全文获取类型
收费全文 | 106篇 |
免费 | 3篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 10篇 |
大气科学 | 11篇 |
地球物理 | 40篇 |
地质学 | 28篇 |
海洋学 | 8篇 |
天文学 | 3篇 |
综合类 | 1篇 |
自然地理 | 11篇 |
出版年
2023年 | 1篇 |
2021年 | 1篇 |
2020年 | 1篇 |
2019年 | 3篇 |
2018年 | 2篇 |
2017年 | 2篇 |
2016年 | 6篇 |
2015年 | 1篇 |
2014年 | 4篇 |
2013年 | 9篇 |
2012年 | 6篇 |
2011年 | 5篇 |
2010年 | 3篇 |
2009年 | 7篇 |
2008年 | 10篇 |
2007年 | 5篇 |
2006年 | 2篇 |
2005年 | 4篇 |
2004年 | 1篇 |
2003年 | 3篇 |
2002年 | 2篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1976年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1890年 | 1篇 |
排序方式: 共有112条查询结果,搜索用时 31 毫秒
91.
Numerous paleoecological questions concern the mobility of ancient fauna in eastern Beringia. Strontium (Sr) isotope ratio (87Sr/86Sr) analysis has emerged as a powerful tracer for determining the provenance of ancient biological materials. However, it is important to characterize 87Sr/86Sr variation across a landscape. We measured the 87Sr/86Sr composition of teeth from present-day, herbivorous rodents (n = 162) sampled from across eastern Beringia to estimate bio-available 87Sr/86Sr values. We compiled these data with the very limited number of previously published 87Sr/86Sr values from the region. We then used this dataset and a machine learning, random-forest regression to predict bio-available 87Sr/86Sr variations across eastern Beringia. As a case study using our new 87Sr/86Sr map (isoscape), we measured the 87Sr/86Sr and oxygen stable isotope values (δ18O) of five radiocarbon-dated steppe bison from eastern Beringia and compared these to our 87Sr/86Sr isoscape and a δ18O isoscape to estimate the probable landscape use of these ancient fauna. Our model and isoscape provide important foundations for a wide range of additional applications, including studies of the paleo-mobility of other fauna, ancient people and present-day fauna in eastern Beringia. 相似文献
92.
Clement TP 《Ground water》2011,49(5):620-629
Groundwater models are routinely used in hindcasting applications to predict the past concentration levels in contaminated aquifers. These predictions are used in risk assessment and epidemiological studies, which are often completed either for resolving a court case or for developing a public‐policy solution. Hindcast groundwater modeling studies utilize a variety of computer tools with complexity levels ranging from simple analytical models to detailed three‐dimensional, multiphase, multispecies, reactive transport models. The aim of this study is to explore the value of using complex reactive transport models in hindcasting studies that have limited historic data. I review a chlorinated solvent exposure problem that occurred at a U.S. Marine Corp Base in Camp Lejeune, North Carolina and use it as an example to discuss the limits of hindcasting modeling exercises. The lessons learned from the study are used to reflect upon the following questions related to model complexity: How should we decide how much is enough? Who should decide when enough is enough? 相似文献
93.
Integrating the Red Edge channel in satellite sensors is valuable for plant species discrimination. Sentinel-2 MSI and Rapid Eye are some of the new generation satellite sensors that are characterized by finer spatial and spectral resolution, including the red edge band. The aim of this study was to evaluate the potential of the red edge band of Sentinel-2 and Rapid Eye, for mapping festuca C3 grass using discriminant analysis and maximum likelihood classification algorithms. Spectral bands, vegetation indices and spectral bands plus vegetation indices were analysed. Results show that the integration of the red edge band improved the festuca C3 grass mapping accuracy by 5.95 and 4.76% for Sentinel-2 and Rapid Eye when the red edge bands were included and excluded in the analysis, respectively. The results demonstrate that the use of sensors with strategically positioned red edge bands, could offer information that is critical for the sustainable rangeland management. 相似文献
94.
95.
Sun Woo ChangT. Prabhakar Clement Matthew J. SimpsonKang-Kun Lee 《Advances in water resources》2011,34(10):1283-1291
Climate change effects are expected to substantially raise the average sea level. It is widely assumed that this raise will have a severe adverse impact on saltwater intrusion processes in coastal aquifers. In this study we hypothesize that a natural mechanism, identified here as the “lifting process,” has the potential to mitigate, or in some cases completely reverse, the adverse intrusion effects induced by sea-level rise. A detailed numerical study using the MODFLOW-family computer code SEAWAT was completed to test this hypothesis and to understand the effects of this lifting process in both confined and unconfined systems. Our conceptual simulation results show that if the ambient recharge remains constant, the sea-level rise will have no long-term impact (i.e., it will not affect the steady-state salt wedge) on confined aquifers. Our transient confined-flow simulations show a self-reversal mechanism where the wedge which will initially intrude into the formation due to the sea-level rise would be naturally driven back to the original position. In unconfined systems, the lifting process would have a lesser influence due to changes in the value of effective transmissivity. A detailed sensitivity analysis was also completed to understand the sensitivity of this self-reversal effect to various aquifer parameters. 相似文献
96.
Linking reforestation policies with land use change in northern Vietnam: Why local factors matter 总被引:1,自引:0,他引:1
In many countries worldwide, similar state policies on land management have been implemented as a response to deforestation in mountainous areas. So far, few studies have examined to which extent these policies have actually contributed to reforestation. This is the focus of our analysis, based on a case study in Vietnam. Because land access and land use were traditionally governed by common rules, we examine land use changes from an institutional perspective. We use the Institutional Analysis and Development (IAD) framework, coupled with an historical perspective and the analysis of actors’ perception and dominant narratives on land management and forests. Results show that national policies significantly interfered with local factors, leading to a complex course of decision-making and action. Substantial reforestation in the area was not a response by farmers to policy incentives but rather the unexpected outcome of the disruption of local institutions by these policies. We argue that, because national interventions have relied on false or exaggerated narratives and beliefs, their implementation is in conflict with the local reality in upland areas, leading to unpredictable and locally dependent outcomes. We defend hence the need for local level studies and also recommend considering local institutions for land use change analysis in contexts where land use systems are characterised by a high degree of human interaction. 相似文献
97.
Interaction of magma with sedimentary wall rock and magnetite ore genesis in the Panzhihua mafic intrusion,SW China 总被引:10,自引:0,他引:10
Clément Ganino Nicholas T. Arndt Mei-Fu Zhou Fabrice Gaillard Catherine Chauvel 《Mineralium Deposita》2008,43(6):677-694
In southwestern China, several large magmatic Fe–Ti–V oxide ore deposits are hosted by gabbroic intrusions associated with
the Emeishan flood basalts. The Panzhihua gabbroic intrusion, a little deformed sill that contains a large titanomagnetite
deposit at its base, concordantly intrudes late-Proterozoic dolostones. Mineralogical and chemical studies of the contact
aureole in the footwall dolostones demonstrate that the metamorphism was largely isochemical but released large quantities
of CO2 as the rocks were converted to marble and skarns during intrusion of the gabbroic magma. Petrological modelling of the crystallization
of the intrusion, using H2O-poor Emeishan basalt as parent magma, shows that under normal conditions, Fe–Ti oxides crystallize at a late stage, after
the crystallization of abundant olivine, clinopyroxene and plagioclase. In order for titanomagnetite to separate efficiently
to form the ore deposit, this mineral must have crystallized earlier and close to the liquidus. We propose that CO2-rich fluids released during decarbonatization of sedimentary floor rocks passed up through the magma. Redox equilibria calculations
show that when magma with the composition of Emeishan basalt is fluxed by a CO2-rich gas phase, its equilibrium oxygen fugacity (fO2) increases from the fayalite–magnetite–quartz buffer (FMQ) to FMQ + 1.5. From experimental constraints on magnetite saturation
in basaltic magma under controlled fO2, such an oxidizing event would allow magnetite to crystallize near to the liquidus, leading to the formation of the deposit. 相似文献
98.
Domenico solution--is it valid? 总被引:1,自引:0,他引:1
The Domenico solution is widely used in several analytical models for simulating ground water contaminant transport scenarios. Unfortunately, many textbook as well as journal article treatments of this approximate solution are full of empirical statements that are developed without mathematical rigor. For this reason, a rigorous analysis of this solution is warranted. In this article, we present a mathematical method to derive the Domenico solution and explore its limits. Our analysis shows that the Domenico solution is a true analytical solution when the value of longitudinal dispersivity is zero. For nonzero longitudinal dispersivity values, the Domenico solution will introduce a finite amount of error. We use an example problem to quantify the nature of this error and suggest some general guidelines for the appropriate use of this solution. 相似文献
99.
Microbial Mn(II) oxidation kinetics in response to oxygen concentration were assessed in suboxic zone water at six sites throughout the Black Sea. Mn(II) oxidation rates increased asymptotically with increasing oxygen concentration, consistent with Michaelis-Menten enzyme kinetics. The environmental half-saturation constant, KE, of Mn(II) removal (oxidation) varied from 0.30 to 10.5 μM dissolved oxygen while the maximal environmental rate, VE−max, ranged from 4 to 50 nM h−1. These parameters varied spatially and temporally, consistent with a diverse population of enzymes catalyzing Mn oxide production in the Black Sea. Coastally-influenced sites produced lower KE and higher VE−max constants relative to the Western and Eastern Gyre sites. In the Bosporus Region, the Mn(II) residence time calculated using our KE and VE−max values with 0.1 μM oxygen was 4 days, 25-fold less than previous estimates. Our results (i) indicate that rapid Mn(II) oxidation to solid phase Mn oxides in the Black Sea’s suboxic zone is stimulated by oxygen concentrations well below the 3-5 μM concentration reliably detected by current oceanographic methods, (ii) suggest the existence of multiple, diverse Mn(II)-oxidizing enzymes, (iii) are consistent with shorter residence times than previously calculated for Mn(II) in the suboxic zone and (iv) cast further doubt on the existence of proposed reactions coupling solid phase Mn oxide production to electron acceptors other than oxygen. 相似文献
100.