全文获取类型
收费全文 | 2919篇 |
免费 | 662篇 |
国内免费 | 1247篇 |
专业分类
测绘学 | 9篇 |
大气科学 | 47篇 |
地球物理 | 244篇 |
地质学 | 4267篇 |
海洋学 | 79篇 |
天文学 | 13篇 |
综合类 | 104篇 |
自然地理 | 65篇 |
出版年
2024年 | 13篇 |
2023年 | 41篇 |
2022年 | 83篇 |
2021年 | 107篇 |
2020年 | 151篇 |
2019年 | 137篇 |
2018年 | 190篇 |
2017年 | 246篇 |
2016年 | 277篇 |
2015年 | 209篇 |
2014年 | 209篇 |
2013年 | 246篇 |
2012年 | 226篇 |
2011年 | 209篇 |
2010年 | 171篇 |
2009年 | 211篇 |
2008年 | 227篇 |
2007年 | 241篇 |
2006年 | 215篇 |
2005年 | 195篇 |
2004年 | 133篇 |
2003年 | 143篇 |
2002年 | 112篇 |
2001年 | 97篇 |
2000年 | 105篇 |
1999年 | 121篇 |
1998年 | 73篇 |
1997年 | 73篇 |
1996年 | 65篇 |
1995年 | 59篇 |
1994年 | 51篇 |
1993年 | 35篇 |
1992年 | 36篇 |
1991年 | 19篇 |
1990年 | 25篇 |
1989年 | 19篇 |
1988年 | 8篇 |
1987年 | 11篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 9篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1976年 | 1篇 |
排序方式: 共有4828条查询结果,搜索用时 234 毫秒
91.
The Adriatic-Dinaridic carbonate platform (ADCP) was one of the largest and relatively well preserved Mesozoic platforms in the Mediterranean region (central Tethys). The peninsula Istria, in the northwestern part of the ADCP, is built up predominantly of shallow-water carbonates of the Middle Jurassic (Dogger) to Eocene age and, to a lesser extent, of Paleogene clastic deposits (flysch and calcareous breccia). This study focuses on a Lower Cretaceous (Barremian to Albian) succession of strata at five localities in western Istria. Stratigraphic determinations are based on identification of nine microfossil assemblages (benthic foraminifera and calcareous algae Dasycladales) and on using their taxa as index fossils. The age of strata with these microfossil assemblages, however, is questionable. Most of the age uncertainties are associated with a regional emersion, which occurred on the ADCP during the Aptian or close to the Aptian-Albian transition. It is unclear what portions of the Upper Aptian and/or Lower Albian are missing along this unconformity. A stable isotope study was conducted on homogenous micritic matrix samples in an attempt to resolve some of these uncertainties. Variations in carbon isotope compositions proved useful for stratigraphic correlation between the examined successions of strata, for improving their age determination, and for relating them to other coeval successions that span an important time interval of major oceanographic changes and carbon-cycle perturbations associated with the Early Aptian oceanic anoxic event (OAE 1a). 相似文献
92.
The hatchling sizes of the Cretaceous nautiloids (Hercoglossa forbesianus and Cimomia angustus; Hercoglossidae) are reported here for the first time as ranging between 20.4-22.2 mm in diameter. These new data for Hercoglossidae and the previously reported data for Nautilidae and Cymatoceratidae suggest that all Cretaceous nautiloids had large hatchlings irrespective of their taxonomy relative to those in ammonoids. Based on the relationship between hatching events and the nepionic constriction in modern nautili, the hatchling size of nautiloids can be recognized by the constriction on the shell surface of the innermost whorl. The range of hatchling diameter in all Cretaceous nautiloids (9-35 mm) is comparable to those of modern nautili (20-32 mm), although the Cretaceous nautiloids are characterized by a wider range with the smaller minimum diameter. The large hatchling size in Cretaceous nautiloids stands in marked contrast to those in ammonoids, which had much smaller hatchling size (mostly <2 mm in diameter) and became extinct at the end of the Cretaceous. Such distinct differences in reproductive strategy (i.e., hatchling and egg sizes) suggest that these differences may have played a significant role in the fate of ammonoids and nautiloids through the K/T mass extinction, as previously hypothesized by several paleontologists. 相似文献
93.
94.
95.
作为理解华南构造演化的关键地区,在华南板块南缘的云开地体和越北的Song Chay地体发育了早中生代的向北东逆冲推覆的韧性变形.在云开地体,经历角闪岩相和绿片岩相变质的矿物指示了产状平缓的面理上发育明显的北东-南西向矿物拉伸线理.沿着这些矿物拉伸线理,具有上部指向北东的剪切变形.同位素年代学的定年结果指示了变形事件发生... 相似文献
96.
97.
Paul D. Taylor Consuelo Sendino 《Proceedings of the Geologists' Association. Geologists' Association》2011,122(3):492-500
When first described in 1922, Dinocochlea ingens, an Early Cretaceous (Valanginian) fossil from the Wadhurst Clay Formation (Wealden) of Hastings in Sussex, England, was regarded as the steinkern of a huge gastropod over 2 m in length and claimed to be the largest gastropod ever to have lived. Most subsequent researchers have doubted the identity of D. ingens as a gastropod, usually believing it to be a pseudofossil, but no convincing explanation has been proposed for its origin. Restudy of the type material leads to the new hypothesis that Dinocochlea originated as a horizontal, corkscrew-shaped burrow resembling Helicodromites that acted as a nucleus for concretion growth. Sectioned specimens of Dinocochlea are filled with concentrically banded sediment interpreted to be growth rings of a concretion. Modification of a computer simulation designed to illustrate gastropod shell growth shows how the development of a concretion around a corkscrew-shaped burrow could lead to the morphology seen in Dinocochlea. The most likely tracemaker for Dinocochlea was a capitellid polychaete (‘threadworm’) with a body just a few mm in diameter. 相似文献
98.
99.
福建锦城187Ma花岗岩的发现——对华南沿海早侏罗世构造演化的制约 总被引:7,自引:2,他引:5
本文对福建沿海福清地区的锦城花岗岩和牛头尾花岗闪长岩进行了岩石地球化学、锆石U-Pb-Hf同位素分析.LA-ICP-MS锆石U-Pb定年结果显示锦城花岗岩的年龄为187±1Ma,为福建沿海首次发现的早侏罗世岩浆活动;牛头尾花岗岩形成时代为130±1Ma,代表燕山晚期(白垩纪)的一期岩浆活动.锦城花岗岩具有高5iO2、偏碱、高Fe2OT3等特征,为高钾钙碱性系列,岩石相对富集轻稀土,亏损Nb、Ta、Ti等高场强元素,富集Ba、Sr等大离子亲石元素,具高Rb/Sr( 1.31 ~ 1.93)比值及低Rb/Ba(0.26 ~0.29)、Eu/Eu*(0.30 ~0.56)、La/Yb(5.69 ~ 18.39),Sr/Y(2.24 ~2.29)比值,为典型的Ⅰ型花岗岩,且具有火山孤花岗岩的地球化学特征,很可能形成于与俯冲相关的构造背景.牛头尾花岗岩相比锦城花岗岩低SiO2,贫钾,而富镁、钙,属于花岗闪长岩.其更亏损Nb、Ta及富集Ba、Sr,具较低Rb/Sr(0.27)、Rb/Ba(0.09)比值和较高的Eu/Eu*(0.98)、La/Yb( 16.87),Sr/Y( 19.08)比值,同样为具有弧特征的钙碱性Ⅰ型花岗岩,也形成于与大洋俯冲有关的构造背景.锦城花岗岩和年头尾花岗闪长岩具有相似的锆石Hf同位素组成,εHf(t)分别为2.18~4.73和1.25 ~4.15,tDMC均为~ 1.0Ga,表明二者起源于相同的新元古代新生地壳.结合已有的资料,本次研究发现锦城花岗岩与南岭地区同时代岩浆岩( 194 ~ 172Ma)在成因上有密切的联系,暗示古太平洋板决的俯冲很可能自早侏罗世开始.这样,华南古特提斯构造域向古太平洋构造域的转换很可能发生在早侏罗世之前,大致在205 ~ 190Ma期间. 相似文献
100.
新疆觉罗塔格小热泉子组火山岩的时代探讨 总被引:1,自引:1,他引:0
新疆东天山觉罗塔格地区广泛分布着石炭系-二叠系火山岩-沉积岩地层,小热泉子组是其中的一套以火山岩为主的地层.前人依据与之接触的底坎尔组中的中石炭世化石,将小热泉子组划为早石炭世.对觉罗塔格南部的小热泉子铜矿的研究,不同学者得出了差别较大的同位素年龄,而北部的小热泉子组目前还没有同位素年龄报道.我们对北部的底坎尔和干沟两地的小热泉子组火山岩进行了锆石SHRIMP测年,得到了271~275 Ma的年龄.这些测年数据与近几年其他学者的数据一致,表明早二叠世的岩浆活动比原来认为的更广,小热泉子组的时代应该属于早二叠世,其他地层单元的时代及其先后关系也需要重新认识. 相似文献