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161.
Recent Environmental Change and Human Impact on Svalbard: The Lake-Sediment Geochemical Record 总被引:1,自引:1,他引:1
As part of a broader investigation into recent environmental change on Svalbard, the inorganic geochemical record of six lake-sediment cores was analysed. The major temporal trends in sediment elemental composition are driven by variations in two contrasting sediment components, both derived from catchment soils: (1) mineral matter, and (2) soil organic matter (SOM), enriched in Fe and Mn oxides and heavy metals. Two environmental impacts are recorded in most or all of the lake sediment sequences. An up-core increase in organic matter can be partly attributed to diagenetic effects, but also requires an enhanced supply of SOM relative to mineral matter. In addition, the central and southern sites all show a ca. 1970 event characterised by an enhanced mineral matter accumulation rate. This requires either an enhanced allochthonous supply or an enhanced mobilisation of littoral sediments. In either case a regional-scale driving force, such as a shift in climate, is required. At five of the lakes the sediment heavy metal concentration profiles can be explained entirely by natural factors. However, at Tenndammen (U), situated close to the Svalbard’s largest settlement at Longyearbyen, possible anthropogenic Pb enrichment is found. Comparison of observed and expected heavy metal profiles (based on Greenland ice-core data) shows that the lakes are generally too insensitive to have recorded a long-transported heavy metal pollution signal. 相似文献
162.
The estuarine chemistry of dissolved humic acids was determined by carrying out both field and laboratory studies. These approaches were combined in an investigation of the Amazon estuary while laboratory mixing experiments were performed using filtered (0.45?0.001 μm) river water fractions of the Water of Luce (Scotland).The results demonstrate that a small fraction of river dissolved organic matter is preferentially and rapidly flocculated during estuarine mixing. This fraction is the high molecular weight component of dissolved humic acids (0.45?0.1 μm filtered). Approximately 60–80% of the dissolved humic acid in these rivers flocculates during estuarine mixing. This represents a removal of only 3–6% of river dissolved organic matter and is responsible for the non-conservative behaviour of dissolved humic acid in the Amazon estuary even though total dissolved organic carbon appears conservative.The salinity dependence with which humic acid flocculates in estuaries is similar to that of iron. This implies that both constituents may be removed from river water by a common mechanism of colloid flocculation. 相似文献
163.
A survey of U.S. east coast estuaries confirms that large-scale rapid removal of iron from river water is a general phenomenon during estuarine mixing. The river-borne ‘dissolved’ iron consists almost entirely of mixed iron oxide-organic matter colloids, of diameter less than 0.45 μm, stabilized by the dissolved organic matter. Precipitation occurs on mixing because the seawater cations neutralize the negatively charged iron-bearing colloids allowing flocculation. The process has been duplicated in laboratory experiments using both natural filtered and unfiltered river water and a synthetic colloidal goethite in 0.05 μm filtered water. The colloidal nature of the iron has been further confirmed by ultracentrifugation and ultrafiltration. A major consequence of the precipitation phenomena is to reduce the effective input of ‘dissolved’ iron to the ocean by about 90% of the primary river value, equivalent to a concentration of less than 1 μmol per liter of river water. 相似文献
164.
Margaret Lois Delaney Allan W.H.Bé Edward A. Boyle 《Geochimica et cosmochimica acta》1985,49(6):1327-1341
Constant-temperature laboratory culture experiments of the planktonic foraminiferal species Globigerinoides sacculifer (Brady) suggest that the ratios of Li and Sr to Ca in the shells are a function of these ratios in the culture solutions. and in the shells did not vary with changes of these ratios in the culture solution. These are the first direct determinations of the relationship between foraminiferal shell chemistry and solution composition.The possibility of temperature dependence for the minor elemental composition of foraminiferal shells was also investigated in the laboratory and by analysis of several planktonic and one benthic foraminiferal species from sediment trap and sediment core samples. The , , and ratios in the natural samples roughly correlate with calcification temperature, whereas differences in the Li/Ca ratios are small and not systematically related to temperature. However, laboratory culture experiments at 20°C and 30°C showed no variation in the , , , and ratios with calcification temperature for the planktonic foraminifera G. sacculifer and Orbulina universa. Therefore, observed differences in the , , and ratios for the sediment trap and core foraminiferal samples cannot be ascribed to direct effects of calcification temperature, but may be due to some other environmental factor which is correlated with temperature. 相似文献
165.
E. Boyle R. Collier A.T. Dengler J.M. Edmond A.C. Ng R.F. Stallard 《Geochimica et cosmochimica acta》1974,38(11):1719-1728
A general model is presented for mixing processes between river and ocean water in which are established criteria for the identification of any non-conservative behavior of the dissolved constituents involved. A review of previous data shows that in no case has removal of silica been demonstrated unambiguously in estuarine regimes. New data for iron which show highly non-conservative behavior are used in an example of the application of the model. 相似文献
166.
The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the 238U/235U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferromanganese crusts/nodules and BIFs) of ∼1.3‰, exceeding by far the analytical precision of our method (≈0.06‰, 2SD). U isotopes were analyzed with MC-ICP-MS using a mixed 236U-233U isotopic tracer (double spike) to correct for isotope fractionation during sample purification and instrumental mass bias. The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Light U isotope compositions (relative to SRM-950a) were observed for manganese crusts from the Atlantic and Pacific oceans, which display δ238U of −0.54‰ to −0.62‰ and for three of four analyzed Banded Iron Formations, which have δ238U of −0.89‰, −0.72‰ and −0.70‰, respectively. High δ238U values are observed for black shales from the Black Sea (unit-I and unit-II) and three Kupferschiefer samples (Germany), which display δ238U of −0.06‰ to +0.43‰. Also, suboxic sediments have slightly elevated δ238U (−0.41‰ to −0.16‰) compared to seawater, which has δ238U of −0.41 ± 0.03‰. Granites define a range of δ238U between −0.20‰ and −0.46‰, but all analyzed basalts are identical within uncertainties and slightly lighter than seawater (δ238U = −0.29‰).Our findings imply that U isotope fractionation occurs in both oxic (manganese crusts) and suboxic to euxinic environments with opposite directions. In the first case, we hypothesize that this fractionation results from adsorption of U to ferromanganese oxides, as is the case for Mo and possibly Tl isotopes. In the second case, reduction of soluble UVI to insoluble UIV probably results in fractionation toward heavy U isotope compositions relative to seawater. These findings imply that variable ocean redox conditions through geological time should result in variations of the seawater U isotope compositions, which may be recorded in sediments or fossils. Thus, U isotopes might be a promising novel geochemical tracer for paleo-redox conditions and the redox evolution on Earth. The discovery that 238U/235U varies in nature also has implications for the precision and accuracy of U-Pb dating. The total observed range in U isotope compositions would produce variations in 207Pb/206Pb ages of young U-bearing minerals of up to 3 Ma, and up to 2 Ma for minerals that are 3 billion years old. 相似文献
167.
168.
Seth G. John Olivier J. Rouxel Paul R. Craddock Alison M. Engwall Edward A. Boyle 《Earth and Planetary Science Letters》2008,269(1-2):17-28
Many of the heaviest and lightest natural zinc (Zn) isotope ratios have been discovered in hydrothermal ore deposits. However, the processes responsible for fractionating Zn isotopes in hydrothermal systems are poorly understood. In order to better assess the total range of Zn isotopes in hydrothermal systems and to understand the factors which are responsible for this isotopic fractionation, we have measured Zn isotopes in seafloor hydrothermal fluids from numerous vents at 9–10°N and 21°N on the East Pacific Rise (EPR), the TAG hydrothermal field on the Mid-Atlantic Ridge, and in the Guaymas Basin. Fluid δ66Zn values measured at these sites range from + 0.00‰ to + 1.04‰. Of the many physical and chemical parameters examined, only temperature was found to correlate with fluid δ66Zn values. Lower temperature fluids (< 250 °C) had both heavier and more variable δ66Zn values compared to higher temperature fluids from the same hydrothermal fields. We suggest that subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite (Zn sulfide), and that this process is a primary cause of Zn isotope variation in hydrothermal fluids. Thermodynamic calculations carried out to determine saturation state of sphalerite in the vent fluids support this hypothesis with isotopically heaviest Zn found in fluids that were calculated to be saturated with respect to sphalerite. We have also measured Zn isotopes in chimney sulfides recovered from a high-temperature (383 °C) and a low-temperature (203 °C) vent at 9–10°N on the EPR and, in both cases, found that the δ66Zn of chimney minerals was lighter or similar to the fluid δ66Zn. The first measurements of Zn isotopes in hydrothermal fluids have revealed large variations in hydrothermal fluid δ66Zn, and suggest that subsurface Zn sulfide precipitation is a primary factor in causing variations in fluid δ66Zn. By understanding how chemical processes that occur beneath the seafloor affect hydrothermal fluid δ66Zn, Zn isotopes may be used as a tracer for studying hydrothermal processes. 相似文献
169.