排序方式： 共有13条查询结果，搜索用时 234 毫秒
1.
Paleoenvironments during Younger Dryas‐Early Holocene retreat of the Greenland Ice Sheet from outer Disko Trough,central west Greenland 
下载免费PDF全文
下载免费PDF全文 2.
BETHAN J. DAVIES DAVID H. ROBERTS COLM Ó COFAIGH DAVID R. BRIDGLAND JAMES B. RIDING EMRYS R. PHILLIPS DEREK A. TEASDALE 《Boreas: An International Journal of Quaternary Research》2009,38(3):555-578
This research reconstructs ice-sheet processes operating during the Late Devensian in northeast England. The article assesses the lithostratigraphy of the Devensian glacial tills of Whitburn Bay, eastern County Durham, and presents the first detailed analysis of petrological, geochemical and biostratigraphical data to reconstruct lithostratigraphy, provenance and iceflow pathways. Two Devensian tractions tills (the Blackhall and Horden tills) are separated by a boulder pavement, pointing to a switch in ice-bed conditions and the production of a melt-out lag prior to deposition of the upper traction till, the Horden Till. The Blackhall Till contains Magnesian Limestone, Carboniferous Limestone, Whin Sill dolerite and Old Red Sandstone, suggesting a northwesterly source, probably from the Midland Valley and the Southern Uplands. The Horden Till contains erratics and heavy minerals derived from crystalline bedrock sources in the Cheviot Hills and northeast Scotland. Within the Horden Till are numerous sand, clay and gravel-filled canals incised downwards into the diamicton which are attributed to a low-energy, distributed, subglacial canal drainage system. Coupled with hydro-fractures and the boulder pavement, this suggests that a partially decoupled, fast-flowing ice stream deposited the Horden Till. The uphill, landward direction of ice movement indicates that the ice stream was confined in the North Sea Basin, possibly by the presence of Scandinavian Ice. 相似文献
3.
STEPHEN J. LIVINGSTONE COLM Ó COFAIGH DAVID J.A. EVANS ADRIAN PALMER 《Boreas: An International Journal of Quaternary Research》2010,39(3):505-527
Livingstone, S. J., Ó Cofaigh, C., Evans, D. J. A. & Palmer, A. 2010: Sedimentary evidence for a major glacial oscillation and proglacial lake formation in the Solway Lowlands (Cumbria, UK) during Late Devensian deglaciation. Boreas, Vol. 39, pp. 505–527. 10.1111/j.1502‐3885.2010.00149.x. ISSN 0300‐9483. This paper is a sedimentological investigation of Late Devensian glacial deposits from the Solway Lowlands, northwest England, in the central sector of the last British–Irish Ice Sheet. In this region, laminated glaciolacustrine sediments occur, sandwiched between diamictons interpreted as subglacial tills. At one location the laminated sediments are interpreted as varves, and indicate the former presence of a proglacial lake. Correlation of these varves with other laminated sediments indicates that the glacial lake was at least 140 km2 in area and probably much larger. Extensive beds of sand, silt and gravel throughout the Solway Basin associated with the lake demonstrate ice‐free conditions over a large area. Based on the number of varves, the lake was in existence for at least 261 years. The stratigraphic sequence of varves bracketed by tills implies a major glacial oscillation prior to the Scottish Re‐advance (16.8 cal. ka BP). This oscillation is tentatively correlated with the Gosforth oscillation at c.19.5 cal. ka BP. Subsequent overriding of these glaciolacustrine sediments during a westward‐moving re‐advance demonstrates rapid ice loss and then gain within the Solway Lowlands from ice‐dispersal centres in the Lake District, Pennines and Southern Uplands. It is speculated that the existence of this and other lakes along the northeastern edge of the Irish Sea Basin would have influenced ice‐sheet dynamics. 相似文献
4.
A late Pleistocene morainal bank is sited in a depocentre to the lee of a major rock ridge, near Greystones, in the western Irish Sea Basin. During deglaciation the ridge provided a pinning point during tidewater wastage northwards. Sedimentation patterns and palaeocurrent data show morainal bank growth by discharge from a single basal efflux located to the east or south-east of the ridge during ice marginal re-equilibration. The four lithofacies associations which are recognized from the western part of the formerly more extensive apron are related largely to variable jet and plume sedimentation. At the base of the 1.6 km long exposure, Lithofacies association 1 (massive mud, muddy diamict and laminated mud) was deposited from turbid plumes, variable ice rafting and traction current activity. Lenticular units of gravels within this mud bank record high energy pulses and sediment fluxes from the efflux jet. Lithofacies association 2 (sands, laminated muds and muddy diamict) is discontinuous and occurs within basins along a marked erosion surface cut in Lithofacies association 1. It is associated with a decrease in jet strength, traction currents and suspension sedimentation. Lithofacies association 3 is a tabular body of interbedded diamicts and gravels which is present along the entire section. It documents the decay phase of re-equilibration as the ice margin disintegrated catastrophically and released large volumes of heterogeneous sediment which was resedimented by quasicontinuous mass flow. Lithofacies association 4 consists of stratified and massive gravels within distributary channels cut into underlying facies and represents the last phase of meltwater activity. Sediment geometries, particularly sedimentary contrasts representing erosion surfaces at a variety of scales and abrupt textural contrasts are attributed to jet switching. Lithofacies association 1 (60%) and Lithofacies association 3 (30%) are the dominant facies. In favourable topographic settings this stratigraphic couplet is a signature for re-equilibrated ice margins in isostatically depressed basins dominated by tidewater fronts, rapid ice flux and high relative sea level. Morainal banks document rapid environmental change and in the Irish Sea Basin they form part of a deglacial event stratigraphy related to unstable tidewater margins and high relative sea level. Deglaciation was therefore controlled primarily by high relative sea level rather than climatic forcing. Facies variations should therefore not be used for stratigraphic correlations in place of direct stratigraphy. This type of situation may be more common than hitherto realized in Late Pleistocene, mid-latitude shelves where most of the preserved stratigraphy is characterized by complex, interbedded sequences formed when isostatic depression exceeded sea-level fall. 相似文献
5.
There has been a huge surge in interest in the preservation of Madagascar's forests in the past two decades, but despite the investment of hundreds of millions of dollars, the goal remains elusive. Recent legislation has given the government the authority to enter into contractual arrangements with communities for the management of the country's public forests, so it has become crucial to grapple with the significant challenges involved. This paper explores the enormity of the challenge of forest governance in Madagascar in an era of decentralization. By examining several forests in one part of the country, it reveals a dizzying range of forest types and forms of use and governance within a fairly small portion of the country. These examples make it apparent that the history of forest management in Madagascar constitutes a broad-ranging experiment with forest governance. Simply monitoring the dynamics of the forest canopy is a significant technical challenge. However, this pales in comparison to the difficulties inherent in explaining those dynamics and assessing the sustainability and equity of different management regimes. Of the forests considered in the study, those where the Malagasy state has partnered with international conservation and development organizations seem to stand out, both in terms of stabilized, or even growing, forest cover, as well as a balance of interests among users. 相似文献
6.
7.
8.
Map and GIS database of glacial landforms and features related to the last British Ice Sheet 总被引：1，自引：0，他引：1
CHRIS D. CLARK DAVID J. A. EVANS ANJANA KHATWA TOM BRADWELL COLM J. JORDAN STUART H. MARSH WISHART A. MITCHELL MARK D. BATEMAN 《Boreas: An International Journal of Quaternary Research》2004,33(4):359-375
A review of the academic literature and British Geological Survey mapping is employed to produce a 'Glacial Map', and accompanying geographic information system (GIS) database, of features related to the last (Devensian) British Ice Sheet. The map (1:625 000) is included in a folder and GIS data are freely available by web download (http://www.shef.ac.uk/geography/staff/clark_chris/britice.html). Emphasis is on information that constrains the last ice sheet. The following are included: moraines, eskers, drumlins, meltwater channels, tunnel valleys, trimlines, limit of key glacigenic deposits, glaciolacustrine deposits, ice-dammed lakes, erratic dispersal patterns, shelf-edge fans and the Loch Lomond Readvance limit of the main ice cap. The GIS contains over 20 000 features split into thematic layers (as above). Individual features are attributed such that they can be traced back to their published sources. Given that the published sources of information that underpin this work were derived by piecemeal effort over 150 years, then our main caveat is of data consistency and reliability. It is hoped that this compilation will stimulate greater scrutiny of published data, assist in palaeoglaciological reconstructions and facilitate use of field evidence in numerical ice-sheet modelling. It may also help direct field workers in their future investigations. 相似文献
9.
Geochemistry and Petrogenesis of Central Lebombo Basalts of the Karoo Igneous Province 总被引：6，自引：3，他引：6
Three distinet basaltic rock types are recognized on the basisof detailed new analyses in the eastern zone (central Lebombo)of the Karoo Igneous Province. The high titanium and zirconiumgroup is composed of low-Fe (henceforth called simply high Ti-Zrbasalts) and high-Fe (henceforth called ‘high-Fe’)varieties and distinguished from the low titanium and zirconiumgroup by 2–4 times greater abundances of Ti, P, Zr, Nb,Y, La, Ce, and Nd (high field strength elements; HFSE), as wellas higher but more variable abundances of K, Rb, Ba, and Sr,All rock types are interbedded to some degree, although thehigh Ti-Zr type (low-Fe) predominates at the base of the sectionand the ‘high-Fe’ type occurs mostly at the topof the section. Stratigraphic relationships show that the highTi-Zr basalts to the south within a distance of 60 km. Eruptionof each rock type from replenished, tapped, and fractionatedmagma chambers can account for much of the major and trace elementvariation within each suite, with the notable exceptions ofthe large ion lithophile elements (LILE; K, Rb, and Ba) andSr. Neither crystal fractionation nor contamination with graniticcrust can produce the variations between the basalt types anddoes not appear to affect significantly the geochemistry withinany basalt type. The North Lebombo high Ti-Zr picrites are themost likely parent magmas to the high Ti-Zr low-MgO group. Considerationof previous studies of the picrites suggested that the highTi-Zr picrite geochemistry requires a mixed source or the mixingof two picrite endmember magmas discriminated on the basis ofdegree of incompatible element enrichment. At least one of theendmembers must be within, or derived from, sub-continentallithospheric mantle. The low-MgO high Ti-Zr basalt geochemistryis consistent with evolution from a picritic parent with abouta 30–44 wt. % mantle lithospheric component. In contrast,the low Ti-Zr basalt group parental composition is outside thecompositional range suggested for the high Ti-Zr picrite endmembers,being more depleted in incompatible elements (e.g., Ti, P, Zr,Nb, LREE, and LILE). Again, by analogy with possible picriticparents, this group may segregate from a source more depletedin incompatible elements than a mid-ocean ridge basalt (MORB)source, leaving a harzburgitic residue. The ‘high-Fe’basalt group is very evolved (e.g., MgO <5%) but may stillbe constrained by Nd-isotope and Zr/Y ratios to have a mantlesource geochemically similar to that from which the Walvis Ridgebasalts were derived. If a mantle plume contributed to basaltgeochemistry significantly, then the late-stage ‘high-Fe’basalts represent this component most closely. 相似文献
10.
Detailed studies of a new, complete Marl Slate core in South Yorkshire have provided information on isotopic (δ13C, δ18O, δ34S) and geochemical variations (trace elements and C/S ratio) which enable the formulation of a model for carbonate and sulphide precipitation in the Late Permian Zechstein Sea. Calcite and dolomite are intimately associated; the fine lamination, organic character and absence of benthos in the sediments are indicative of anoxic conditions. Lithologically the core can be divided into a lower, predominantly sapropelic Marl Slate (2 m) and an upper Transition Zone (0·65 m) of alternating sapropel and calcite-rich and dolomite-rich carbonates. C/S ratios are 2·22 for the Marl Slate and 1·72 for the Transition Zone respectively, both characteristic of anoxic environments. δ18O in the carbonates shows a large and systematic variation closely mirrored by variations in calcite/dolomite ratio. The results suggest a fractionation factor equivalent to a depletion of 3·8% for 18O and 1·5% for 13C in calcite. The δ34S values of pyrite are isotopically light (mean value = - 32·7%) suggesting a fractionation factor for the Marl Slate of almost 44%, typical of anoxic basins. The results are related to stratification in the early Zechstein Sea. Calcite was precipitated in oxic upper layers above the halocline. Below the oxic/anoxic boundary framboidal pyrite was precipitated, resulting in lower sulphate concentration and elevated Mg/Ca ratio (due to calcite precipitation). As a result of this, dolomite formation occurred below the oxic/anoxic interface, within the anoxic water column and in bottom sediments. Variations in calcite/dolomite ratios, and isotopic variations, are thus explained by fluctuations in the relative level of the oxic/anoxic boundary in the Zechstein Sea. 相似文献