全文获取类型
收费全文 | 4724篇 |
免费 | 382篇 |
国内免费 | 295篇 |
专业分类
测绘学 | 91篇 |
大气科学 | 122篇 |
地球物理 | 2298篇 |
地质学 | 1196篇 |
海洋学 | 307篇 |
天文学 | 19篇 |
综合类 | 154篇 |
自然地理 | 1214篇 |
出版年
2024年 | 46篇 |
2023年 | 93篇 |
2022年 | 163篇 |
2021年 | 170篇 |
2020年 | 172篇 |
2019年 | 185篇 |
2018年 | 163篇 |
2017年 | 164篇 |
2016年 | 183篇 |
2015年 | 180篇 |
2014年 | 239篇 |
2013年 | 267篇 |
2012年 | 229篇 |
2011年 | 230篇 |
2010年 | 184篇 |
2009年 | 239篇 |
2008年 | 238篇 |
2007年 | 273篇 |
2006年 | 250篇 |
2005年 | 189篇 |
2004年 | 195篇 |
2003年 | 165篇 |
2002年 | 142篇 |
2001年 | 101篇 |
2000年 | 108篇 |
1999年 | 111篇 |
1998年 | 158篇 |
1997年 | 106篇 |
1996年 | 97篇 |
1995年 | 75篇 |
1994年 | 78篇 |
1993年 | 67篇 |
1992年 | 61篇 |
1991年 | 22篇 |
1990年 | 23篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 4篇 |
排序方式: 共有5401条查询结果,搜索用时 15 毫秒
901.
Tracking eutrophication in Taihu Lake using the diatom record: potential and problems 总被引:7,自引:0,他引:7
Xuhui Dong Helen Bennion Rick Battarbee Xiangdong Yang Handong Yang Enfeng Liu 《Journal of Paleolimnology》2008,40(1):413-429
Taihu Lake is the third largest freshwater lake in China and has been experiencing eutrophication problems for several decades.
Diatoms in short sediment cores from three bays in northern Taihu Lake were studied in addition to 1-year of seasonal phytoplankton
samples in order to evaluate the rate and magnitude of nutrient enrichment. The dominant species found in the phytoplankton
samples appeared in high percentages in the surface sediment samples, suggesting that the latter faithfully record the modern
diatom flora. The diatom preservation status varied among the three cores, while in all cores the preservation deteriorated
with sediment depth. Due to the superior diatom preservation in the core from Mashan Bay, the fossil diatom record of this
core and an established diatom total phosphorus (TP) transfer function were used to reconstruct the nutrient history of Taihu
Lake. Diatom assemblages changed from Aulacoseira-dominated to other eutrophic planktonic species, such as Stephanodiscus minutulus, Cyclostephanos tholiformis, Cyclotella atomus, C. meneghiniana and S. hantzschii in ca. 1980. Diatom-inferred TP concentrations exhibited little change prior to 1980, with values around 50 μg/l. However,
after 1980 TP concentrations increased significantly and remained in excess of 100 μg/l, reflecting eutrophication of Taihu
Lake. Comparison with TP measurements in the water column from 1988 to 2004, as well as the analogue analysis among fossil
and modern samples, demonstrates that the diatom-TP inference model can reliably hindcast past TP concentrations. Therefore,
the baseline TP value of about 50 μg/l, can be used as a restoration target for Taihu Lake. However, due to the complexity
of this very large, shallow aquatic ecosystem, caution should be exercised when employing the diatom record to track eutrophication.
Further studies on the mechanism of diatom distribution, evolution and preservation are recommended for Taihu Lake. 相似文献
902.
Vegetation cycles in a disturbed sequence around the Cobb-Mountain subchron in Catalonia (Spain) 总被引:1,自引:0,他引:1
Suzanne A. G. Leroy 《Journal of Paleolimnology》2008,40(3):851-868
903.
Integration of reflection seismic and sediment grain-size data from Lake Khubsugul (Northern Mongolia): a reply to Prokopenko and Kendall 总被引:1,自引:0,他引:1
Prokopenko and Kendall (J Paleolimnol doi:, 2008) criticise the work presented in Fedotov et al. (J Paleolimnol 39:335–348, 2008), and instead propose an alternative
interpretation for the grain-size evolution recorded in the KDP-01 core, retrieved from the central part of Lake Khubsugul.
Their interpretation is based (i) on a seismic-stratigraphic re-interpretation of sparker seismic profile khub012 (which they
copied from Fedotov et al. (EOS Trans 87:246–250, 2006)), (ii) on the presupposition that changes in lake level are the dominant
control on facies distribution in Lake Khubsugul, and (iii) on the invalidation of our age-depth model. In this reply to their
comment, we demonstrate that they interpreted seismic artefacts and geometries caused by changes in profile orientation as
true stratigraphic features and that the lake-level reconstruction they derive from this interpretation is therefore incorrect.
We also demonstrate that their grain-size predictions, which they consider to be predominantly driven by changes in lake level,
are inconsistent with the measured sulphate concentration, which is a demonstrated proxy of lake level in Lake Khubsugul,
and with the measured grain-size record. Finally, we point out that even if there would be a problem with the age-depth model,
this problem would not affect the part of the sedimentary sequence discussed in Fedotov et al. (J Paleolimnol 39:335–348,
2008). 相似文献
904.
The lipids in a sediment core from Lake Valencia, a hypereutrophic freshwater lake in Venezuela, are examined to understand
environmental changes over the last ∼13,000 years. From the latest Pleistocene to the earliest Holocene, total organic carbon
(TOC) substantially increased from 2.2 to 10%, while total organic carbon over total nitrogen (TOC/TN) decreased from as high
as 34 to as low as 10. Correspondingly, the concentration of terrestrially derived triterpenoids markedly decreased, and the
dominant n-alkane shifted from C31 to C23 or C25. During the same period, algal biomarkers such as botryococcenes, dinosterol, isoarborinol, C20 HBIs and 1,15C32 keto-ol markedly increased in abundance. These changes suggested a greater contribution of algal organic matter at the onset
of the Holocene, which was concurrent with increasing rainfall and the formation of a permanent lake (Lake Valencia) in the
Aragua Valley, Venezuela. The age profile of Paq, a n-alkane based proxy, showed large oscillations (0.20–0.81), reflecting historical variations in source strength of submerged/floating
vs. terrestrial/emergent OM inputs. An abrupt increase in tetrahymanol abundance at ∼7,260 cal years BP suggests the establishment
of an oxic–anoxic boundary in the lake’s water column. After reaching its maximum abundance at ∼2,100 cal year BP, botryococcenes,
a biomarker of Botryococcus braunii, gradually decreased to below the detection limit in the uppermost sediments, while different algal/microbial biomarkers
such as diploptene, dinosterol and isoarborinol substantially increased. These different historical profiles of algal/microbial
biomarkers reflect different responses of source organisms to environmental changes throughout this period. The δ13C determinations presented exceptionally enriched values for botryococcene isomers (−7.7 to −15.1‰), indicating the utilization
of bicarbonate as carbon sources in an extremely productive ecosystem. 相似文献
905.
Janice Brahney John J. Clague Brian Menounos Thomas W. D. Edwards 《Journal of Paleolimnology》2008,40(1):489-505
The level of Kluane Lake in southwest Yukon Territory, Canada, has fluctuated tens of metres during the late Holocene. Contributions
of sediment from different watersheds in the basin over the past 5,000 years were inferred from the elemental geochemistry
of Kluane Lake sediment cores. Elements associated with organic material and oxyhydroxides were used to reconstruct redox
fluctuations in the hypolimnion of the lake. The data reveal complex relationships between climate and river discharge during
the late Holocene. A period of influx of Duke River sediment coincides with a relatively warm climate around 1,300 years BP.
Discharge of Slims River into Kluane Lake occurred when Kaskawulsh Glacier advanced to the present drainage divide separating
flow to the Pacific Ocean via Kaskawulsh and Alsek rivers from flow to Bering Sea via tributaries of Yukon River. During periods
when neither Duke nor Slims river discharged into Kluane Lake, the level of the lake was low and stable thermal stratification
developed, with anoxic and eventually euxinic conditions in the hypolimnion. 相似文献
906.
907.
Glacial Lake Benson formed in west-central Minnesota as the Des Moines lobe of the Laurentide ice sheet retreated north of a small moraine in the Minnesota River lowland. Although previous research has constrained the timing of glacial Lake Agassiz immediately to the north, little age control is available for the formation of glacial Lake Benson and ice-marginal positions to the south. In order to constrain the age of glacial Lake Benson and test the application of single-grain optically stimulated luminescence (OSL) dating to ice-marginal deposits, seven OSL samples were collected from a variety of depositional settings. These included deltaic deposits linked to specific lake levels, pro-glacial fluvial, ice-contact and supra-glacial deposits. Single-grain OSL results indicate evidence for incomplete resetting (partial bleaching) of the luminescence signal, as expected for glacial environments, and therefore ages were calculated using a minimum age model. OSL results constrain the timing of ice-margin retreat and lake formation to 14.4–14.8 ka. Analysis of single-grain equivalent dose distributions indicates that deposits created by glacial-dominated processes typically had higher over-dispersion (>50%) and greater positive skew (>0.9) than deposits originating from fluvial processes. These results suggest that water-lain deposits should be targeted for OSL sampling over those created by glacial processes when dating ice-proximal settings. 相似文献
908.
山东西南部南四湖水位变化特征及其与气候要素的相关分析 总被引:2,自引:0,他引:2
气候变化对水资源的影响是水文领域的一个重要的研究方向, 研究水位变化与气候要素的相关分析, 对预测湖面水位意义重大. 利用1981-2013年山东西南部南四湖水位和沿湖5个国家级气象观测站逐月平均气温、 相对湿度、 风速、 降水和蒸发量等资料, 分析了近33 a来南四湖水位变化特征及气象影响因子. 结果表明: 近33 a南四湖水位升高趋势显著, 平均每10 a升高0.46 m; 水位变化整体分两个阶段, 1989年以前为下降态势, 1989年以后为上升态势, 1994年是水位升高的突变时间点; 平均最高水位出现在3月为33.04 m, 最低出现在12月为32.03 m. 各气象要素对南四湖水位的影响呈明显的季节性, 降水量是影响年水位变化的重要气象因子, 年降水量每增加100 mm, 水位升高0.21 m, 夏季降水量对水位的影响更为显著; 蒸发量在夏、 秋季与水位呈极显著负相关; 水位在夏季与风速、 在冬季与相对湿度均呈显著负相关. 相似文献
909.
基于1956-2015年洞庭湖主要控制站实测水文数据,运用Mann-Kendall检验法、主成分分析法对比分析了近60 a来洞庭湖东、南、西三个湖区水位演变特征及其影响因素。结果表明:从调弦口堵口至葛洲坝截流后,南咀和城陵矶站同流量下水位均升高,但南咀站平均水位受三口分流能力减弱而下降(0.03 m),城陵矶站平均水位受湖盆泥沙淤积和长江干流顶托作用而上升(1.33 m);三峡水库运行后,湖盆冲淤基本持平,湖泊同流量下水位基本不变,由于该时段长江流域整体为相对枯水期,因而与葛洲坝截流后相比湖泊年平均水位下降约0.31~0.58 m。近60 a来南咀站平均水位呈显著下降趋势(p<0.05),而城陵矶站水位呈显著上升趋势(p<0.01),说明湖泊水位影响因素作用存在空间异质性。洞庭湖年内水位存在涨(4-5月)~丰(6-9月)~退(10-11月)~枯(12月-次年3月)的变化特征,葛洲坝运行期丰水期水位上涨明显,三峡运行期各月水位均有下降,受水库调度方式影响7-10月水位降幅最大。洞庭湖流域降水量、四水入湖和出湖径流大小以及长江干流水情是洞庭湖水位变化的主要影响因素,三口来沙变异条件下的洞庭湖冲淤量变化是湖泊水位变化的次要因素。 相似文献
910.
采用典型测站观测资料与水力学原理相结合,分析不同情况下城陵矶水位与洞庭湖区水位关联性强弱转化的机理和临界条件以及三峡水库对其影响等问题。理论解析表明,固定流量下,湖区水位与城陵矶水位相关关系应为单调指数函数,受到区间距离、湖槽形态等多因素影响,据此提出和率定了各湖区水位的经验计算模式。利用经验计算模式对实测数据进行延展,构建了各种可能出现的湖区来流和干流水位组合下的湖区水位特征曲线族,发现湖区水位与城陵矶水位之间的关联强度存在无影响区、影响区和决定区等状态区间,通过对临界条件的定义和计算,实现了各状态区间的定量划分,并提出了各状态区间内洞庭湖区水位的估算方法。通过对三峡水库蓄水后湖床冲淤和水文条件变化的影响分析,论证了以上方法和认识在水库蓄水后的适用性。 相似文献