全文获取类型
收费全文 | 3013篇 |
免费 | 153篇 |
国内免费 | 46篇 |
专业分类
测绘学 | 67篇 |
大气科学 | 259篇 |
地球物理 | 836篇 |
地质学 | 1019篇 |
海洋学 | 249篇 |
天文学 | 525篇 |
综合类 | 9篇 |
自然地理 | 248篇 |
出版年
2022年 | 24篇 |
2021年 | 57篇 |
2020年 | 62篇 |
2019年 | 63篇 |
2018年 | 116篇 |
2017年 | 88篇 |
2016年 | 124篇 |
2015年 | 85篇 |
2014年 | 99篇 |
2013年 | 177篇 |
2012年 | 114篇 |
2011年 | 153篇 |
2010年 | 133篇 |
2009年 | 169篇 |
2008年 | 155篇 |
2007年 | 171篇 |
2006年 | 130篇 |
2005年 | 109篇 |
2004年 | 98篇 |
2003年 | 94篇 |
2002年 | 85篇 |
2001年 | 44篇 |
2000年 | 48篇 |
1999年 | 45篇 |
1998年 | 41篇 |
1997年 | 33篇 |
1996年 | 37篇 |
1995年 | 33篇 |
1994年 | 39篇 |
1993年 | 23篇 |
1992年 | 35篇 |
1991年 | 33篇 |
1990年 | 40篇 |
1989年 | 26篇 |
1988年 | 22篇 |
1987年 | 18篇 |
1986年 | 19篇 |
1985年 | 30篇 |
1984年 | 36篇 |
1983年 | 29篇 |
1982年 | 21篇 |
1981年 | 33篇 |
1980年 | 24篇 |
1979年 | 24篇 |
1978年 | 24篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 13篇 |
1974年 | 14篇 |
1973年 | 16篇 |
排序方式: 共有3212条查询结果,搜索用时 0 毫秒
21.
On October 3 and 4, 1986, DSRVAlvin dives encountered a strong current at 2,300 m in South Wilmington Canyon. The current, estimated at 1 knot, transported surficial
sediment and constructed and modified bedforms. It appears to have been constant in its direction of flow from 30 to 40°.
The observed current was probably a burst of fast flow in a region of slow average currents in the Deep Western Boundary Undercurrent.
Such episodic events may have a greater influence on the stratigraphic record than the temporally longer more tranquil flow
conditions. 相似文献
22.
Charles W. Holmes 《Geo-Marine Letters》1981,1(3-4):237-241
The plot of210Pb activity against depth in carbonate sands on the Virgin Island Bank is a negative asymmetric hyperbolic curve. As depth
increases, an initial rapid decrease in210Pb activity caused by the decay of unsupported210Pb and226Ra is followed by increasing activity as a result of210Pb achieving equilibrium with ingrowing230Th. As this curve is time dependent, an estimate of the relative ages in carbonate sequences and the rates of net carbonate
accumulation can be made. The ease of210Pb activity determinations makes this procedure an attractive method in obtaining carbonate sand accumulation rates. 相似文献
23.
24.
The North Atlantic right whale, a seriously endangered species, is found in Cape Cod Bay (Massachusetts, USA) during the winter and early spring. During their residency in these waters, these whales are frequently observed feeding. This study evaluated spatial and temporal changes in the chemical composition (carbon weight and C/N ratio) of the food resource targeted by the right whales in Cape Cod Bay. The three taxa measured (Centropages typicus, Pseudocalanus spp., and Calanus finmarchicus) had highly variable chemical compositions resulting from the different life strategies and from fluctuations in their surrounding environment. The impact of seasonal variability in the energy densities of the food resource of right whales was calculated and compared to the energetic requirements of these whales. Calculations indicated that differences in the nutritional content of the zooplankton prey in Cape Cod Bay could have a considerable effect on the nutrition available to the right whales. Therefore, it is likely that using more precise estimates of the energetic densities of the prey of right whales would lead to a re‐evaluation of the adequacy of the food resource available to these whales in the North Atlantic. 相似文献
25.
26.
27.
Lysosomal destabilization assays have been used as valuable biomarkers of pollutant exposures in a variety of bivalve and fish species. The responses of oysters, Crassostrea virginica, deployed at and native to various reference and degraded sites were evaluated for lysosomal destabilization during both summer and winter seasons. In both native and deployed oysters, lysosomal destabilization rates tended to be higher during the winter at both reference and polluted sites. There are at least two hypothetical explanations. Greater lysosomal destabilization rates may be related to physiological changes associated with mobilization of nutrient reserves during the winter and gametogenesis. However, lysosomal destabilization in deployed oysters was correlated with tissue metal concentrations. These data also support a second hypothesis that seasonal differences in physico-chemical factors (such as reduced levels of acid volatile sulfides) may increase the bioavailability of metals during the winter so that adverse effects are more pronounced. 相似文献
28.
29.
Hayes RA Regondi S Winter MJ Butler PJ Agradi E Taylor EW Kevin Chipman J 《Marine environmental research》2004,58(2-5):665-669
Metallothionein has been assayed in a range of aquatic animal tissues as an indicator of metal exposure. We sequenced chub (Leuciscus cephalus) metallothionein cDNA which showed over 90% homology to common carp, goldfish and stone loach and 77% homology to rainbow trout sequences for metallothionein. We then used the extended primer method to develop an accurate quantitative competitive RT-PCR assay for metallothionein mRNA. RT-PCR was used to measure metallothionein mRNA in feral chub from a range of field sites, with different levels of heavy metal pollution, in the West Midlands, UK. Measurements were complemented by analysis of liver and gill metallothionein protein by capillary electrophoresis. There was no significant difference in the metallothionein protein levels between fish of different rivers and there was no evidence of elevation of mRNA at the sites of highest metal exposure. The level of metal exposure (e.g. zinc, nickel and cadmium each ranging between 15 and 28 microg/l ) at the pH (7.5-8.5) of these rivers appears insufficient to elevate hepatic or gill metallothionein in chub. A lack of elevation of hepatic metallothionein mRNA in chub exposed to zinc, copper and manganese for 24 h and 10 days in the laboratory also suggests a non-responsiveness of this species. 相似文献
30.
本文报导了应用纤维素乙酸平板电泳方法,测定采于中国山东省东方红盐场卤虫(Artemia)的同功酶。经测定,8种同功酶其中有6种是多态的,即:苹果酸脱氢酶(MDH)、异柠檬酸酶(IDH)、谷氨酸草醋酸转移酶(GOT)、葡糖-6-磷酸盐脱氢酶(D6PHD)、6-磷酸萄糖脱氢酶(6PGDH)和磷酸葡糖异构酶(PGI)。本文所提供的研究方法,不仅适用于基础研究,而且在水产养殖业上有助于对卤虫的筛选和确证。 相似文献