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151.
152.
Synchronous variability changes in Alpine temperature and tree-ring data over the past two centuries
DAVID FRANK ROBERT WILSON JAN ESPER 《Boreas: An International Journal of Quaternary Research》2005,34(4):498-505
The understanding of extremes and their temporal distribution is useful in characterizing the behaviour of the climate system, and necessary for understanding their social and economic costs and risks. This task is analogous to the study of pointer years in dendrochronological investigations. Commonly used dendroclimatological methods, however, tend to result in an equalization of variance throughout the record by normalizing variability within moving windows. Here, we analyse a larger network of high-elevation temperature-sensitive tree sites from the European Alps processed to preserve the relative frequency and magnitude of extreme events. In so doing, temporal changes in year-to-year tree-ring width variability were found. These decadal length periods of increased or decreased likelihood of extremes coincide with variability measures from a long-instrumental summer temperature record representative of high-elevation conditions in the Alps. Intervention analysis, using an F-test to identify shifts in variance, on both the tree-ring and instrumental series, resulted in the identification of common transitional years. Based on a well-replicated network of sites reflecting common climatic variation, our study demonstrates that the annual growth rings of trees can be utilized to quantify past frequency and amplitude changes in extreme variability. Furthermore, the approach outlined is suited to address questions about the role of external forcing, ocean-atmosphere interactions, or synoptic scale changes in determining patterns of observed extremes prior to the instrumental period. 相似文献
153.
DAVID H. ROBERTS ROGER V. DACKOMBE GEOFF S. P. THOMAS 《Boreas: An International Journal of Quaternary Research》2007,36(2):115-129
Late Devensian glacial sediments and landforms of the Isle of Man record the advance and deglacial signature of the central sector of the British-Irish Ice Sheet. Evidence from the area, gathered from striae, erratic trains and drift limits, show ice was routed over and around the island in two flow phases post-36 kyr BP. In the south of the island, streamlined depositional bedforms with low elongation ratios suggest low ice-flow velocities resulting from one or more of (i) the up-ice location of the island within a regional onset zone, (ii) flow retardation of ice interacting with the margins of the island and (iii) localized drainage of the deforming bed. The deglacial landform assemblage of lateral marginal sandurs and drainage diversions, coupled with a lack of dead-ice features, suggests ice did not downwaste in situ but retreated intact along the coastal margins as Manx Upland ice thinned. In the north of the island, however, the Bride Moraine complex indicates a change in deglacial ice-sheet dynamics, with temporary re-advance and marginal oscillation causing proglacial tectonism and thrusting of the glacial sediment pile, possibly during the Killard Point Stadial event (18.8-16.4 cal. kyr BP). From a basin-wide perspective, the Irish Sea Basin sector of the British-Irish Ice Sheet had many of the characteristics of an ice stream, such as a zone of flow convergence up-ice, a grounding line in the southern Celtic Sea and recessional limits characterized by proglacially tectonized and thrust dead-ice landscapes indicative of a rapidly oscillating ice margin. 相似文献
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155.
DENIS-DIDIER ROUSSEAU DAVID HENRY KEEN 《Boreas: An International Journal of Quaternary Research》1989,18(1):61-66
New sampling of Mollusca from Portelet, Jersey, Channel Islands, has allowed characterization of local conditions during deposition of the pre-Kesselt Weichselian loess. The Portelet fauna also allows a reassessment of the distribution of the Columella fauna in western Europe. Comparison of the Portelet fauna with those of eastern France and central Europe shows a decrease of the number of species present in this far-western locality which appears to be either a result of the operation of a typical 'peninsula' effect comparable to that seen in other zoological groups or a consequence of their occurrence in a marginal part of the European loess belt. 相似文献
156.
DAVID LOWENTHAL 《Geographical review》2007,97(2):202-229
ABSTRACT. Geography, experience, and imagination are all crucial to how we take measure of islands. Prime foci of legend and invention, islands have haunted humanity since the dawn of history. Why are they so intensely loved and loathed, desired and rejected, minutely scrutinized yet often perilously misjudged? On islands we feel alternately landed and adrift, magnified and reduced, fulfilled and voided, at home and in exile. These and other polarities are reviewed here with examples from Caribbean, Atlantic, Mediterranean, antipodean, and imaginary islands fancied by the creator. 相似文献
157.
Groundwater silcretes have been recognized recently as major terrestrial silicon sinks and yet their origins are poorly understood. This paper aims to further the understanding of the micro‐fabric, geochemistry and formation of groundwater silcretes, through detailed analyses of silcrete boulders from the South Downs, Sussex, UK. In‐depth petrological investigations of silcrete blocks identified three varieties of silcrete across the study area (saccharoid, hard and pebbly conglomeratic or silcrete breccia), similar to those elsewhere in England. Silcrete fabrics were universally simple and preserved host material structures. Optically continuous quartz overgrowths were the dominant cement and developed on framework grains in the absence of silt‐ and clay‐sized particles. Finer‐grained silica cements occurred in isolated patches and as vein‐ or cap‐like geopetal features. These caps and veins formed through the silicification of illuviated clay‐rich material that entered the host sediment prior to, or in the early stages of, overgrowth formation. Titanium contents were related to the amount of fine‐grained silica and appear to reflect variations in host material chemistry. Subsequent to silicification, the silcretes were altered by at least two phases of ferruginization, characterized by the ingress of iron minerals and partial fabric dissolution or replacement. The study reveals strong similarities in the micromorphology and geochemistry of groundwater silcretes in the study area and those developed in sandy host sediments in neighbouring parts of northwest Europe. Variations that do occur could be explained by differences in the host sediment, geological setting or tectonic history of the respective basins (or sub‐basins), suggesting that there may be a common mechanism for silcrete formation across north‐west Europe. Silcrete development appears to be related to the release of silica accompanying acid leaching of the host material or adjacent strata. In the South Downs, this probably resulted from oxidation of pyrite in the argillaceous and lignitic sediments overlying the host sands. It is envisaged that palaeoenvironmental conditions were of limited importance for silcrete development and that there was no specific ‘era’ of formation, with phased development accompanying landscape evolution through the Neogene into the Pleistocene. 相似文献
158.
159.
DAVID E. SMITH CALLUM R. FIRTH ROBIN A. CULLINGFORD 《Boreas: An International Journal of Quaternary Research》2002,31(2):185-202
Previously published radiocarbon-dated horizons relating to early and middle Holocene relative sea-level change along the eastern coast of mainland Scotland are examined and trends determined. The data are modified to ensure comparability and are compared against the pattern of glacio-isostatic uplift in the area. Results show that the rate of relative sea-level rise during the Main Postglacial Transgression in the middle Holocene becomes greater towards the edge of the uplifted area, whilst the age of the Main Postglacial Shoreline becomes younger in the same direction. Linear and quadratic regression analyses disclose trends which indicate that at the 0 m HWMOST isobase of the Main Postglacial Shoreline the rate of relative sea level rise between c. 8400 and c . 7000 14 C years BP ( c . 9500 to c . 7900 cal. BP) was 5-11 mm/radiocarbon year or 6-11 mm/calibrated year, whilst at the same isobase the Main Postglacial Shoreline was reached between 5500 and 6100 14 C years BP (between 6300 and 7000 cal. BP). The relative sea-level changes identified are compatible with a rising sea surface level offshore, which may have involved three episodes, possibly related to regional and wider deglaciation. 相似文献
160.