全文获取类型
收费全文 | 291篇 |
免费 | 23篇 |
国内免费 | 35篇 |
专业分类
测绘学 | 47篇 |
大气科学 | 54篇 |
地球物理 | 68篇 |
地质学 | 98篇 |
海洋学 | 35篇 |
天文学 | 6篇 |
综合类 | 15篇 |
自然地理 | 26篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 6篇 |
2020年 | 4篇 |
2019年 | 5篇 |
2018年 | 8篇 |
2017年 | 9篇 |
2016年 | 7篇 |
2015年 | 4篇 |
2014年 | 10篇 |
2013年 | 35篇 |
2012年 | 14篇 |
2011年 | 14篇 |
2010年 | 6篇 |
2009年 | 26篇 |
2008年 | 22篇 |
2007年 | 19篇 |
2006年 | 19篇 |
2005年 | 23篇 |
2004年 | 15篇 |
2003年 | 7篇 |
2002年 | 13篇 |
2001年 | 9篇 |
2000年 | 4篇 |
1999年 | 8篇 |
1998年 | 6篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 10篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
排序方式: 共有349条查询结果,搜索用时 31 毫秒
81.
82.
Oxygen depletion off the Changjiang (Yangtze River) Estuary 总被引:25,自引:0,他引:25
In a survey on the Yellow Sea and the East China Sea on August 20-30 of 1999, we found a hypoxic zone (<2 mg/L) of 13700 km2 with an average thickness of 20m at the bottom of the Changjiang (Yangtze River) Estuary, with an oxygen minimum value of
1 mg/L. The extension of the dissolved oxygen deficiency extended to the 100m isobath in a southeastward direction along the
bottom of the continental shelf of the East China Sea. During the last two decades, the minimum dissolved oxygen values in
the low oxygen region of the Changjiang Estuary have decreased from 2.85 mg/L to 1 mg/L. In the hypoxic zone, the apparent
oxygen utilization (AOU) was 5.8 mg/L and the total oxygen depletion approximately 1.59 × 106t. The strong halocline above the hypoxic zone, as a result of affluent water from the Changjiang, Taiwan Warm Current (TWC),
and the high concentrations of particle organic carbon (POC) and nitrogen (PON) are the major factors causing the formation
of the hypoxic zone. The POC: PON ratios and nutrient concentration distributions in the hypoxic zone suggest that the oxygen
deficiency in the bottom water during the summer in the East China Sea off the Changjiang is the result of organic carbon
production enhanced by nutrients from the Changjiang and fluvial organic matter input, followed by a shift in regeneration
of nutrients in the East China Sea. 相似文献
83.
C. J. Farrugia H. K. Biernat N. V. Erkaev L. M. Kistler G. Le C. T. Russell 《Annales Geophysicae》1998,16(5):518-527
We compare numerical results obtained from a steady-state MHD model of solar wind flow past the terrestrial magnetosphere with documented observations made by the AMPTE/IRM spacecraft on 24 October, 1985, during an inbound crossing of the magnetosheath. Observations indicate that steady conditions prevailed during this about 4 hour-long crossing. The magnetic shear at spacecraft entry into the magnetosphere was 15°. A steady density decrease and a concomitant magnetic field pile-up were observed during the 40 min interval just preceding the magnetopause crossing. In this plasma depletion layer (1) the plasma beta dropped to values below unity; (2) the flow speed tangential to the magnetopause was enhanced; and (3) the local magnetic field and velocity vectors became increasingly more orthogonal to each other as the magnetopause was approached (Phan et al., 1994). We model parameter variations along a spacecraft orbit approximating that of AMPTE/IRM, which was at slightly southern GSE latitudes and about 1.5 h postnoon Local Time. We model the magnetopause as a tangential discontinuity, as suggested by the observations, and take as input solar wind parameters those measured by AMPTE/IRM just prior to its bow shock crossing. We find that computed field and plasma profiles across the magnetosheath and plasma depletion layer match all observations closely. Theoretical predictions on stagnation line flow near this low-shear magnetopause are confirmed by the experimental findings. Our theory does not give, and the data on this pass do not show, any localized density enhancements in the inner magnetosheath region just outside the plasma depletion layer. 相似文献
84.
85.
De-bugging the 'millipede' porphyroblast microstructure: a serial thin-section study and 3-D computer animation 总被引:1,自引:0,他引:1
ABSTRACT Seventy-five spatially orientated, serial thin sections cut from a single rock containing ‘millipede’ porphyroblast microstructure from the Robertson River Metamorphics, Australia, reveal the three-dimensional (3-D) geometry of oppositely concave microfolds (OCMs) that define the microstructure. Electronic animations showing progressive serial sections of the 3-D microstructure are made available via the World Wide Web. The OCM amplitudes decrease regularly from a maximum in near-central sections to a minimum in near-marginal sections, whereas the OCM interlimb angles increase regularly from a minimum in near-central sections to a maximum in near-marginal sections. These observations illustrate that the OCMs are noncylindrical surfaces with culminations located in near-central sections. Because the porphyroblast cores appear to have been present before significant development of the syn-OCM foliation, all of the OCMs were formed by heterogeneous extension around these cores. The overall geometry of the OCMs described here reflects the strain state, and cannot be used to constrain deformation paths. 相似文献
86.
着重介绍了国际二号码与ASCII码相互转换及用TURBOBASIC实现异步串行通信时的一些关键技术的解决办法。 相似文献
87.
Rate constants for the reaction of hydroxyl radicals with dibromomethanehave been measured by discharge flow-resonance fluorescence technique(DF-RF) over the temperature range 288–368 K. The derived Arrheniusequation is k1=(1.51 ± 0.37)× 10-12 exp(-(720 ±60)/T) cm3 molec.-1 s-1.The tropospheric lifetime of dibromomethane has been estimated to be 0.29years. An ozone depletion potential (ODP) value of 0.10 for dibromomethanehas been obtained. 相似文献
88.
Implications of ozone depletion for surface-water photochemistry: Sensitivity of clear lakes 总被引:1,自引:0,他引:1
Norman M. Scully Warwick F. Vincent David R. S. Lean William J. Cooper 《Aquatic Sciences - Research Across Boundaries》1997,59(3):260-274
Increased ultraviolet-B radiation (UV-B) in aquatic ecosystems brought about by stratospheric ozone depletion may result in
increased formation rates of photochemical reaction products in the surface waters of aquatic ecosystems. In this study, the
potential impact of increases in lower wave-length radiation on the formation of hydrogen peroxide (H2O2) and singlet oxygen (1O2) was modelled for lakes over a range of dissolved organic carbon (DOC) concentrations. The impact of increased UV-B radiation
on the production of carbon monoxide (CO) was also calculated for two humic stained systems. The relative increases of H2O2 and1O2 production were greater in the clear lakes (70% increase in photochemical reaction rates near the surface) than the coloured
systems (25%). Production of CO in the study sites also increased under depleted ozone conditions implying increased DOC losses
to the overlying atmosphere. 相似文献
89.
Previous zero-dimensional photochemical calculations indicate that multiple tropospheric steady states may exist, in which different NO
x
(NO+NO2) levels could be supported by the same source of NO
x
. To investigate this possibility more closely, a one-dimensional photochemical model has been used to estimate the rate of removal of atmospheric NO
x
compounds at different NO
x
levels. At low NO
x
levels NO
x
is photochemically converted to HNO3, which is removed by either wet or dry deposition. At high NO
x
levels formation of HNO3 is inhibited, and NO
x
is removed by a variety of other processes, including rainout of N2O4 and N2O5, surface deposition of NO and NO2, and direct dissolution of NO and NO2 in rainwater. Multiple steady states are possible if surface deposition of NO
x
is relatively inefficient. The NO
x
source required to trigger high atmospheric NO
x
levels is approximately 10 to 15 times the present global emission rate-less than half the source strength predicted by the zero-dimensional model. NO
x
mixing ratios in excess of 10-7 would cause severe damage to the ozone layer and could result in either a climatic warming or cooling, depending upon the amount of NO2 present. 相似文献
90.
江苏东海芝麻房预先导孔(CCSD-PPl)橄榄岩及其反映的上地幔亏损和交代事件 总被引:10,自引:2,他引:10
芝麻房预先导孔橄榄岩岩石类型有二辉橄榄岩、方辉橄榄岩、单辉橄榄岩和纯橄榄岩,其中前3种多数含有石榴子石。这些岩石在垂向上显示成层性,没有某种方向的韵律变化,它们之间呈渐变过渡关系。对样品进行主量元素、微量元素、稀土元素和Sr、Nd、O同位素分析表明,随着样品MgO含量的增高,Al2O3、CaO逐渐降低,呈良好的相关关系,并且大致上从石榴橄榄岩类到不含石榴子石的橄榄岩类主量元素的亏损程度增高,两者一般均比原始地幔主量元素亏损。稀土元素配分型式表现为右倾的轻稀土富集型,在微量元素蛛网图上,也表现为强不相容元素和微量元素到适度不相容元素的总体右倾型,与原始地幔相比,均富集强不相容元素。富集程度的不同与主量元素的亏损无对应关系,说明主量元素和微量元素各自的特点不是同一种事件造成的,而是先前地幔部分熔融作用之后又发生了富含不相容元素流体的交代作用。Sr、Nd同位素具有富集特点,变化范围较大,而且主量元素较亏损的岩石更富集Sr同位素,显示了交代富集的特点。稀土元素(Ce/Yb)x比值和εNd之间的负相关关系和Rb丰度与Sr同位素之间的正相关关系表明,富集Sr、Nd同位素的流体和富集不相容微量元素的流体均来自地幔。 相似文献