全文获取类型
收费全文 | 4333篇 |
免费 | 176篇 |
国内免费 | 92篇 |
专业分类
测绘学 | 158篇 |
大气科学 | 302篇 |
地球物理 | 1128篇 |
地质学 | 1528篇 |
海洋学 | 389篇 |
天文学 | 709篇 |
综合类 | 22篇 |
自然地理 | 365篇 |
出版年
2022年 | 31篇 |
2021年 | 48篇 |
2020年 | 47篇 |
2019年 | 64篇 |
2018年 | 85篇 |
2017年 | 105篇 |
2016年 | 121篇 |
2015年 | 112篇 |
2014年 | 137篇 |
2013年 | 260篇 |
2012年 | 132篇 |
2011年 | 233篇 |
2010年 | 166篇 |
2009年 | 229篇 |
2008年 | 239篇 |
2007年 | 212篇 |
2006年 | 193篇 |
2005年 | 175篇 |
2004年 | 136篇 |
2003年 | 164篇 |
2002年 | 138篇 |
2001年 | 88篇 |
2000年 | 90篇 |
1999年 | 63篇 |
1998年 | 70篇 |
1997年 | 56篇 |
1996年 | 67篇 |
1995年 | 53篇 |
1994年 | 55篇 |
1993年 | 43篇 |
1992年 | 49篇 |
1991年 | 42篇 |
1990年 | 35篇 |
1989年 | 39篇 |
1988年 | 37篇 |
1987年 | 40篇 |
1986年 | 45篇 |
1985年 | 59篇 |
1984年 | 47篇 |
1983年 | 52篇 |
1982年 | 52篇 |
1981年 | 58篇 |
1980年 | 48篇 |
1979年 | 40篇 |
1978年 | 47篇 |
1977年 | 28篇 |
1976年 | 30篇 |
1975年 | 25篇 |
1974年 | 31篇 |
1973年 | 30篇 |
排序方式: 共有4601条查询结果,搜索用时 31 毫秒
991.
Dounia Hamoutene Anne Mathieu Paul Hofmann Jean-Pierre Salaun Marc Lafaurie 《Marine environmental research》1995,39(1-4)
The capacity of the mammalian liver microsomal P-450-dependent systems to metabolize a wide variety of endogenous and exogenous compounds is thought to reflect the presence of multiple forms of P-450 haemoproteins with broad and overlapping substrate specificity. In plants, the functions and specificity of cytochrome P-450 systems are less well known.This study was designed to prepare and characterize subcellular fractions from fresh sheaths (basal parts of leaves) of a mediterranean seagrass Posidonia oceanica (Linnaeus) Delile, the aim being the preparation of a microsomal fraction suitable for studying xenobiotic metabolism. The purity of the different fractions obtained by centrifugation, as well as the recovery of different organelles, was determined using enzyme markers (cytochrome c oxidase, alkaline phosphatase, glucose-6-phosphatase) and morphological examination by transmission electron microscopy. Some assays of enzymes involved in xenobiotic metabolism (cytochrome c reductase, laurate hydroxylase, ethoxyresorufin O-deethylase and glutathione-S-transferase) were also performed on different fractions of the preparation. The subcellular distribution for drug metabolism and marker enzymes showed a loss of endoplasmic reticulum in the pellet obtained after the first centrifugation, but the microsomal fraction was relatively free of mitochondria and fragments of the plasma membrane.Some assays are still being performed to avoid the small loss of endoplasmic reticulum experienced with the first pellet. However, the microsomes prepared in this study from sheaths of Posidonia oceanica appear suitable for further investigation of xenobiotic metabolism. 相似文献
992.
Bioassays were conducted with sea urchin and sand dollar sperm to determine the toxicity of chlorinated and unchlorinated sewage effluent and chlorinated and brominated seawater. The sperm cells were exposed to seawater dilutions of each toxicant for 5–15 min. The fertilisation of eggs served as the indicators of sperm viability. The effective concentrations which reduced fertilisation success by 50% (EC50) averaged 2·2 and 4·8% chlorinated and unchlorinated sewage in seawater, respectively. The sperm cells were extremely sensitive to chlorinated seawater at concentrations from 0·002 to 0·020 mg/litre total residual oxidant (TRO). Brominated seawater proved toxic to sperm in one test at 0·015 mg/litre TRO. Results of the sperm bioassays are compared with previous acute and chronic bioassays with fish. 相似文献
993.
Paul Farrimond Paul Comet Geoffrey Eglinton Richard P. Evershed Michael A. Hall David W. Park Anthony M.K. Wardroper 《Marine and Petroleum Geology》1984,1(4):340-354
Results are presented from an organic geochemical investigation of a suite of rock samples taken from the Upper Kimmeridge Clay near Kimmeridge, Dorset. All samples contain immature organic matter of marine origin, although one horizon, the Whitestone Band, contains an additional secondary input of partially biodegraded mature hydrocarbons, due to an oil seepage of unknown origin. With the exception of increased relative abundances of 4-methylsteroidal hydrocarbons in the more organic-rich samples, the immature molecular distributions are very similar, suggesting a consistent source of organic matter. The results are in agreement with the palaeoenvironmental model proposed by Tyson et al. (1979) for the deposition of the Kimmeridge Clay, where the different lithologies are controlled by a fluctuating oxic/anoxic boundary, with only the organic-poor mudstones being deposited in relatively oxygenated waters. 相似文献
994.
Barun K. Sen Gupta Michel Pujos J.- C. Pons James J. Galluzzo Paul Aharon M. Parra 《Geo-Marine Letters》1991,11(1):2-8
Clay mineralogy and trace-element geochemistry of two abyssal cores indicate that the primary source of surface-current-transported
detrital material in the southern Grenada Basin changed from a continental, South American terrane to a volcanic, Lesser Antillean
terrane at the end of the Pleistocene. The record of benthic foraminiferal assemblages demonstrates that the Caribbean Bottom
Water (CBW) was relatively oxygen poor and less corrosive in late Pleistocene glacial times than in interglacial times. The
change in the properties of CBW in the Holocene was related to a renewed influx of North Atlantic Deep Water in the Caribbean. 相似文献
995.
Paul J. Harrison 《Journal of Oceanography》2002,58(2):259-264
The Northeast Pacific has one of the longest time series of any open ocean station, primarily as a result of the weathership
station at Station P from the 1950s to 1981. This review summarizes our understanding of the plankton ecosystem for this station
and examines interannual variability for the primary producers. The weathership era characterized a period of high temporal
sampling resolution with a limited number of parameters being measured. In contrast, the post-weathership period focussed
on seasonal sampling (usually three times per year), but a wider range of parameters were measured and sediment traps were
deployed to estimate carbon and opal flux rates. The mixed layer depth is shallow compared to the Atlantic Ocean, ranging
from 40 to 120 m in late summer and winter respectively. Nitrate, silicate and phosphate are saturating year round with only
a few exceptions in the 1970s. Winter and summer Si:N ratios are the same (1.5:1). Ammonium and urea are 0.5 uM in winter
and near detection limits (∼0.1 uM) in late summer. Iron is limiting (∼0.05 nM) in late spring and summer for the growth of
large diatoms, but iron is co-limiting with irradiance in winter. Chlorophyll and primary productivity are low and show little
seasonal variation (about 2 times). Summer chl is about 20 mg m−2 while primary productivity ranges from 400–850 mg C m−2d−1. The f-ratio of 0.25 does not vary with seasons and indicates that primary productivity is fueled by regenerated nitrogen
(e.g. NH4 and urea). Small cells (<5 um) are normally abundant and they utilize regenerated nitrogen produced by the micrograzers;
they do not appear to be Fe-limited, but rather controlled by the micrograzers. Shipboard carboy experiments indicate that
large diatoms become dominant when iron is added. Therefore top down control is exerted by the micrograzers on the small cells,
while there is bottom up control of the large phytoplankton due to low Fe concentrations.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
996.
997.
Eucampia antarctica (Castr.) Mangin abundance curves for two piston cores from the western Agulhas Basin (southeast Atlantic sector of the Southern Ocean) were used to place volcanic glass shard and ice-rafted debris abundance curves into a stratigraphic framework for the last 100,000 years. A direct correlation is shown between increased abundance of E. antarctica, tephra and ice-rafted debris; low abundances are often characterized by calcareous sediment in the northern core. Peaks in abundance of E. antarctica have been interpreted as an indication of glacial periods and the increased ice-rafted debris and tephra during glacial periods is evidence for an ice-rafted origin for the tephra.Microprobe geochemical analysis of volcanic glass shards provides no single distinct source for the ash. The geochemical data is consistent, but not definitive, with a primary source in the Scotia Arc and a minor source from Bouvet Island. Between core correlations of geochemically “fingerprinted” dispersed tephra zones were not successful due to variability of glass shard geochemistry and limited sample size. 相似文献
998.
999.
Paul A. Hwang 《Journal of Oceanography》2004,60(5):835-841
Surface waves are the roughness element of the ocean surface, the air-sea interaction processes are influenced by the wave
conditions. The dynamic influence of surface waves decays exponentially with distance from the air-water interface. The relevant
length scale characterizing the decay rate is the wavelength. The parameterization of drag coefficient and surface roughness
can be significantly improved by using wavelength as the reference length scale of atmospheric measurements. The wavelength
scaling of drag coefficient and dynamic roughness also receives support from theoretical studies of wind and wave coupling. 相似文献
1000.