全文获取类型
收费全文 | 3565篇 |
免费 | 99篇 |
国内免费 | 39篇 |
专业分类
测绘学 | 54篇 |
大气科学 | 316篇 |
地球物理 | 821篇 |
地质学 | 1257篇 |
海洋学 | 339篇 |
天文学 | 494篇 |
综合类 | 10篇 |
自然地理 | 412篇 |
出版年
2021年 | 45篇 |
2020年 | 42篇 |
2019年 | 51篇 |
2018年 | 59篇 |
2017年 | 65篇 |
2016年 | 71篇 |
2015年 | 75篇 |
2014年 | 73篇 |
2013年 | 193篇 |
2012年 | 97篇 |
2011年 | 145篇 |
2010年 | 148篇 |
2009年 | 168篇 |
2008年 | 136篇 |
2007年 | 121篇 |
2006年 | 123篇 |
2005年 | 111篇 |
2004年 | 98篇 |
2003年 | 88篇 |
2002年 | 86篇 |
2001年 | 75篇 |
2000年 | 47篇 |
1999年 | 56篇 |
1998年 | 57篇 |
1997年 | 45篇 |
1996年 | 68篇 |
1995年 | 46篇 |
1994年 | 50篇 |
1993年 | 45篇 |
1992年 | 54篇 |
1991年 | 55篇 |
1990年 | 49篇 |
1989年 | 47篇 |
1988年 | 50篇 |
1987年 | 51篇 |
1986年 | 39篇 |
1985年 | 70篇 |
1984年 | 85篇 |
1983年 | 83篇 |
1982年 | 68篇 |
1981年 | 48篇 |
1980年 | 53篇 |
1979年 | 52篇 |
1978年 | 51篇 |
1977年 | 48篇 |
1976年 | 40篇 |
1975年 | 32篇 |
1974年 | 33篇 |
1973年 | 34篇 |
1972年 | 25篇 |
排序方式: 共有3703条查询结果,搜索用时 15 毫秒
321.
322.
323.
David C. Gerlach Hans G. AvéLallemant William P. Leeman 《Earth and Planetary Science Letters》1981,53(2):255-265
The Canyon Mountain ophiolite, Oregon, is exceptional in lacking sheeted dikes, basaltic pillow lavas, and sediments that are characteristic of many other ophiolites. Instead, the uppermost portion of the complex consists of a significant volume of plagiogranites, which, in addition to minor basalts, intrude a large section of keratophyres believed to be of volcanic origin. The trend of intrusive rocks and of bedding in the keratophyres is mostly parallel to layering in the underlying gabbroic cumulates and to contacts between units in the remainder of the ophiolite. It is suggested that the plagiogranites, basalts, and keratophyres comprise a sill complex. Both the plagiogranites and the keratophyres are similar, respectively, to low-K2O plutonic and extrusive rocks of island arcs. The mineralogy and penetrative deformation structures of the ultramafic and some of the gabbroic rocks of the ophiolite indicate greater depth of formation, related to magmatism and diapirism above a Benioff zone. Radiometric age dates of plagiogranites confine the minimum age of the complex to the Early Permian. The Canyon Mountain ophiolite may thus be correlative with other fragments of a Lower Permian arc terrane throughout northeastern Oregon which were chaotically mixed during renewed subduction in middle to late Triassic time. 相似文献
324.
325.
Surficial stratigraphic units of Aroostook County, Maine, have been mapped and formal stratigraphic names for these units are proposed. Evidence exists for at least two distinct glacial phases which are represented by three tills. Two of these tills were deposited penecontemporaneously either as the result of coalescing ice sheets or as the result of the thermal regime existing within a single ice sheet. The oldest till is named the St. Francis and is correlated with the Chaudière Till of southeastern Quebec. The other tills are named the Mars Hill and Van Buren tills, respectively, and are correlated with the Lennoxville till of southeastern Quebec. Interbedded stratified sediments associated with the St. Francis till are correlated with the Gayhurst Formation. Stratified sediments associated with Van Buren and Mars Hill tills are correlated with post Lennoxville sediments of Quebec. Granite-gneiss erratics of Canadian Shield provenance in the Van Buren till indicate advance of the Laurentide ice into northern Maine during late Wisconsinan time. Moraines in southern Aroostook County with associated outwash and eskers record general recession from coastal Maine. Recession occurred after the formation of the Pineo Ridge moraine in Maine and the St. Antonin-Highland Front moraine complex in Quebec. The Caribou-Winterville moraine complex in northern Maine marks the boundary between the penecontemporaneously deposited Van Buren and Mars Hill surface tills and is correlated with the Grand Falls moraine at Grand Falls, New Brunswick. 相似文献
326.
Broecker presented a steady-state, two-box model which emphasizes the role of kinetic factors in determining the chemical composition of sea water. Unlike thermodynamic models, Broecker's model suggests that sizable temporal variation in the composition of sea water and rates of sedimentation may have taken place in the past 100 million years. To describe the evolution of the ocean's chemical system and interpret variations observed in the sedimentary record, we have formulated a dynamical model. Mass-balance consideration leads to a set of coupled nonlinear differential equations. The equations are linearized and solved for step function changes in the rate of river input and in the rate of vertical mixing. This simple model of the ocean's chemical system is shown to be stable against oscillations. Using data for the modern ocean, the response times for P, C, Si, Ca, and Ba are calculated to be of the order of 104 to 106 yr for changes in river and other inputs and of the order 101 to 102 yr for a change in the rate of vertical mixing. Analog-simulation techniques, discussed for the situation of the two-box model, provide a powerful tool for treating nonlinearities and systems with more than two components. In the final section, consequences of the dynamical formulation are compared with parameters appearing in Broecker's steady-state formulation. 相似文献
327.
328.
Edward B. Evenson William R. Farrand Donald F. Eschman David M. Mickelson Louis J. Maher 《Quaternary Research》1976,6(3):411-424
New evidence from recent field and seismic investigations in the Lake Michigan basin and in the type areas of the Valders, Two Creeks and Two Rivers deposits necessitates revision of late-glacial ice-front positions, rock- and time-stratigraphic nomenclature and climatic interpretations and deglaciation patterns for the period ca. 14,000–7,000 radiocarbon years B.P. The previously reported and long accepted pattern of deglaciation for the Lake Michigan basin started with a regular retreat from the Lake Border Morainic System, with a minor oscillation marked by the Port Huron moraine(s) and then an extensive Twocreekan deglaciation followed by a major (320 km) post-Twocreekan advance (Valders). However, we now record a major retreat between the times of the Lake Border and Port Huron moraines, followed by a gradual retreat from the Port Huron limit and interrupted by a minor standstill (deposition of Manitowoc Till), a retreat (Twocreekan) and a readvance (Two Rivers Till). No Woodfordian or younger readvance was as extensive as had been the preceding one. This sequence argues for a normal, climatically controlled, progressive deglaciation rather than one interrupted by a major post-Twocreekan (formerly Valderan) surge. This revision appears finally to harmonize the geologic evidence and the palynological record for the Great Lakes region. Our investigations show that Valders Till from which the Valderan Substage was named is late-Woodfordian in age. We propose the term “Greatlakean” as a replacement for the now misleading time-stratigraphic term “Valderan”. The type section and the definition of the upper and lower boundaries of the Greatlakean Substage remain the same as those originally proposed for the Valderan Substage but the name is changed. 相似文献
329.
330.
William S. Reeburgh 《Earth and Planetary Science Letters》1976,28(3):337-344
Detailed measurements of CH4 in the water column and sediments of the Cariaco Trench show that CH4 is non-conservative in both environments. Concentration differences between the sediments and adjacent overlying water suggest that the sediments are the source of the water column CH4. Co-metabolism of CH4 by sulfate reducers appears to be the CH4 sink in anoxic environments. 相似文献