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281.
Holocene millennial-scale climate variations documented by multiple lake-level proxies in sediment cores from Hurleg Lake,Northwest China 总被引:2,自引:0,他引:2
Cheng Zhao Zicheng Yu Yan Zhao Emi Ito Kenneth P. Kodama Fahu Chen 《Journal of Paleolimnology》2010,44(4):995-1008
Millennial-scale climate variability has not been well documented in arid northwest China due to the scarcity of high-resolution,
well-dated paleoclimate records. Here we present multi-proxy records from sediment cores taken in freshwater Hurleg Lake on
the northeastern Tibetan Plateau, which reveal millennial-scale lake-level and climate variations over the past 8,000 years.
This high-elevation region is very sensitive to large-scale climate change, thus allowing us to better understand Holocene
climate variations in East Asia. The lake-level record, derived from lithology, magnetic mineralogy, carbonate isotopes, ostracode
shell isotopes and trace elements, X-ray fluorescence (XRF), and gray scale data, indicates a highly variable and generally
dry climate from 7.8 to 1 ka (1 ka = 1,000 cal year BP), and a relatively stable and wet climate after 1 ka. Superimposed
on this general trend, six dry intervals at 7.6–7.2 ka, 6.2–5.9 ka, 5.3–4.9 ka, 4.4–3.8 ka, 2.7–2.4 ka, and 1.7–1.1 ka were
detected from the high-resolution carbonate content and XRF data. The generally dry climate between 7.8 and 1 ka was almost
synchronous with the decrease of East Asian and Indian monsoon intensities shortly after 8 ka. The six dry intervals can be
correlated with weak monsoon events recorded in the East Asia and Indian monsoon regions, as well as the North Atlantic cold
events. Our data suggest that millennial-scale monsoon variations could cause highly variable climate conditions in arid northwest
China during the Holocene. These millennial-scale climate variations may reflect changes in solar variation and/or changes
in oceanic and atmospheric circulation. 相似文献
282.
Isotopic evolution and climate paleorecords: modeling boundary effects in groundwater-dominated lakes 总被引:1,自引:0,他引:1
We used an isotopic mass-balance model to examine how the hydrogeologic setting of lakes influences isotopic response of evaporating
lake water to idealized hydroclimatic changes. The model uses a monthly water and isotope balance approach with simplified
water-column structure and groundwater exchanges. The framework for comparative simulations is provided by lakes in a region
of the Northern Rocky Mountains that display high interlake geochemical variability, thought to be controlled by groundwater
hydraulics. Our analysis highlights several isotopic effects of flow between aquifers and lakes, leading to possible divergence
of isotopic paleorecords formed under a common climate. Amplitude of isotopic variation resulting from simulated climate forcing
was greatly damped when high groundwater fluxes and/or low lake volume resulted in low lake fluid residence time. Differing
precipitation and evaporation scenarios that are equivalent in annual fluid balance (P−E) resulted in different isotopic signatures,
interpreted as a result of evaporation kinetics. Concentrating low-δ groundwater inflow during spring months raised springtime
lake δ values, a counterintuitive result of coincidence between times of high groundwater inflow and the evaporation season.
Transient effects of reduced fluid balance caused excursions opposite in sign from eventual steady-state isotopic shifts resulting
from enhanced groundwater inflow dominance. Lags in response between climate forcing and isotopic signals were shortened by
high groundwater fluxes and resulting short lake residence times. Groundwater-lake exchange exerts control over patterns of
lake isotopic response to evaporation through effects on lake residence time, inflow composition, and seasonal timing of inflow
and outflow. Sediments from groundwater-linked lakes, often used for paleoenvironmental analysis, should be expected to reflect
isotopic complexities of the type shown here.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
283.
Eiji Ohtani Naohisa Hirao Tadashi Kondo Masayoshi Ito Takumi Kikegawa 《Physics and Chemistry of Minerals》2005,32(1):77-82
We observed a direct reaction of metallic iron with water to form iron hydride and iron oxide, 3Fe + H2O–>2FeHx + FeO, at pressures from 6 GPa to 84 GPa and temperatures above 1,000 K in diamond anvil cell (DAC). Iron hydride is dhcpFeHx or -FeHx, and iron oxide has the rhombohedral or B1 structure at pressures at least up to 37 GPa. The formation of an assembly composed of dhcpFeHx and FeO with the B8 structure was observed at 84 GPa. In primordial Earth, water formed by dehydration of the low temperature primitive materials reacts with metallic iron in the high temperature component to form iron hydride FeHx and iron oxide FeO. The former would be incorporated in the iron forming the core. Thus hydrogen could be an important element of the Earths core. This reaction would be essential for transport of hydrogen into the core in the accretion stage of the Earth. 相似文献
284.
285.
286.
We report new petrological, phase equilibria modeling, and fluid inclusion data for pelitic and mafic granulites from Rundv?gshetta in the highest-grade region of the Neoproterozoic Lützow-Holm Complex(LHC),East Antarctica, and provide unequivocal evidence for fluid-rock interaction and high-temperature metasomatism in the presence of brine fluid. The studied locality is composed dominantly of well-foliated pelitic granulite(K-feldspar+quartz+sillimanite+garnet+ilmenite) with foliation-parallel bands and/or layers of mafic granulite(plagioclase+orthopyroxene+garnet+ilmenite+quartz+biotite). The boundary between the two lithologies is defined by thin(about 1 -20 cm in thick) garnet-rich layers with a common mineral assemblage of garnet+plagioclase+quartz+ilmenite+biotite ? orthopyroxene. Systematic increase of grossular and decrease of pyrope contents in garnet as well as decreasing Mg/(Fe+Mg) ratio of biotite from the pelitic granulite to garnet-rich rock and mafic granulite suggest that the garnet-rich layer was formed by metasomatic interaction between the two granulite lithologies. Phase equilibria modeling in the system NCKFMASHTO demonstrates that the metasomatism took place at 850 -860℃, which is slightly lower than the peak metamorphism of this region, and the modal abundance of garnet is the highest along the metapeliteemetabasite boundary(up to 40%), which is consistent with the field and thin section observations. The occurrence of brine(7.0 -10.9 wt.% Na Cleqfor ice melting or 25.1 -25.5 wt.% NaC leqfor hydrohalite melting) fluid inclusions as a primary phase trapped within plagioclase in the garnet-rich layer and the occurrence of Cl-rich biotite(Cl = 0.22 -0.60 wt.%) in the metasomatic rock compared to that in pelitic(0.15 -0.24 wt.%) and mafic(0.06-0.13 wt.%) granulites suggest infiltration of brine fluid could have given rise to the high-temperature metasomatism. The fluid might have been derived from external sources possibly related to the formation of major suture zones formed during the Gondwana amalgamation. 相似文献
287.
288.
Jingeng Sha Masaki Matsukawa Huawei Cai Baoyu Jiang Makoto Ito Chengquan He Zhiwei Gu 《Cretaceous Research》2003,24(6):715-728
In eastern Heilongjiang, the Upper Jurassic is marine and restricted to the Suibin and Dong’an areas, where it is characterized faunally by Callovian–Volgian (Tithonian) bivalves and florally by dinoflagellates. The Lower Cretaceous is widely distributed in eastern Heilongjiang, and characterized faunally by Berriasian–Valanginian bivalves, Barremian–Albian ammonites and Aucellina, and florally by dinoflagellates. To the west, the marine facies grade into non-marine beds. Thus, in the east, for example in the Dong’an and Dajiashan areas, near the northwestern Palaeo-Pacific, the Lower Cretaceous is marine; westward, in the Yunshan, Longzhaogou, Peide, and Zhushan areas, marine and non-marine deposits alternate, whereas further west still, e.g. in the Jixi Basin, non-marine facies are intercalated with marine beds. This regional distribution is indicative of a large, shallow embayment opening eastwards to the Palaeo-Pacific; during the Early Cretaceous successive transgressive-regressive events influenced the climate and biota of eastern Heilongjiang and northeastern China. Many of the Lower Cretaceous sections contain abundant coals, demonstrating that in this region the Early Cretaceous was an important coal-forming period. Some non-marine bivalve species are common to the Lower Cretaceous Jixi Group of eastern Heilongjiang, the Jehol Group of western Liaoning and the Transbaikalian Group of Siberia, suggesting that these groups are of comparable Early Cretaceous age. 相似文献