全文获取类型
收费全文 | 444篇 |
免费 | 150篇 |
国内免费 | 133篇 |
专业分类
测绘学 | 36篇 |
大气科学 | 30篇 |
地球物理 | 106篇 |
地质学 | 418篇 |
海洋学 | 21篇 |
天文学 | 8篇 |
综合类 | 34篇 |
自然地理 | 74篇 |
出版年
2023年 | 12篇 |
2022年 | 24篇 |
2021年 | 12篇 |
2020年 | 20篇 |
2019年 | 30篇 |
2018年 | 28篇 |
2017年 | 14篇 |
2016年 | 14篇 |
2015年 | 19篇 |
2014年 | 25篇 |
2013年 | 18篇 |
2012年 | 20篇 |
2011年 | 27篇 |
2010年 | 27篇 |
2009年 | 31篇 |
2008年 | 29篇 |
2007年 | 13篇 |
2006年 | 13篇 |
2005年 | 12篇 |
2004年 | 24篇 |
2003年 | 18篇 |
2002年 | 17篇 |
2001年 | 19篇 |
2000年 | 12篇 |
1999年 | 13篇 |
1998年 | 20篇 |
1997年 | 19篇 |
1996年 | 19篇 |
1995年 | 7篇 |
1994年 | 13篇 |
1993年 | 17篇 |
1992年 | 11篇 |
1991年 | 10篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 3篇 |
1987年 | 10篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 6篇 |
1983年 | 9篇 |
1959年 | 4篇 |
1939年 | 5篇 |
1937年 | 3篇 |
1934年 | 4篇 |
1932年 | 4篇 |
1930年 | 3篇 |
1927年 | 2篇 |
1926年 | 7篇 |
1924年 | 6篇 |
排序方式: 共有727条查询结果,搜索用时 15 毫秒
1.
2.
We have previously demonstrated that medaka CYP3A is associated with metabolism of several endobiotics including steroids and bile acids. In this study, we demonstrate that medaka CYP3A catalysis exhibits unusual kinetic behaviors consistent with allosteric interaction which cannot be described by hyperbolic kinetic models. Using 7-benzyloxy-4-(trifluoromethyl)-coumarin (BFC) and nonylphenol as CYP3A substrates, we describe both homotropic and heterotropic cooperative interactions. Given the role of CYP3A in maintaining the homeostatic balance for numerous endobiotics, enzymatic activation/inhibition by endocrine disruptors (EDCs) represents a putative (non-genomic) mechanism for endocrine disruption. 相似文献
3.
4.
5.
6.
邹平铜矿处于齐河-广饶深大断裂带南部的邹平火山岩盆地中,形成于破火山口火山通道充填的石英正长闪长岩岩颈中央上部,包括伟晶岩型铜矿和细脉浸染状斑铜矿床两种类型。前者矿体较小,但品位高;后者品位较低,但规模中等。含矿石英正长闪长岩等密度小、磁性弱,故在火山岩系中呈现高背景重力场上的重力低和杂乱高磁场背景中的低负异常,即“重磁同低”,且高极化。重磁同低异常区和高极化率异常带,是本区寻找铜矿的有利部位。 相似文献
7.
2月15日.当人们尚沉浸在年味儿未尽的喜庆氛围时,位于九江市的二六七大队接到了市政府应急管理办的紧急通知:根据江西省政府指示,派员支援兄弟地市电力抢修工作。 相似文献
8.
Complete budgets for carbon and oxygen have been constructed for cyanobacterial mats dominated by Microcoleus chthonoplastes from the evaporating ponds of a salt works located in Guerrero Negro, Baja California Sur, Mexico. Included in the budget are measured rates of O2 production, sulfate reduction, and elemental exchange across the mat/brine interface, day and night, at various temperatures and times of the year. We infer from this data the various sinks for O2, as well as the sources of carbon for primary production. To summarize, although seasonal variability exists, a major percentage of the O2 produced during the day did not diffuse out of the mat but was used within the mat to oxidize both organic carbon and the sulfide produced by sulfate reduction. At night, most of the O2 that diffused into the mat was used to oxidize sulfide, with O2 respiration of minor importance. During the day, the internal mat processes of sulfate reduction and O2 respiration generated as much or more inorganic carbon (DIC) for primary production as diffusion into the mat. Also, oxygenic photosynthesis was the most important process of carbon fixation, although anoxygenic photosynthesis may have been important at low light levels during some times of the year. At night, the DIC lost from the mat was mostly from sulfate reduction. Elemental fluxes across the mat/brine interface indicated that carbon with an oxidation state of greater than zero was taken up by the mat during the day and liberated from the mat at night. Overall, carbon with an average oxidation state of near zero accumulated in the mat. Both carbon fixation and carbon oxidation rates varied with temperature by a similar amount. These mats are thus closely coupled systems where rapid rates of photosynthesis both require and fuel rapid rates of heterotrophic carbon oxidation. 相似文献
9.
Sulfur isotope fractionation during bacterial sulfate reduction in organic-rich sediments 总被引:18,自引:0,他引:18
Isotope fractionation during sulfate reduction by natural populations of sulfate-reducing bacteria was investigated in the cyanobacterial microbial mats of Solar Lake, Sinai and the sediments of Logten Lagoon sulfuretum, Denmark. Fractionation was measured at different sediment depths, sulfate concentrations, and incubation temperatures. Rates of sulfate reduction varied between 0.1 and 37 micromoles cm-3 d-1, with the highest rates among the highest ever reported from natural sediments. The depletion of 34S during dissimilatory sulfate reduction ranged from 16% to 42%, with the largest 34S-depletions associated with the lowest rates of sulfate reduction and the lowest 34S-depletions with the highest rates. However, at high sulfate reduction rates (>10 micromoles cm-3 d-1) the lowest fractionation was 20% independent of the rates. Overall, there was a similarity between the fractionation obtained by the natural populations of sulfate reducers and previous measurements from pure cultures. This was somewhat surprising given the extremely high rates of sulfate reduction in the experiments. Our results are explained if we conclude that the fractionation was mainly controlled by the specific rate of sulfate reduction (mass cell-1 time-1) and not by the absolute rate (mass volume-1 time-1). Sedimentary sulfides (mainly FeS2) were on average 40% depleted in 34S compared to seawater sulfate. This amount of depletion was more than could be explained by the isotopic fractionations that we measured during bacterial sulfate reduction. Therefore, additional processes contributing to the fractionation of sulfur isotopes in the sediments are indicated. From both Solar Lake and Logten Lagoon we were able to enrich cultures of elemental sulfur-disproportionating bacteria. We suggest that isotope fractionation accompanying elemental sulfur disproportionation contributes to the 34S depletion of sedimentary sulfides at our study sites. 相似文献
10.
Canfield DE 《Geochimica et cosmochimica acta》1997,61(16):3349-3365