluster features with different spatiotemporal densities in geographic information data set. This paper presents an effective density-based spatiotemporal clustering algorithm (DBSTC). First, we propose a method to measure the degree of similarity of a core point to the geometric center of its spatiotemporal reachable neighborhood, which can effectively solve the isolated noise point misclassification problem that exists in the shared nearest neighbor methods. Second, we propose an ordered reachable time window distribution algorithm to calculate the reachable time window for each spatiotemporal point in the data set to solve the problem of different clusters with different temporal densities. The effectiveness and advantages of the DBSTC algorithm are demonstrated in several simulated data sets. In addition, practical applications to seismic data sets demonstrate the capability of the DBSTC algorithm to uncover clusters of foreshocks and aftershocks and help to improve the understanding of the underlying mechanisms of dynamic spatiotemporal processes in digital earth. 相似文献
844.
Camila N. Signori Jean L. Valentin Ricardo C. G. Pollery Alex Enrich-Prast 《Estuaries and Coasts》2018,41(4):1089-1101
Dark carbon fixation (DCF) is considered an important energy source in aquatic environments, although it has been neglected for a long time. DCF is known to be relevant in ecosystems associated with redoxclines, shallow-water sulfide-rich habitats, deep-sea vents, cold seeps, and even in coastal waters associated with upwelling events. The aim of this study was to evaluate the relative importance of DCF in relation to heterotrophic bacterial production (BP), as well as how these rates affect each other, and how they are influenced by the environmental factors. This study was conducted monthly during 2 years in a tropical eutrophic bay (Guanabara Bay), where two stations were sampled and compared. DCF and BP were measured by 14C-bicarbonate and 3H-leucine incorporation, respectively, and incubations in the dark. Our results showed that DCF is not a quantitatively relevant process in this estuarine system, when compared to heterotrophic BP, and possibly occurred via anaplerotic reactions. Relatively higher DCF rates were associated with less oxygenated waters at the more polluted station and during the wet summer-spring, when the water column is more stratified. BP rates presented clear spatial patterns, according to pollution and depth gradients, with higher rates in more polluted areas, and also at surface waters. The hydrodynamics combined with other environmental conditions (precipitation, temperature, dissolved organic carbon, and nutrients) may control the distribution of DCF and BP over space and time. The allochthonous inputs of organic matter are more important than DCF-derived organic carbon to bacterioplankton in this polluted and eutrophic bay, where the heterotrophic metabolism prevails. 相似文献
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Landry Soh Tamehe Chongtao Wei Sylvestre Ganno Carlos Alberto Rosiere Jean Paul Nzenti Chesther Gatse Ebotehouna Guanwen Lu 《地学前缘(英文版)》2021,12(2):549-572
The discovery of the Gouap banded iron formations(BIFs)-hosted iron mineralization in the northwestern of the Nyong Group(Ntem Complex)in southwestern Cameroon provides unique insights into the geology of this region.In this contribution,we firstly report detailed study of geochemistry,isotopic and geochronology of well preserved samples of the Gouap BIFs collected from diamond drillcores.The Gouap BIFs consist mainly of amphibole BIFs and amphibole-pyrite BIFs characterized by dominant Fe2O3+SiO2contents and variable contents of CaO,MgO and SO3,consistent with the presence of amphibole,chlorite,epidote and pyrite,formed during amphibolite facies metamorphism and overprinted hydrothermal event.The amphibole–pyrite BIFs are typically enriched in trace and rare earth elements(REE)compared to the amphibole BIFs,suggesting the influence of detrital materials as well as secondary hydrothermal alteration.The Post Archean Australian Shale(PAAS)-normalized REE–Y profiles of the Gouap BIFs display positive La,Eu anomalies,weak negative Ce anomalies,indicating a mixture of low-temperature hydrothermal fluids and relatively oxic conditions probably under relative shallow seawater.We present here the first isotopic data of BIFs within the Ntem Complex.Theδ30SiNBS28values of the quartz from the Gouap BIFs vary from-1.5‰to-0.3‰and from-0.8‰to-0.9‰for the amphibole BIFs and amphibole–pyrite BIFs,respectively.The quartz hasδ18OV-SMOW values of 6.8‰–9.5‰(amphibole BIFs)and 9.2‰–10.6‰(amphibole–pyrite BIFs).The magnetite from the Gouap BIFs showsδ18O values ranging from-3.5‰to-1.8‰and from-3‰to-1.7‰for the amphibole BIFs and amphibole–pyrite BIFs,respectively.Moreover,the pyrite grains in the amphibole–pyrite BIFs displayδ34S values of 1.1‰–1.8‰.All isotopic data of the Gouap BIFs confirm that they might have precipitated from low-temperature hydrothermal fluids with detrital input distant from the volcanic activity.According to their geochemical and isotopic characteristics,we propose that the Gouap BIFs belong to the Superior type.In situ U–Pb zircon dating of BIFs was conducted to assess the BIF depositional age based on strong evidence of zircon in thin section.The Gouap BIFs were probably deposited at 2422±50 Ma in a region where sediments extended from continental shelf to deep-water environments along craton margins like the Caue Formation of the Minas Supergroup,Brazil.The studied BIFs have experienced regional hydrothermal activity and metamorphism at 2089±8.3 Ma during the Eburnean–Transamazonian orogeny.These findings suggest a physical continuity between the protocratonic masses of both Sao Francisco and Congo continents in the Rhyacian Period. 相似文献
848.
Teutsong Tessontsap Temga Jean Pierre Enyegue Armelle Ayissi Feuwo Nicodème Noel Bitom Dieudonné 《中国地球化学学报》2021,40(2):163-175
Acta Geochimica - Iron ore deposits hosted by Precambrian banded iron formation (BIF) are the most important source of mineable iron. In Cameroon, they are located in the southern part of the... 相似文献
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