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21.
High-resolution paleoenvironmental data from a peat profile with a small pollen source area are used to reconstruct the impacts of landnám on vegetation and soils at a Norse farm complex (∅2 at Tasiusaq) comprising two farms in the Eastern Settlement of Greenland. Analyses include the AMS 14C dating of plant macrofossil samples and the use of Bayesian radiocarbon calibration to construct improved age-depth models for Norse cultural horizons. The onset of a regional landnám may be indicated by the clearance of Betula pubescens woodland immediately prior to local settlement. The latter is dated to AD 950-1020 (2σ) and is characterised by possible burning of Betula glandulosa scrub to provide grassland pasture for domestic stock. Clearance and grazing resulted in accelerated levels of soil erosion at a westerly farm. This was followed by an easterly migration of settlement and agriculture. Site constraints prevent an assessment of the demise of the easterly farm, but pressures of overgrazing and land degradation may have been the major factors responsible for the abandonment of the earlier farm. 相似文献
22.
We study the morphology of the major rivers draining the Eastern Alps to test whether the active tectonics of this part of the orogen is reflected in the shape of channel profiles of the river network. In our approach we compare channel profiles measured from digital elevation models with numerically modelled channel profiles using a stream power approach. It is shown that regions of high stream power coincide largely with regions of highest topography and largest uplift rates, while the forelands and the Pannonian Basin are characterised by a significantly lower stream power. From stream power modelling we conclude that there is young uplift at the very east of the Eastern Alps, in the Bohemian Massif and in the Pohorje Range. The impact of the Pleistocene glaciations is explored by comparing properties of rivers that drain in proximal and distal positions relative to the ice sheet during the last glacial maximum. Our analysis shows that most knick points, wind gaps and other non-equilibrium features of catchments covered by ice during the last glaciations (Salzach, Enns) can be correlated with glacial processes. In contrast the ice free catchments of the Mur and Drava are characterized by channels in morphological equilibrium at the first approximation and are showing only weak evidence of the strong tectonic activity within these catchments. Finally, the channel profiles of the Adige and the divide between the upper Rhine and Danube catchments differ significantly from the other catchments. We relate this to the fact that the Adige and the Rhine respond to different base levels from the remainder of the Eastern Alps: The Adige may preserve a record from the Messininan base level change and the Rhine is subject to the base level lowering in the Rhine Graben. 相似文献
23.
The uptake and release of trace metals (Cu, Ni, Zn, Cd, and Co) in estuaries are studied using river and sea end-member waters
and suspended particulate matter (SPM) collected from the Changjiang Estuary, China. The kinetics of adsorption and desorption
were studied in terms of environmental factors (pH, SPM loading, and salinity) and metal concentrations. The uptake of the
metals studied onto SPM occurred mostly within 10 h and reached an asymptotic value within 40 h in the Changjiang Estuary.
As low pH river water flows into the high pH seawater and the water become more alkaline as it approaches to the seaside of
estuary, metals adsorb more on SPM in higher pH water, thus, particulate phase transport of metal become increasingly important
in the seaward side of the estuary. The percentage of adsorption recovery and the distribution coefficients for trace metals
remained to be relatively invariable and a significant reduction only occurred in very high concentrations of metals (>0.1 mg L−1). The general effect of salinity on metal behavior was to decrease the degree of adsorption of Cu, Zn, Cd, Co, and Ni onto
SPM but to increase their adsorption equilibrium pH. The adsorption–desorption kinetics of trace metals were further investigated
using Kurbatov adsorption model. The model appears to be most useful for the metals showing the conservative behavior during
mixing of river and seawater in the estuary. Our work demonstrates that dissolved concentration of trace metals in estuary
can be modeled based on the metal concentration in SPM, pH and salinity using a Kurbatov adsorption model assuming the natural
SPM as a simple surfaced molecule. 相似文献
24.
Lake Sapanca in NW Turkey is a fault originated freshwater basin fed by seasonally variable flows of 15 streams. Considerations of lake–river interaction, supported by statistical measures of 47 bottom samples, suggest that sediment transport and deposition within the lake is controlled by two types of human constructed structures in addition to natural factors: (1) the dykes constructed in the front of streams, which feed the lake by strong flows, to prevent the filling of lakebed by coarse-grained sediments; (2) the constructed regulator on the outflowing Çarksuyu stream results in a higher sedimentation rate of clay-sized material on the NE corner of the lake, which is extraordinary in the shelf environment. 相似文献
25.
The shallow water wave simulation model-SWAN incorporated with a simple fine sediment erosion model is applied to Hangzhou
Bay, China, to model the horizontal distribution of the maximum bottom orbital velocity and corresponding fine sediment erosion
rates induced by: (1) southeasterly steady winds (5, 20 and 30 m/s), (2) southwesterly steady winds (5 and 20 m/s); (3) northwesterly
steady winds (5 and 20 m/s); (4) east-southeasterly steady winds (5 and 20 m/s); (5) easterly steady winds (5 and 20 m/s)
under closed and unclosed boundaries; and (6) unsteady winds during the slack water periods. Results suggest: (1) the steady
wind wave-induced maximum bottom orbital velocities and corresponding fine sediment erosion rates generally increased with
the increasing steady winds; (2) closed and unclosed boundary conditions had more significant influences on modeled fine sediment
erosion rates under 5 m/s easterly steady winds than 20 m/s; and (3) steady and unsteady wind wave-induced maximum bottom
currents could be significant in eroding fine sediment bed in Hangzhou Bay. The results show implications for geomorphology,
sedimentology, coastal erosion, and environmental pollution mitigation in Hangzhou Bay. 相似文献
26.
Hong-Fu Zhang Steven L. Goldstein Xin-Hua Zhou Min Sun Jian-Ping Zheng Yue Cai 《Contributions to Mineralogy and Petrology》2008,155(3):271-293
The ages of subcontinental lithospheric mantle beneath the North China and South China cratons are less well-constrained than
the overlying crust. We report Re–Os isotope systematics of mantle xenoliths entrained in Paleozoic kimberlites and Mesozoic
basalts from eastern China. Peridotite xenoliths from the Fuxian and Mengyin Paleozoic diamondiferous kimberlites in the North
China Craton give Archean Re depletion ages of 2.6–3.2 Ga and melt depletion ages of 2.9–3.4 Ga. No obvious differences in
Re and Os abundances, Os isotopic ratios and model ages are observed between spinel-facies and garnet-facies peridotites from
both kimberlite localities. The Re–Os isotopic data, together with the PGE concentrations, demonstrate that beneath the Archean
continental crust of the eastern North China Craton, Archean lithospheric mantle of spinel- to diamond-facies existed without
apparent compositional stratification during the Paleozoic. The Mesozoic and Cenozoic basalt-borne peridotite and pyroxenite
xenoliths, on the other hand, show geochemical features indicating metasomatic enrichment, along with a large range of the
Re–Os isotopic model ages from Proterozoic to Phanerozoic. These features indicate that lithospheric transformation or refertilization
through melt-peridotite interaction could be the primary mechanism for compositional changes during the Phanerozoic, rather
than delamination or thermal-mechanical erosion, despite the potential of these latter processes to play an important role
for the loss of garnet-facies mantle. A fresh garnet lherzolite xenolith from the Yangtze Block has a Re depletion age of
∼1.04 Ga, much younger than overlying Archean crustal rocks but the same Re depletion ages as spinel lherzolite xenoliths
from adjacent Mesozoic basalts, indicating Neoproterozoic resetting of the Re–Os system in the South China Craton. 相似文献
27.
Gary Bridge 《Geoforum》2008,39(4):1570-1584
This paper explores the radical possibilities of pragmatism for geography using the illustration of arguments concerning a renewed (urban) public realm through the exchange of validity claims in communication. Pressing further the pragmatist possibilities of Habermas’s idea of communicative action it draws on John Dewey’s work, and a range of contemporary pragmatist philosophers, to consider human communicability in its widest sense. This is then explored using an example of the spatiality and performativity of body-minds in a range of communicative spaces of the city. Then the paper moves on to consider the radical implications of pragmatism for geography in general in terms of body-mind/environment relations; a transactional view of space; experience, rationality and radical democracy. 相似文献
28.
Prakash C. Sinha Indu Jain Neetu Bhardwaj Ambarukhana D. Rao Shishir K. Dube 《Natural Hazards》2008,45(3):413-427
The Orissa coast of India is one of the most vulnerable regions of extreme sea levels associated with severe tropical cyclones.
There was extensive loss of life and property due to the October 1999 super cyclone, which devastated large part of the Orissa
coast. The shallow nature of the head bay, presence of a large number of deltas formed by major rivers of Orissa such as Mahanadi
and Dhamra, and high tidal range are responsible for storm surge flooding in the region. Specifically, rising and falling
tidal phases influence the height, duration, and arrival time of peak surge along the coast. The objective of the present
study is to evaluate the tide-surge interaction during the 1999 Orissa cyclone by using nonlinear vertically integrated numerical
models. The pure tidal solution for the head bay region of the Bay of Bengal provides the initial condition for the fine resolution
nested grid Orissa model. However, the feedback from the Orissa model does not affect the head bay model as the study provides
a one-way interaction. Numerical experiments are performed to study the tide-surge interaction by considering various relative
phases of the tidal waves with the surge-wave produced by 1999 Orissa cyclone. The comparison, although utilizing only the
limited estimates of tidal data, appears adequate to assert that the principal features are reproduced correctly. 相似文献
29.
南海西部海域构造复杂,主要发育有北东—北东东向、北西向和近南北向3组深大断裂。其中,北西向断裂与板块汇聚、碰撞有关,多具走滑性质;北东—北东东向断裂具有与中国东部裂谷盆地相似的发育特点,呈张扭性质;近南北向断裂可能是南海在扩张活动期间于洋、陆壳过渡部位形成的走滑调节断裂,是洋盆扩张的西部边界。新生代里,南海经历了4次成盆事件与3期扩张活动,盆地经历了古新世—中始新世陆缘断陷、渐新世—早中新世扩展与中中新世以来的热沉降3个演化阶段。陆缘断陷阶段的充填系列主要是北东—北东东向与北西向的河流—冲积扇、湖泊沼泽等陆相沉积及火山岩等;盆地扩展阶段表现为中-小型断陷、断-坳陷逐渐复合与联合为大-中型坳陷,古地理格局逐渐由河流与湖沼陆相环境演变为滨海至浅海相的沉积环境;热沉降阶段的成盆活动逐渐减弱以至停止,地层表现超覆,盆地出现联合迹象。结合以往勘探与油气资源调查成果分析,认为南海西部海域陆架陆坡区发育的大-中型沉积盆地石油地质条件良好,蕴藏着丰富的油气资源,勘探潜力巨大。 相似文献
30.
断层对盖层的破坏主要表现为两个方面:一是减小了盖层的连续封盖面积,二是减小了盖层的厚度。盖层被断层破坏的主要影响因素有断层的断距、倾角和盖层厚度。通过对影响因素研究,提出了盖层有效断接厚度的新概念和计算方法。根据我国部分与断层有关的大-中型气田气柱高度资料,发现了有效断接厚度与所能封闭的最大气柱高度的对数线性关系,并由此提出了评价被断层破坏的盖层封闭能力的新方法。通过对库车坳陷库姆格列木群膏泥岩盖层的应用研究,所得结论与勘探实践完全吻合,证明所提出的评价方法是可行的。 相似文献