全文获取类型
收费全文 | 111篇 |
免费 | 11篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 2篇 |
大气科学 | 3篇 |
地球物理 | 32篇 |
地质学 | 45篇 |
海洋学 | 4篇 |
天文学 | 31篇 |
自然地理 | 6篇 |
出版年
2023年 | 1篇 |
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2018年 | 3篇 |
2017年 | 6篇 |
2016年 | 3篇 |
2015年 | 3篇 |
2014年 | 3篇 |
2013年 | 2篇 |
2012年 | 2篇 |
2011年 | 7篇 |
2010年 | 8篇 |
2009年 | 6篇 |
2008年 | 8篇 |
2007年 | 6篇 |
2006年 | 11篇 |
2005年 | 6篇 |
2004年 | 4篇 |
2003年 | 7篇 |
2002年 | 1篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1989年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1977年 | 3篇 |
1976年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有123条查询结果,搜索用时 15 毫秒
1.
The persistence effect contribution of legacy nutrients is often cited as a reason for little or no improvement in water quality following extensive implementation of watershed nutrient mitigation actions, yet there is limited knowledge concerning factors influencing this response, often called the “persistence effect.” Here, we adopted detrended fluctuation analysis and Spearman analysis methods to assess the influence of land use on the watershed phosphorus (P) persistence effect, using monthly water quality records during 2010–2016 in 13 catchments within a drinking water reservoir watershed in eastern China. Detrended fluctuation analysis was used to calculate the Hurst exponent α to assess watershed legacy P characteristics (α ≈ 0.5, α > 0.5, and α < 0.5 indicate white noise, persistence, and anti‐persistence, respectively). Results showed weak to strong P persistence (0.60–0.81) in the time series of riverine P in the 13 catchments. The Hurst exponent α had negative relationships with agricultural land (R = ?.47, p = .11) and developed land (R = ?.67, p = .01) and a positive relationship with forest land cover (R = .48, p = .10). The persistence effect of riverine P was mainly determined by retention ability (biogeochemical legacy) and migration efficiency (hydrological legacy). A catchment with strong retention capacity (e.g., biomass uptake/storage and soil PO4 sorption) and low migration efficiency results in a stronger persistence effect for riverine P. In practice, source control is more effective in catchments with weak persistence, whereas sink control (e.g., riparian buffers and wetlands) is preferred in catchments with strong persistence effects. 相似文献
2.
Axel Wittmann Randy L. Korotev Bradley L. Jolliff Paul K. Carpenter 《Meteoritics & planetary science》2019,54(2):379-394
Magnesium‐rich spinel assemblages occur in the two lunar vitric breccia meteorites—Dhofar (Dho) 1528 and Graves Nunataks (GRA) 06157. Dho 1528 contains up to ~0.7 mm cumulate Mg‐rich spinel crystals associated with Mg‐rich olivine, Mg‐ and Al‐rich pyroxene, plagioclase, and rare cordierite. Using thermodynamic calculations of these mineral assemblages, we constrain equilibration depths and discuss an origin of these lithologies in the upper mantle of the Moon. In contrast, small, 10 to 20 μm spinel phenocryst assemblages in glassy melt rock clasts in Dho 1528 and GRA 06157 formed from the impact melting of Mg‐rich rocks. Some of these spinel phenocrysts match compositional constraints for spinel associated with “pink spinel anorthosites” inferred from remote sensing data. However, such spinel phenocrysts in meteorites and Apollo samples are typically associated with significant amounts of olivine ± pyroxene that exceed the compositional constraints for pink spinel anorthosites. We conclude that the remotely sensed “pink spinel anorthosites” have not been observed in the collections of lunar rocks. Moreover, we discuss impact‐excavation scenarios for the spinel‐bearing assemblages in Dhofar 1528 and compare the bulk rock composition of Dho 1528 to strikingly similar compositions of Luna 20 samples that contain ejecta from the Crisium impact basin. 相似文献
3.
Hong Huang Xiaoliang Ji Fang Xia Shuhui Huang Xu Shang Han Chen Minghua Zhang Randy A. Dahlgren Kun Mei 《水文研究》2020,34(5):1213-1227
Regression-based methods are commonly used for riverine constituent concentration/flux estimation, which is essential for guiding water quality protection practices and environmental decision making. This paper developed a multivariate adaptive regression splines model for estimating riverine constituent concentrations (MARS-EC). The process, interpretability and flexibility of the MARS-EC modelling approach, was demonstrated for total nitrogen in the Patuxent River, a major river input to Chesapeake Bay. Model accuracy and uncertainty of the MARS-EC approach was further analysed using nitrate plus nitrite datasets from eight tributary rivers to Chesapeake Bay. Results showed that the MARS-EC approach integrated the advantages of both parametric and nonparametric regression methods, and model accuracy was demonstrated to be superior to the traditionally used ESTIMATOR model. MARS-EC is flexible and allows consideration of auxiliary variables; the variables and interactions can be selected automatically. MARS-EC does not constrain concentration-predictor curves to be constant but rather is able to identify shifts in these curves from mathematical expressions and visual graphics. The MARS-EC approach provides an effective and complementary tool along with existing approaches for estimating riverine constituent concentrations. 相似文献
4.
Axel Wittmann Randy L. Korotev Bradley L. Jolliff Kunihiko Nishiizumi A. J. Timothy Jull Marc W. Caffee Michael Zanetti Anthony J. Irving 《Meteoritics & planetary science》2019,54(10):2167-2188
Oued Awlitis 001 is a highly feldspathic, moderately equilibrated, clast‐rich, poikilitic impact melt rock lunar meteorite that was recovered in 2014. Its poikilitic texture formed due to moderately slow cooling, which judging from textures of rocks in melt sheets of terrestrial impact structures, is observed in impact melt volumes at least 100 m thick. Such coherent impact melt volumes occur in lunar craters larger than ~50 km in diameter. The composition of Oued Awlitis 001 points toward a crustal origin distant from incompatible‐element‐rich regions. Comparison of the bulk composition of Oued Awlitis 001 with Lunar Prospector 5° γ‐ray spectrometer data indicates a limited region of matches on the lunar farside. After its initial formation in an impact crater larger than ~50 km in diameter, Oued Awlitis 001 was excavated from a depth greater than ~50 m. The cosmogenic nuclide inventory of Oued Awlitis 001 records ejection from the Moon 0.3 Ma ago from a depth of at least 4 m and little mass loss due to ablation during its passage through Earth's atmosphere. The terrestrial residence time must have been very short, probably less than a few hundred years; its exact determination was precluded by a high concentration of solar cosmic ray‐produced 14C. If the impact that excavated Oued Awlitis 001 also launched it, this event likely produced an impact crater >10 km in diameter. Using petrologic constraints and Lunar Reconnaissance Orbiter Camera and Diviner data, we test Giordano Bruno and Pierazzo as possible launch craters for Oued Awlitis 001. 相似文献
5.
Petrology and geochemistry of feldspathic impact‐melt breccia Abar al' Uj 012, the first lunar meteorite from Saudi Arabia 下载免费PDF全文
Marianna Mészáros Beda A. Hofmann Pierre Lanari Randy L. Korotev Edwin Gnos Nicolas D. Greber Ingo Leya Richard C. Greenwood A. J. Timothy Jull Khalid Al‐Wagdani Ayman Mahjoub Abdulaziz A. Al‐Solami Siddiq N. Habibullah 《Meteoritics & planetary science》2016,51(10):1830-1848
Abar al' Uj (AaU) 012 is a clast‐rich, vesicular impact‐melt (IM) breccia, composed of lithic and mineral clasts set in a very fine‐grained and well‐crystallized matrix. It is a typical feldspathic lunar meteorite, most likely originating from the lunar farside. Bulk composition (31.0 wt% Al2O3, 3.85 wt% FeO) is close to the mean of feldspathic lunar meteorites and Apollo FAN‐suite rocks. The low concentration of incompatible trace elements (0.39 ppm Th, 0.13 ppm U) reflects the absence of a significant KREEP component. Plagioclase is highly anorthitic with a mean of An96.9Ab3.0Or0.1. Bulk rock Mg# is 63 and molar FeO/MnO is 76. The terrestrial age of the meteorite is 33.4 ± 5.2 kyr. AaU 012 contains a ~1.4 × 1.5 mm2 exotic clast different from the lithic clast population which is dominated by clasts of anorthosite breccias. Bulk composition and presence of relatively large vesicles indicate that the clast was most probably formed by an impact into a precursor having nonmare igneous origin most likely related to the rare alkali‐suite rocks. The IM clast is mainly composed of clinopyroxenes, contains a significant amount of cristobalite (9.0 vol%), and has a microcrystalline mesostasis. Although the clast shows similarities in texture and modal mineral abundances with some Apollo pigeonite basalts, it has lower FeO and higher SiO2 than any mare basalt. It also has higher FeO and lower Al2O3 than rocks from the FAN‐ or Mg‐suite. Its lower Mg# (59) compared to Mg‐suite rocks also excludes a relationship with these types of lunar material. 相似文献
6.
Large numbers of soil samples are routinely analyzed using methylene blue (MB) titration to determine clay content. Although, the MB filter paper spot test titration is simple, it is labor-intensive, subjective, and leaves no record for subsequent re-examination. Preliminary thermometric monitoring of the titration indicated a switch from endothermic to exothermic reaction during titration. We examined this nearly universal property of physicochemical processes as a potential replacement method to automate the MB clay titration, making use of literature data.We translated the isothermal enthalipmetric titration of montmorillonite using MB by Rytwo and Ruiz-Hitzky to an adiabatic thermometric titration (2003). The plot of temperature versus titrant volume gave two intersecting linear segments dependent on the titrated percentage of the clay's cation exchange capacity (CEC). The MB filter paper spot test endpoint reflects the first excess MB that persists in the solid suspension, which also happens to be a function of the clay's CEC. The spot test endpoint is correlated to the thermometric indicator point and, for montmorillonite, the two are related by an exact factor of two. Although this factor will naturally be dependent on the clay minerals in the samples, the analyses show that the thermometric indicator titration method is a viable replacement for the spot test. The proposed automation is commonplace as it is practiced with other types of titrations. 相似文献
7.
奥巴马政府2011年1月即将面对共和党领导的众议院,日前,总统科学顾问JohnHoldren就广泛话题接受了美国地球物理联合会(AGU)会刊Eos的独家专访。在此之前,他曾在2010年12月13日旧金山举行的AGU秋季会议上发表过政策演讲。 相似文献
8.
白宫推迟发布行政部门的科研诚信计划这一举动导致了最近的一场诉讼,并使公众产生对完善白宫2009年签署的科研诚信备忘录的担忧,尽管政府已采取了一些措施并在备忘录方面取得了进展。科研诚信备忘录发布于2009年3月9 相似文献
9.
10.
Compound‐Specific Isotope Analyses to Assess TCE Biodegradation in a Fractured Dolomitic Aquifer 下载免费PDF全文
The potential for trichloroethene (TCE) biodegradation in a fractured dolomite aquifer at a former chemical disposal site in Smithville, Ontario, Canada, is assessed using chemical analysis and TCE and cis‐DCE compound‐specific isotope analysis of carbon and chlorine collected over a 16‐month period. Groundwater redox conditions change from suboxic to much more reducing environments within and around the plume, indicating that oxidation of organic contaminants and degradation products is occurring at the study site. TCE and cis‐DCE were observed in 13 of 14 wells sampled. VC, ethene, and/or ethane were also observed in ten wells, indicating that partial/full dechlorination has occurred. Chlorine isotopic values (δ37Cl) range between 1.39 to 4.69‰ SMOC for TCE, and 3.57 to 13.86‰ SMOC for cis‐DCE. Carbon isotopic values range between ?28.9 and ?20.7‰ VPDB for TCE, and ?26.5 and ?11.8‰ VPDB for cis‐DCE. In most wells, isotopic values remained steady over the 15‐month study. Isotopic enrichment from TCE to cis‐DCE varied between 0 and 13‰ for carbon and 1 and 4‰ for chlorine. Calculated chlorine‐carbon isotopic enrichment ratios (?Cl/?C) were 0.18 for TCE and 0.69 for cis‐DCE. Combined, isotopic and chemical data indicate very little dechlorination is occurring near the source zone, but suggest bacterially mediated degradation is occurring closer to the edges of the plume. 相似文献