排序方式： 共有14条查询结果，搜索用时 125 毫秒
1.
Emplacement of a large igneous province is usually accompanied by kilometre-scale uplift over an area of ∼106 km2. We have developed a method for mapping the dynamically supported swell associated with the North Atlantic Igneous Province by inverting palaeo-topographic information from continental margins. In the forward model, latest Palaeocene palaeo-topography around Britain and Ireland is calculated by correcting present-day topography for global sea-level change, denudation and dynamic support. We initially assume a Gaussian, axially symmetric dynamic support profile. Modelled coastlines are compared with palaeo-coastlines mapped on 2D and 3D reflection seismic data. In the inverse model, the amplitude, width and centre of the dynamically supported swell are determined by minimising misfit between modelled and observed coastlines. Uncertainties associated with global sea-level variation and denudation have little effect on this calculation. The best-fit dynamic support profile from inverting palaeo-coastline positions is in good agreement with dynamic support estimates from subsidence anomalies measured in extensional sedimentary basins fringing Britain and Ireland. However, a circular planform of dynamic support cannot simultaneously account for palaeo-coastlines, subsidence anomalies and the spatial extent of the North Atlantic Igneous Province. In combination, these data suggest that the swell was more irregular in planform. This inference can be tested in future by modelling stratigraphic data from offshore Norway, Greenland and Canada. The large areal extent and short time interval for inflation of the dynamically supported swell are best explained by rapid convective emplacement of an abnormally hot mantle layer horizontally beneath the lithosphere, during the starting phase of the Icelandic convective system. We emphasise the need to interpret the igneous record jointly with the dynamic support history when discussing models of large igneous province formation and mantle convection. 相似文献
2.
3.
Surface solar radiation measurements have been made at many meteorological sites for long periods. These data potentially provide higher temporal resolution cloud amount information than traditional estimations of cloud amount recorded in eighths of the sky obscured (oktas). To utilise existing solar radiation datasets for this, two quantities are derived from the surface measurements of global (G) and diffuse (D) solar irradiance, and top of atmosphere solar irradiance (E) — the Diffuse Fraction (DF = D/G) and the Opaqueness (Op = 1 - G/E). These are compared with subjective cloud observations made at Reading daily during 1997 to 2006. This shows that DF measurements are sensitive to cloud amount, particularly for low and moderate cloud coverage. Complimentary information is available in Op, which is particularly sensitive to cloud amount in moderate to overcast cloud coverage. In overcast conditions, DF = 1, during which Op provides a measure of overcast cloud thickness. As well as cloud amount, the variability in DF and Op provides a basis for discriminating between cumuliform and stratiform cloud: large variability indicates convective cloud whereas only small variability occurs under stratiform cloud. 相似文献
4.
Quartile-deviation (QDa) and median-diameter (Md) values have been calculated from the grain-size distributions of a variety of Pleistocene and modern glacial and para-glacial sediments. The QDa—Md pairs are plotted on double-log graph paper to determine whether their graphic distributions are distinctive, and to examine their sedimentological implications. Three distributions emerge. The first has a shallow gradient and represents coarse bi-modal tills consisting largely of rock fragments — distribution (a). The second, with an intermediate gradient, represents fine uni- or non-modal tills consisting mainly of monomineralic grains — distribution (b). The third (c) has a steep gradient and consists of bi-modal tills with a dominant proportion of monomineralic grains. Each of these distributions is explained by using the grain-size characteristics of physically weathered rock debris as a starting point for discussion. By a process dominated by the continual addition of weathered-grain populations (population addition), tills with (a) characteristics are formed. By a process dominated by comminution and the selective removal of grains by ablation (population subtraction), tills with (b) characteristics are formed. Any of these tills, when subjected to flowing water, rapidly lose their glacial characteristics and assume QDa—Md characteristics of fluvial deposits (although they are fluvioglacial). 相似文献
5.
Environmental perceptions and practices of livestock keepers on the Namaqualand Commons challenge conventional rangeland management 总被引：1，自引：1，他引：1
Nicky Allsopp Catherine Laurent Laure M.C. Debeaudoin M. Igshaan Samuels 《Journal of Arid Environments》2007,70(4):740
In South Africa, interventions in communal rangelands proposed by most agriculture-sector institutions are still dominated by the belief that communal herders have little technical skills and by the tenets of “the tragedy of the Commons”, assuming that individual livestock keepers are selfish, norm free, and aiming at maximising short-term offtake and that there is no consistent management of the commons. In this study we show that practices of a diversity of livestock keepers on the Leliefontein Commons of Namaqualand are at odds with this viewpoint: access to rangeland and its use are structured by collective norms and concerns regarding both the sharing of resources and their long-term sustainability. We also show that part of the management of the commons relies on grazing practices which involve tacit and formalised technical knowledge that can be described and modelled. Livestock keepers assess the heterogeneity of the grazing quality of the commons. They are aware of the toxicity and unpalatability of some plants, of the way this differs according to the amount ingested and the availability of other forage, and of the variations of these two factors over seasons of grazing. They classify grazing areas and decide on daily grazing routes according to a complex set of criteria. Carrying capacity of each area is not considered as a fixed parameter but rather as a variable dependant on rainfall. Seasonal movements between areas are designed accordingly. These findings offer a new perspective for research agendas on technical models and extension measures for communal rangeland management particularly in arid and semi-arid areas. 相似文献
6.
Jasper A. Slingsby Abri de Buys Adrian D. A. Simmers Eric Prinsloo Greg G. Forsyth Julia Glenday Nicky Allsopp 《水文研究》2021,35(4):e14101
The Jonkershoek Forestry Research Station was established in the Mediterranean climate region of South Africa in 1935 to implement a multiple catchment experiment to determine the effects of afforestation on water yield. The experiment consists of six neighbouring catchments previously supporting indigenous fynbos shrublands, five of which were sequentially afforested with Pinus radiata plantations every 8 years from 1940 to 1980 and one kept as the control. They conclusively demonstrated a significant impact of afforestation on streamflow. The treatment catchments have seen subsequent plantation rotations since 1980 and rainfall and streamflow observations have been continued to date. Here we describe the site, experimental design, rainfall and streamflow records, instrumentation, and how to access the data. We also provide details of recently installed instrumentation, including full weather stations, fog gauges, and an eddy covariance flux tower. The Jonkershoek catchment experiment was the core of a globally significant interdisciplinary research programme (1935–1992) that shaped most environmental policies and practices relating to biodiversity, forestry, fire and water in South Africa and beyond. The South African Environmental Observation Network (SAEON) inherited the experiment in 2010 and is maintaining it as a long-term eco-hydrological research platform and global change observatory. 相似文献
7.
In Namaqualand, heavy grazing of low-lying areas has resulted in the reduction of palatable perennial shrubs and the dominance of the unpalatable shrub, Galenia africana. This study investigates the process of rehabilitation and how it can be used to gain an understanding of the ecological factors that may be sustaining altered vegetation states. We intervened to change the biophysical environment through the construction of microcatchments and brushpacks to act as traps for water, seed and organic material combined with the introduction of seeds and seedlings were implemented. Two years after implementation brushpacked microhabitats improved natural recruitment of herbaceous species compared to bare areas at two out of three sites. Microhabitats did not influence natural recruitment of perennial seedlings. Emergence of palatable perennial seed was not influenced by microhabitat manipulation. Seedlings transplanted under adult G. africana shrubs showed higher survival rates at two out of three sites compared to seedlings in unmodified areas. 相似文献
8.
Simon Williams Nicky M.Wright John Cannon Nicolas Flament R.Dietmar Müller 《地学前缘(英文版)》2021,12(2):769-780
Reconstructions of past seafloor age make it possible to quantify how plate tectonic forces,surface heat flow,ocean basin volume and global sea level have varied through geological time.However,past ocean basins that have now been subducted cannot be uniquely reconstructed,and a significant challenge is how to explore a wide range of possible reconstructions.Here,we investigate possible distributions of seafloor ages from the late Paleozoic to present using published full-plate reconstructions and a new,efficient seafloor age reconstruction workflow,all developed using the open-source software GPlates.We test alternative reconstruction models and examine the influence of assumed spreading rates within the Panthalassa Ocean on the reconstructed history of mean seafloor age,oceanic heat flow,and the contribution of ocean basin volume to global sea level.The reconstructions suggest variations in mean seafloor age of~15 Myr during the late Paleozoic,similar to the amplitude of variations previously proposed for the Cretaceous to present.Our reconstructed oceanic age-area distributions are broadly compatible with a scenario in which the long-period fluctuations in global sea level since the late Paleozoic are largely driven by changes in mean seafloor age.Previous suggestions of a constant rate of seafloor production through time can be modelled using our workflow,but require that oceanic plates in the Paleozoic move slower than continents based on current reconstructions of continental motion,which is difficult to reconcile with geodynamic studies. 相似文献
9.
The Weissliegend is a European sandstone unit of largely late Early Permian age. It is underlain by the Early Permian Rotliegend red desert sandstones and is overlain by the conventionally accepted basal bed of the Zechstein-the bituminous marine shales of the Kupferschiefer. The Weissliegend sandstones are characteristically white or grey in colour and have been recognised beneath the North Sea, in Germany and in Poland. Equivalents, which are red or yellow in colour, occur in NE England and at the southern edge of the Moray Firth Basin in Scotland.From an examination of cliff and quarry exposures in Britain, and of drill cores from southern North Sea gas wells, it is now believed that the bulk of the Weissliegend sandstones (and their equivalents) were originally deposited as aeolian dunes. These dune sands, however, were later modified by a widespread event, the Zechstein transgression, which caused their partial homogenisation, the creation of large-scale soft-sediment deformation structures, and the local and minor reworking of some of the dune flanks.The preferred mechanism of deformation is interpreted as: (1) entrapment of large pockets of air within the bodies of the dunes by flanking and overlying wetted dune sands; (2) venting of the air pockets when the rising internal air pressures overcame the weight of the hydrostatic head of water and the capillary (cohesive) strength of the overlying wetted sands; (3) the rapid replacement of air by water, which caused liquidisation of the original dune laminae; and (4) the associated collapse and final consolidation of the sands into a tigher packing configuration.Deformations seem to be more developed in former transverse dunes than in seif dunes. The reason may be that the relatively tightly packed low-angle accretion bedding common on the flanks of seif dunes is more resistant to deformation than the looser avalanche sands that form a major part of transverse dunes. Limited reworking of former dune sands was probably best developed on the steep lee slopes of transverse dunes and the steeper upper slopes of seif dunes.The lack of reddening of the Weissliegend sandstones-proper is attributed to a combination of their accumulation above the Rotliegend water table, to the rapidity of the Zechstein transgression, and to the anoxic state of the early Zechstein sea floor. The Weissliegend sands, unlike the underlying Rotliegend into which they grade, were thus never in a diagenetic environment that was conducive to reddening.Finally, it is recommended that the term Weissliegend be dropped in any formational sense. It should only be retained for the Weissliegend proper, and their equivalents, to denote a complex facies association dominated by (1) the uppermost Early Permian Rotliegend dune sands (now partly deformed) that lay above the water table just prior to the Zechstein transgression, together with (2) the minor erosional marine products caused by that transgression. The latter, sensu stricto, are Zechstein sandstones of earliest Late Permian age. 相似文献
10.
Borehole and geophysical data have been collated for two sections across the Tay estuary on the lines of the road and rail bridges at Dundee. Geological reconstructions reveal a complex late-glacial and Holocene fill resting on a basement of Devonian sandstones and lavas. Comparative analyses of bathymetric charts (1816–1970) have enabled areas of present-day relatively stable and unstable bed to be identified. Stable areas are underlain by either gravels or partially compacted clays; unstable areas by loose, coarse to fine, sands. Non-migratory channels coincide with the stable areas; shifting sand banks and migratory channels occur elsewhere. 相似文献