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111.
Mark E. Hines Wm. Berry Lyons Peter B. Armstrong William H. Orem Mary Jo Spencer Henri E. Gaudette Galen E. Jones 《Marine Chemistry》1984,15(2):173-187
Concentrations of dissolved iron, manganese, molybdenum, copper, and organic carbon (DOC) were measured in the pore waters from surficial sediments of a temperate estuary to delineate seasonal metal remobilization from 1978 through 1980. Iron and DOC data were collected for 31 months and covaried inversely and exponentially. Iron dissolution occurred during the spring and during periods of active bioturbation with concentrations as high as 18 mg 1?1. Iron values were low during winter due to oxidation to ferric oxides. The lack of active bioturbation during the summer of 1978 allowed for the nearly complete removal of iron as a monosulfide precipitate. However, bioturbation resumed during the summer of 1979 and 1980 and dissolved iron concentrations as high as 10 mg 1?1 were observed at those times. The iron and DOC data were a qualitative measure of bioturbation activity. Dissolved manganese, molybdenum, and copper data were collected for 18 months during 1978 and 1979. All three metals displayed spring maxima covariate with iron, suggesting that they behaved chemically like iron and/or were associated with iron- or manganese-rich phases during this time of the year. In general, manganese and molybdenum varied temporally with iron while copper concentrations mimicked iron variations only during the spring. 相似文献
112.
A reconstruction of the hydrological and environmental evolution of the crater lake at Malha (Northern Darfur, Sudan) resulted from the mineralogical and biological study of a 9.21 m section of lake sediments, representing an uninterrupted sequence of lacustrine deposition since 8 290 14C years BP.Important changes in water supply and conditions of sedimentation are reflected in the nature of the sediments and the morphology and stratigraphical distribution of various salt minerals. Additional information on lake circulation patterns and salinity conditions are obtained from associated benthic paleocommunities, represented by ostracods and dipterid larvae. Combining both lines of evidence, the studied sequence can be divided in six distinct sections, which correspond to six successive periods in the lake's Holocene history. The first three periods, generally characterized by high lake levels, represent three generations of a meromictic lake, two of which have ended with a complete desiccation of the lake basin. Meromixis was stable during Period I, due to wind shelter and pronounced density stratification. In the course of Periods II and III stratification was repeatedly interrupted. During Period II, the disruptions were accompanied by important water budget fluctuations; a superimposed gradual decrease in net water supply eventually resulted in holomictic conditions terminating this period. Evidence of turbulence periodically affecting profundal waters is recorded in the sediments of Period III, suggesting that disruptions of stratification were now initiated by very strong winds. Between Period I and Period III, the littoral mixolimnion gradually evolved from near fresh to mesosaline. In Periods IV to VI, lake level was intermediate to low. The lake was holomictic for most of the time and meso- to hypersaline; during Period V, it repeatedly shrunk to a shallow brine pool.The Holocene evolution of Malha Crater Lake illustrates the progressive increase in aridity over most of North Africa following a well-established, early- to mid-Holocene major humid episode. The uninterrupted sedimentary sequence lends itself for detailed reconstruction of Holocene climatic evolution in arid Northeast Africa, a region where records of continuous lacustrine deposition are extremely scarce. As the chronology of critical events in the lake's history remains as yet unsupported by radiocarbon dates, correlation with other Holocene sequences in the eastern Sahara is highly tentative at this point. 相似文献
113.
Nathalie Volland-Tuduri Michel Brossard Ary Bruand Henri Garreau 《Comptes Rendus Geoscience》2004,336(11):1017-1024
Subrounded microaggregates 100 to 500 μm in diameter were collected in a Ferralsol located in the state of Goiàs. Observation of these microaggregates in dry conditions in scanning electron microscopy showed morphological characteristics that are representative of microaggregates described earlier in many Ferralsols. The shrinkage for drying of initially water saturated microaggregates was studied using an environmental scanning electron microscope. Results showed very small shrinkage for the microaggregates usually studied. However, the shrinkage varies since the volume of dried microaggregates was found to be 93 to 99% of their volume when they are saturated. To cite this article: N. Volland-Tuduri et al., C. R. Geoscience 336 (2004). 相似文献
114.
Tree-ring records from western juniper (Juniperus occidentalis var. occidentalis Hook.) growing throughout the interior Pacific Northwest identify extreme climatic pointer years (CPYs) (i.e., severe single-year droughts) from 1500–1998. Widespread and extreme CPYs were concentrated in the 16th and early part of the 17th centuries and did not occur again until the early 20th century. The 217-yr absence of extreme CPYs may have occurred during an extended period of low variance in the Pacific Decadal Oscillation. We mapped climatic boundaries for the interior Pacific Northwest based on the location of sites with similar precipitation variability indices. Three regions, the Northwest (based on chronologies from nine sites), the Southwest (four sites), and the East (five sites) were identified. Our results suggest that western juniper radial growth indices have substantial interannual variability within the northwestern range of the species (central Oregon), particularly when compared with western juniper growing in its eastern range (eastern Oregon, southeastern Idaho, and northern Nevada) and southwestern range (southern Oregon and northeast California). We suspect that the substantial differences in the variability of western juniper radial growth indices are linked to the influence of ENSO events on winter/spring precipitation amounts. 相似文献
115.
Gravity-induced stresses in stratified rock masses 总被引:1,自引:1,他引:1
Bernard Amadei Henri S. Swolfs William Z. Savage 《Rock Mechanics and Rock Engineering》1988,21(1):1-20
Summary This paper presents closed-form solutions for the stress field induced by gravity in anisotropic and stratified rock masses. These rocks are assumed to be laterally restrained. The rock mass consists of finite mechanical units, each unit being modeled as a homogeneous, transversely isotropic or isotropic linearly elastic material. The following results are found. The nature of the gravity induced stress field in a stratified rock mass depends on the elastic properties of each rock unit and how these properties vary with depth. It is thermodynamically admissible for the induced horizontal stress component in a given stratified rock mass to exceed the vertical stress component in certain units and to be smaller in other units; this is not possible for the classical unstratified isotropic solution. Examples are presented to explore the nature of the gravity induced stress field in stratified rock masses. It is found that a decrease in rock mass anisotropy and a stiffening of rock masses with depth can generate stress distributions comparable to empirical hyperbolic distributions previously proposed in the literature. 相似文献
116.
François Guichard Thomas M. Church Michel Treuil Henri Jaffrezic 《Geochimica et cosmochimica acta》1979,43(7):983-997
A large variety of barites collected from marine and continental environments was analyzed by neutron activation for the rare-earth elements (REE) La, Ce, Sm, Eu and Dy. Relative to chondrites, all barites show a decrease of the lighter REE from La toward Eu. The abundance and distribution of rare earths in barites show a distinction of barite types. Deep-sea barites have large REE concentrations as do other authigenic deep-sea minerals and display the chondrite normalized Eu minimum, but not the negative Ce anomaly, of sea water. Other barites, mostly on land, some hydrothermal, and others of shallow marine origin, display lower total Ree concentrations. Chondrite normalized positive Eu anomalies are displayed by those varieties of reducing sedimentary and metamorphic origin.Distribution of REE in barite can be attributed both to crystallographic constraints of substitution, and to solution complexing of REE in the precipitating medium. Plots of rare earth partitioning versus effective ion size suggest that the decreasing enrichment toward Eu for all barite types is caused by crystallographic constraints due to contraction of the substituting REE ion sizes relative to the size of the host Ba ion. Solution effects on REE substitution in barite can be evaluated by writing solid solution distribution equations based on mass action of REE and Ba sulfates and the lanthanide (Ln) solution species Ln (CO3)?54), LnSO+4, LnCl+2 and LnF+2. Under normal sea water conditions, solution complexing plays a minor role. However, increased alkalinities of reducing sediments and increased brine chlorinities could cause significant complexing and deplete REE heavier than Eu. Besides Dy in barites, this could be true for aqueous precipitation of REE in general. 相似文献
117.
Henri E. Gaudette Annie Vitrac-Michard Claude J. Allègre 《Earth and Planetary Science Letters》1981,54(2):248-260
Zircons separated from the Cambrian Potsdam sandstone of New York yield four distinct populations which can be defined by a number of analytical techniques. U-Pb isotopic analyses of small samples and monozircons of each population reveal a fine chronology not apparent in milligram-sized sample analysis, and define source area ages of 1180, 1320, 2100 and 2700 m.y. for the Cambrian detrital suite. These ages correspond to well-defined sources in the Superior and Grenville Provinces of the Canadian Shield (2700 and 2100 m.y.) and the well established Grenville age rocks of the Adirondack Mountains (1180 m.y.). The 1320-m.y. age appears to be derived from the Adirondacks, and suggests the existence of pre-Grenville basement in that massif. Our techniques allow the interpretation of the Precambrian history of a large portion of eastern North America from a single sample, and thus should be valuable in the definition of source areas in paleogeographic reconstruction, and in studies of continental crustal evolution. 相似文献
118.
Benoît Villemant Henri Jaffrezic Jean-Louis Joron Michel Treuil 《Geochimica et cosmochimica acta》1981,45(11):1997-2016
Major and seventeen trace element distribution coefficients between main phenocrysts (olivine, clinopyroxene, amphibole, mica, feldspars and Fe-Ti oxides) and groundmass have been measured in the alkali basalt suite of Chaîne des Puys (Massif Central, France). The suite appears to be a well behaved crystal fractionation series. We pinpoint key elements whose behavior is closely related to the appearance or disappearance of specific crystal phases in the fractionation process. Ta, for instance, clearly indicates the role of hydrous silicates (amphiboles and micas). Distribution coefficients are shown to vary systematically along the differentiation trend. Significantly the hygromagmaphile tendency (Treuilet al., 1979) of U, Th, Ta and La is variable along the series.The mass balance equations, where Di and Dji are the bulk and mineral/liquid distribution coefficients respectively, and xj the weight fractions of the fractionating phases, are solved by least square resolution of the overdetermined system, taking into account the analytical errors on data. The solution applied to the Chaîne des Puys suite leads to a coherent and quantitative model of the fractional crystallization process. The suite has apparently evolved in three stages. Each stage is characterized by constant bulk distribution coefficients and a specific mineral assemblage. Amphibole fractionation plays an important role in the early stages. Some intensive parameters (T, ? ?O2, PH2O) as well as f (weight fraction of residual liquid) are also estimated. 相似文献
119.
120.
Pure and Applied Geophysics - Cette brève communication est simplement destinée à faire connaître le début d'expériences de convection artificielle en... 相似文献