全文获取类型
收费全文 | 3445篇 |
免费 | 556篇 |
国内免费 | 862篇 |
专业分类
测绘学 | 50篇 |
大气科学 | 502篇 |
地球物理 | 615篇 |
地质学 | 1902篇 |
海洋学 | 752篇 |
天文学 | 98篇 |
综合类 | 158篇 |
自然地理 | 786篇 |
出版年
2024年 | 16篇 |
2023年 | 76篇 |
2022年 | 194篇 |
2021年 | 164篇 |
2020年 | 154篇 |
2019年 | 181篇 |
2018年 | 147篇 |
2017年 | 147篇 |
2016年 | 178篇 |
2015年 | 193篇 |
2014年 | 250篇 |
2013年 | 292篇 |
2012年 | 257篇 |
2011年 | 276篇 |
2010年 | 166篇 |
2009年 | 255篇 |
2008年 | 259篇 |
2007年 | 189篇 |
2006年 | 218篇 |
2005年 | 170篇 |
2004年 | 165篇 |
2003年 | 123篇 |
2002年 | 111篇 |
2001年 | 103篇 |
2000年 | 110篇 |
1999年 | 84篇 |
1998年 | 81篇 |
1997年 | 65篇 |
1996年 | 39篇 |
1995年 | 40篇 |
1994年 | 26篇 |
1993年 | 21篇 |
1992年 | 11篇 |
1991年 | 9篇 |
1990年 | 10篇 |
1989年 | 5篇 |
1988年 | 16篇 |
1987年 | 3篇 |
1986年 | 15篇 |
1985年 | 15篇 |
1984年 | 9篇 |
1983年 | 3篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 1篇 |
排序方式: 共有4863条查询结果,搜索用时 39 毫秒
81.
We measured the concentrations of dissolved inorganic carbon (DIC) and major ions and the stable carbon isotope ratios of DIC (δ13CDIC) in two creeks discharging from carbonate‐rich sulphide‐containing mine tailings piles. Our aim was to assess downstream carbon evolution of the tailings discharge as it interacted with the atmosphere. The discharge had pH of 6.5–8.1 and was saturated with respect to carbonates. Over the reach of one creek, the DIC concentrations decreased by 1.1 mmol C/l and δ13CDIC increased by ~4.0‰ 200 m from the seep source. The decrease in the DIC concentrations was concomitant with decreases in the partial pressure of CO2(aq) because of the loss of excess CO2(aq) from the discharge. The corresponding enrichment in the δ13CDIC is because of kinetic isotope fractionation accompanying the loss of CO2(g). Over the reach of the other creek, there was no significant decrease in the DIC concentrations or notable changes in the δ13CDIC. The insignificant change in the DIC concentrations and the δ13CDIC is because the first water sample was collected 160 m away from the discharge seep, not accessible during this research. In this case, most of the excess CO2(aq) was lost before our first sampling station. Our results indicate that neutral discharges from tailings piles quickly lose excess CO2(aq) to the atmosphere and the DIC becomes enrich in 13C. We suggest that a significant amount of carbon cycling in neutral discharges from tailings piles occur close to the locations where the discharge seeps to the surface. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
82.
A long‐term study of stable isotopes as tracers of processes governing water flow and quality in a lowland river basin: the upper Thames,UK 下载免费PDF全文
A long‐term study of O, H and C stable isotopes has been undertaken on river waters across the 7000‐km2 upper Thames lowland river basin in the southern UK. During the period, flow conditions ranged from drought to flood. A 10‐year monthly record (2003–2012) of the main River Thames showed a maximum variation of 3‰ (δ18O) and 20‰ (δ2H), although interannual average values varied little around a mean of –6.5‰ (δ18O) and –44‰ (δ2H). A δ2H/δ18O slope of 5.3 suggested a degree of evaporative enrichment, consistent with derivation from local rainfall with a weighted mean of –7.2‰ (δ18O) and –48‰ (δ2H) for the period. A tendency towards isotopic depletion of the river with increasing flow rate was noted, but at very high flows (>100 m3/s), a reversion to the mean was interpreted as the displacement of bank storage by rising groundwater levels (corroborated by measurements of specific electrical conductivity). A shorter quarterly study (October 2011–April 2013) of isotope variations in 15 tributaries with varying geology revealed different responses to evaporation, with a well‐correlated inverse relationship between Δ18O and baseflow index for most of the rivers. A comparison with aquifer waters in the basin showed that even at low flow, rivers rarely consist solely of isotopically unmodified groundwater. Long‐term monitoring (2003–2007) of carbon stable isotopes in dissolved inorganic carbon (DIC) in the Thames revealed a complex interplay between respiration, photosynthesis and evasion, but with a mean interannual δ13C‐DIC value of –14.8 ± 0.5‰, exchange with atmospheric carbon could be ruled out. Quarterly monitoring of the tributaries (October 2011–April 2013) indicated that in addition to the aforementioned factors, river flow variations and catchment characteristics were likely to affect δ13C‐DIC. Comparison with basin groundwaters of different alkalinity and δ13C‐DIC values showed that the origin of river baseflow is usually obscured. The findings show that long‐term monitoring of environmental tracers can help to improve the understanding of how lowland river catchments function. Copyright © NERC 2015. Hydrological Processes © 2015 John Wiley & Sons, Ltd. 相似文献
83.
Mercury and methylmercury biogeochemistry in a thawing permafrost wetland complex,Northwest Territories,Canada 下载免费PDF全文
In arctic and sub‐arctic environments, mercury (Hg), more specifically toxic methylmercury (MeHg), is of growing concern to local communities because of its accumulation in fish. In these regions, there is particular interest in the potential mobilization of atmospherically deposited Hg sequestered in permafrost that is thawing at unprecedented rates. Permafrost thaw and the resulting ground surface subsidence transforms forested peat plateaus into treeless and permafrost‐free thermokarst wetlands where inorganic Hg released from the thawed permafrost and draining from the surrounding peat plateaus may be transformed to MeHg. This study begins to characterize the spatial distribution of MeHg in a peat plateau–thermokarst wetland complex, a feature that prevails throughout the wetland‐dominated southern margin of thawing discontinuous permafrost in Canada's Northwest Territories. We measured pore water total Hg, MeHg, dissolved organic matter characteristics and general water chemistry parameters to evaluate the role of permafrost thaw on the pattern of water chemistry. A gradient in vegetation composition, water chemistry and dissolved organic matter characteristics followed a toposequence from the ombrotrophic bogs near the crest of the complex to poor fens at its downslope margins. We found that pore waters in poor fens contained elevated levels of MeHg, and the water draining from these features had dissolved MeHg concentrations 4.5 to 14.5 times higher than the water draining from the bogs. It was determined through analysis of historical aerial images that the poor fens in the toposequence had formed relatively recently (early 1970s) as a result of permafrost thaw. Differences between the fens and bogs are likely to be a result of their differences in groundwater function, and this suggests that permafrost thaw in this landscape can result in hotspots for Hg methylation that are hydrologically connected to downstream ecosystems. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
84.
AKIHIRO KANO YOKO KUNIMITSU TETSUHIRO TOGO CHIDURU TAKASHIMA FUMITO SHIRAISHI WEI WANG 《Island Arc》2011,20(2):280-293
Oxygenation of the ocean is presumed to be an important factor stimulating the evolution of multicellular animals. The appearance of the Ediacaran‐type biota (ca 575 Ma) was assigned to the aftermath of the Gaskiers glaciation (ca 580 Ma), when substantial oceanic oxygenation is believed to have started. However, several lines of evidence reveal that at least sponges evolved before this oxygenation. For understanding the first stage of animal evolution, we propose the hypothesis that Dissolved Organic Carbon (DOC) Stimulated the evolution for Animal Multicellularity (DOXAM). Recent geochemical studies of the Ediacaran sedimentary sequences have indicated that a substantial DOC mass was developed in the stratified ocean after the Marinoan glaciation (655–635 Ma), and this was supported by the inorganic and organic carbon isotope profiles of the Doushantuo Formation in South China. The DOC mass was an oxygen consumer in the water column; however, it could have provided a food source for filter‐feeding animals such as sponges and cnidarians, and established a primitive food‐web. Such an ecological structure is recognized in modern deep‐sea coral mounds. Results from the integrated ocean drilling program (IODP) Expedition 307 for a mound in northeastern Atlantic suggested that organic carbon suspended around the density boundary in the water column is the key feature to feed the heterotrophic deep‐sea coral community. Our hypothesis is consistent with the fact that the two most primitive animal phyla (Porifera and Cnidaria) are filter feeders. The evolution of filter feeding ecosystems removed the DOC mass and may have contributed to ocean oxygenation in the terminal Neoproterozoic when animal evolution passed into the second stage, with the appearance of bilaterians. 相似文献
85.
Tewodros Alemayehu Albrecht Leis Anton Eisenhauer Martin Dietzel 《Chemie der Erde / Geochemistry》2011,(2):177-187
Isotopic and chemical composition of groundwater from wells and springs, and surface water from the basalt-dominated Axum area (northern Ethiopia) provides evidence for the origin of water and dissolved species. Shallow (depth < 40 m) and deep groundwater are distinguished by both chemical and isotopic composition. Deep groundwater is significantly enriched in dissolved inorganic carbon up to 40 mmol l−1 and in concentrations of Ca2+, Mg2+, Na+ and Si(OH)4 compared to the shallow type.The δ2H and δ18O values of all solutions clearly indicate meteoric origin. Shifts from the local meteoric water line are attributed to evaporation of surface and spring water, and to strong water–rock interaction. The δ13CDIC values of shallow groundwater between −12 and −7‰ (VPDB) display the uptake of CO2 from local soil horizons, whereas δ13CDIC of deep groundwater ranges from −5 to +1‰. Considering open system conditions with respect to gaseous CO2, δ13CDIC = +1‰ of the deep groundwater with highest PCO2 = 10−0.9 atm yields δ13CCO2(gas) ≈ −5‰, which is close to the stable carbon isotopic composition of magmatic CO2. Accordingly, stable carbon isotope ratios within the above range are referred to individual proportions of CO2 from soil and magmatic origin. The uptake of magmatic CO2 results in elevated cations and Si(OH)4 concentrations. Weathering of local basalts is documented by 87Sr/86Sr ratios of the groundwater from 0.7038 to 0.7059. Highest values indicate Sr release from the basement rocks. Besides weathering of silicates, neoformation of solids has to be considered, which results in the formation of, e.g., kaolinite and montmorillonite. In several solutions supersaturation with respect to calcite is reached by outgassing of CO2 from the solution leading to secondary calcite formation. 相似文献
86.
高精度准确测定氯代烃单体碳同位素对示踪污染物来源,了解污染物的生物降解过程具有重要意义。在环境转化过程中,有机污染物的同位素组成可能是稳定不变的,也有可能发生改变。若污染物的同位素组成在迁移转化过程中不变,根据其同位素组成可以示踪污染物的来源;若同位素组成变化,根据同位素分馏结果,可以评价环境中有机污染物降解发生的可能性和程度。本文综述了固相微萃取、静态顶空进样、吹扫-捕集、多级串联技术等前处理方法与气相色谱-燃烧-同位素比值质谱仪(GC-C-IRMS)联用分析水中氯代烃单体碳同位素的研究进展,比较了分析方法的优缺点。液-液萃取较少用于水中氯代烃的单体同位素分析。静态顶空进样、固相微萃取、吹扫-捕集都是无溶剂富集技术,与GC-C-IRMS联用分析水中氯代烃单体同位素过程中不存在或存在小且恒定的可校正的同位素分馏,分析精度一般优于1‰,没有二次污染,降低了杂质干扰,提高了GC-C-IRMS的分辨率和分析精度,降低了检测限。从静态顶空进样、固相微萃取、吹扫-捕集,到多级串联等技术与GC-C-IRMS联用分析水中氯代烃单体同位素比值,检测限逐渐降低。目前,吹扫-捕集-GC-C-IRMS在分析水中氯代烃中应用最广泛,重现性好、检测限低。针内微萃取、管内微萃取、搅动棒吸附萃取和顶空进样吸附萃取等前处理方法与GC-C-IRMS仪联用具有一定的应用前景。 相似文献
87.
88.
89.
Makoto Yamada Shinji Ohsawa Kohei Kazahaya Masaya Yasuhara Hiroshi Takahashi Kazuhiro Amita Hideo Mawatari Shin Yoshikawa 《Journal of Geochemical Exploration》2011
To understand deep groundwater flow systems and their interaction with CO2 emanated from magma at depth in a volcanic edifice, deep groundwater samples were collected from hot spring wells in the Aso volcanic area for hydrogen, oxygen and carbon isotope analyses and measurements of the stable carbon isotope ratios and concentrations of dissolved inorganic carbon (DIC). Relations between the stable carbon isotope ratio (δ13CDIC) and DIC concentrations of the sampled waters show that magma-derived CO2 mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ13CDIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ18O) reflect the meteoric-water origin of that region's deep groundwater. A negative correlation was found between the altitude of the well bottom and the altitude of groundwater recharge as calculated using the equation of the recharge-water line and δD value. This applies especially in the Aso-dani area, where deeper groundwater correlates with higher recharge. Groundwater recharged at high altitude has higher δ13CDIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO2 is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO2 mixes into deeper groundwater flowing nearer the magma conduit or chamber. 相似文献
90.
广东凡口铅锌矿碳、氧同位素地球化学特征及其地质意义 总被引:3,自引:0,他引:3
广东凡口超大型铅锌矿床产于泥盆系台地碳酸盐岩中,矿床成因仍存在较大争议。通过含矿地层岩石以及矿石系统的碳、氧同位素分析研究,发现紫色砂岩的褪色蚀变和碳酸盐岩的白云石化与盆地卤水作用密切相关,褪色蚀变砂岩中的碳酸盐胶结物具有显著低的δ^13C和δ^18O值,与矿石中碳酸盐矿物相似,白云石化的碳酸盐岩之δ^13c和δ^18... 相似文献