全文获取类型
收费全文 | 4106篇 |
免费 | 585篇 |
国内免费 | 1031篇 |
专业分类
测绘学 | 60篇 |
大气科学 | 541篇 |
地球物理 | 717篇 |
地质学 | 2190篇 |
海洋学 | 1005篇 |
天文学 | 100篇 |
综合类 | 194篇 |
自然地理 | 915篇 |
出版年
2024年 | 26篇 |
2023年 | 91篇 |
2022年 | 211篇 |
2021年 | 187篇 |
2020年 | 182篇 |
2019年 | 204篇 |
2018年 | 164篇 |
2017年 | 181篇 |
2016年 | 201篇 |
2015年 | 222篇 |
2014年 | 283篇 |
2013年 | 341篇 |
2012年 | 300篇 |
2011年 | 319篇 |
2010年 | 203篇 |
2009年 | 290篇 |
2008年 | 305篇 |
2007年 | 228篇 |
2006年 | 258篇 |
2005年 | 208篇 |
2004年 | 204篇 |
2003年 | 153篇 |
2002年 | 137篇 |
2001年 | 116篇 |
2000年 | 128篇 |
1999年 | 101篇 |
1998年 | 98篇 |
1997年 | 81篇 |
1996年 | 45篇 |
1995年 | 53篇 |
1994年 | 30篇 |
1993年 | 33篇 |
1992年 | 15篇 |
1991年 | 10篇 |
1990年 | 13篇 |
1989年 | 5篇 |
1988年 | 17篇 |
1987年 | 9篇 |
1986年 | 22篇 |
1985年 | 16篇 |
1984年 | 11篇 |
1983年 | 3篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 1篇 |
排序方式: 共有5722条查询结果,搜索用时 15 毫秒
21.
Roles of Biogeochemical Processes in the Oceanic Carbon Cycle Described with a Simple Coupled Physical-Biogeochemical Model 总被引:1,自引:0,他引:1
Masahiko?FujiiEmail author Motoyoshi?Ikeda Yasuhiro?Yamanaka 《Journal of Oceanography》2005,61(5):803-815
To evaluate the contribution of biogeochemical processes to the oceanic carbon cycle and to calculate the ratio of calcium
carbonate to organic carbon downward export, we have incorporated biological and alkalinity pumps in the yoked high-latitude
exchange/interior diffusion-advection (YOLDA) model. The biogeochemical processes are represented by four parameters. The
values of the parameters are tuned so that the model can reproduce the observed phosphate and alkalinity distributions in
each oceanic region. The sensitivity of the model to the biogeochemical parameters shows that biological production rates
in the euphotic zone and decomposition depths of particulate matters significantly influence horizontal and vertical distributions
of biogeochemical substances. The modeled vertical fluxes of particulate organic phosphorus and calcium carbonate are converted
to vertical carbon fluxes by the biological pump and the alkalinity pump, respectively. The downward carbon flux from the
surface layer to the deep layer in the entire region is estimated to be 3.36 PgC/yr, which consists of 2.93 PgC/yr from the
biological pump and 0.43 PgC/yr from the alkalinity pump, which is consistent with previous studies. The modeled rain ratio
is higher with depth and higher in the Pacific and Indian Oceans than in the Atlantic Ocean. The global rain ratio at the
surface layer is calculated to be 0.14 to 0.15. This value lies between the lower and higher ends of the previous estimates,
which range widely from 0.05 to 0.25. This study indicates that the rain ratio is unlikely to be higher than 0.15, at least
in the surface waters. 相似文献
22.
Fluxes and sources of suspended organic matter in an estuarine turbidity maximum region during low discharge conditions 总被引:6,自引:1,他引:6
Miguel A. Goni Mary W. Cathey Yong H. Kim George Voulgaris 《Estuarine, Coastal and Shelf Science》2005,63(4):683-700
Water column concentrations of total suspended solids (TSS), particulate organic carbon (POC) and particulate nitrogen (PN) were measured at three different depths in four different locations bracketing the estuarine turbidity maximum (ETM) along the main channel of a temperate riverine estuary (Winyah Bay, South Carolina, USA). Measurements were carried out over full tidal cycle (over 24 h). Salinity, temperature, current magnitude and direction were also monitored at the same time throughout the water column. Tidally averaged net fluxes of salt, TSS, POC and PN were calculated by combining the current measurements with the concentration data. Under the extreme low river discharge conditions that characterized the study period, net landward fluxes of salt were measured in the lower part of the study area, suggesting that the landward transport through the main channel of the estuary was probably balanced by export out through the sides. In contrast, the net fluxes of salt in the upper reaches of the study area were near zero, indicating a closed salt balance in this part of the estuary. In contrast to salt, the net fluxes of TSS, POC and PN in the deeper parts of the water column were consistently landward at all four sites in Winyah Bay indicating the non-conservative behavior of particulate components and their active transport up the estuary in the region around the ETM.The carbon contents (%POC), carbon:nitrogen ratios (org[C:N]a) and stable carbon isotopic compositions (δ13CPOC) of the suspended particles varied significantly with depth, location and tidal stage. Tidally averaged compositions showed a significant increase up the estuary in the %POC and org[C:N]a values of suspended particles consistent with the preferential landward transport of carbon-rich particles with higher vascular plant debris content. The combination of tidal resuspension and flood-dominated flow appeared to be responsible for the hydrodynamic sorting of particles along the estuary that resulted in denser, organic-poor particles being transported landward less efficiently. The elemental and isotopic compositions indicated that vascular C3 plants and estuarine algae were the major sources of the particulate organic matter of all the samples, without any significant contributions from salt marsh C4 vegetation (Spartina alterniflora) and/or marine phytoplankton. 相似文献
23.
利用雨生红球藻生产虾青素的研究进展 总被引:20,自引:0,他引:20
作为天然虾青素的最佳生物来源,雨生红球藻已成为近年来国内外微藻研究的热点之一.本文综述了国内外雨生红球藻培养及虾青素积累的研究进展,介绍了国外天然虾青素商业生产的现状,并对现存的主要问题和发展方向做出阐述,以期推动国内利用雨生红球藻生产虾青素的商业化进程。 相似文献
24.
Organic matter origin and distribution in suspended particulate materials and surficial sediments from the western Adriatic Sea (Italy) 总被引:5,自引:2,他引:5
T. Tesi S. Miserocchi M.A. Goi L. Langone A. Boldrin M. Turchetto 《Estuarine, Coastal and Shelf Science》2007,73(3-4):431-446
In this study, organic carbon (OC), total nitrogen (TN), stable carbon isotopic (δ13COC) and CuO reaction product compositions were used to identify the sources of organic matter (OM) and to quantify the relative importance of allochthonous and autochthonous contributions to the western Adriatic Sea, Italy. Suspended particulate material (195 samples) and surficial sediments (0–1 cm, 70 samples) from shallow cross-shelf transects were collected in February and May 2003, respectively. Vertical water column profiles were acquired along the same transects. Data include depth, potential temperature, salinity, density and chlorophyll fluorimetry.Along the western Adriatic shelf in the near-shore region, the phytoplankton growth was influenced by dynamics of the buoyant plumes from the Po and Appennine rivers. A small amount of very fine terrigenous material remained suspended within the coastal current and was exported southward along the shelf to the slope. High variability in the bulk composition was detected in the Po prodelta surficial sediments, whereas the western Adriatic shelf, although a larger area, exhibited a narrower range of values.A significant decoupling was observed between suspended particles in the water column and surficial deposits. The organic material collected in the water column was compositionally heterogeneous, with contributions from marine phytoplankton, riverine–estuarine phytoplankton and soil-derived OM. Frequent physical reworking of surficial sediments likely leads to the efficient oxidation of marine OC, resulting in the observed accumulation and preservation of refractory soil-derived OC delivered by the Po and Appennine rivers. 相似文献
25.
26.
INTRODUCTIONDissolvedorganiccarbon (DOC)makesupthesecondlargestofthebioactivepoolsofcar bonintheocean ,secondtothelargestpoolofdissolvedinorganiccarbon .Theglobaldissolvedorganiccarbonpoolisestimatedtobe 6 85Gt,avaluecomparabletothemassofCO2 intheat mosphere (Hedges,1 992 ) .Thesizeofthereservoir,aswellasitsdynamics ,indicatesthatDOCplaysacentralroleintheoceancarboncycle .AsitrelatestogreenhousegasessuchasCO2 andassociatesclimatecycle ,oceanicDOCbiogeochemicalcycleshavebeenoneoftheh… 相似文献
27.
对虾养殖生态系中有机碳的初步研究本研究由 总被引:1,自引:0,他引:1
对于围隔养殖对虾生态系中有机碳的变动进行了研究 ,结果表明 :溶解有机碳 (DOC)含量波动在 5 .2 99~ 13.39mg/ L之间 ,平均为 8.5 3mg/ L± 2 .2 5 mg/ L;颗粒有机碳 (POC)含量波动在0 .6 5~ 6 .6 3mg/ L之间 ,平均为 3.2 5 mg/ L± 1.76 mg/ L;总有机碳 (TOC)含量波动在 6 .92~ 2 0 .0 2mg/ L之间 ,平均为 11.78mg/ L± 3.82 mg/ L ;其中 DOC∶ POC∶ TOC为 0 .72∶ 0 .2 8∶ 1。各种有机碳组分的含量明显高于自然海水中各种有机碳的含量 ;各种有机碳组分的含量在养殖期间总体上呈上升的趋势 ;其变化与 DCOD的变化呈显著线性正相关的关系。 相似文献
28.
赤潮生物种类特征光谱获取技术 总被引:1,自引:0,他引:1
较为系统地研究了赤潮生物水体反射率曲线获取方法。通过对 2类反射率反演方法得出的反射率曲线与实际反射率曲线进行比较 ,得出在不具备大气环境参数的情况下 ,经验线性法的反演结果与实际反射率曲线最接近的结论 ,并进一步对正常海水和赤潮生物水体的光谱特性进行分析 ,提取出赤潮生物水体的特征光谱。这是进行赤潮监测和识别的重要前提 相似文献
29.
Stable carbon isotope ratios have been used to study the sources of particulate organic carbon (POC) and total dissolved inorganic carbon in the Orinoco Basin. The isotopic composition of total dissolved inorganic carbon shows a range of from -8·1 to -23·0 ppt, an indication of dominance of biological processes. The isotopic composition of POC exhibits a range of from -24·1 to -34·6 ppt with little seasonal variation. The isotopic evidence indicates that the POC is predominantly of terrestrial origin rather than a result of in situ planktonic production. The similarity of isotopic composition of POC and coastal sediments suggests that riverine organic detritus has been transported 30-50 km offshore in a direction parallel to the Orinoco river channel. 相似文献
30.
Radiocarbon and total carbonate data were obtained near the 1973 GEOSECS stations in the North Pacific along 30°N and along 175°E between 1993 and 1994. In these stations, we estimated radiocarbon originating from atomic bomb tests using tritium, trichlorofluoromethane and silicate contents. The average penetration depth of bomb radiocarbon during the two decades has deepened from 900 m to 1300 m. Bomb radiocarbon inventories above the average value for the whole North Pacific were found widely in the western subtropical region around 30°N both in the 1970s and 1990s, and its area in the 1990s was broader than that in the 1970s. In most of the North Pacific, while the bomb radiocarbon has decreased above 25.4, the bomb radiocarbon flux below 25.4 was over 1 × 1012 atom m-2yr-1 in the subtropical region around 30°N. In the tropical area south of 20°N, the bomb radiocarbon inventory below 25.4 increased from zero to over 10 × 1012 atom m-2 during the last three decades. These distributions suggest that the bomb radiocarbon removed from the surface is currently accumulated with bomb 14C flux of over 1 × 1012 atom m-2yr-1 below 25.4 in the subtropical region, mainly by advection from the higher latitude, and that part of the accumulated bomb 14C gradually spread southward with about 30 years. 相似文献