摘要:

AbstractGlacial geological studies in tropical mountain areas of the Southern Hemisphere can be used to address two issues of late Pleistocene climate change: the global synchroneity of deglaciation and the magnitude of temperature reduction in the tropics. Radiocarbon dates from the Cordillera Real and from other areas in Perú and Bolivia suggest that late Pleistocene glaciation culminated between 14 000 and 12 000 yr BP, followed by rapid deglaciation. Because deglaciation was apparently synchronous with that in Northern Hemisphere regions, insolation change at high latitudes may not have been the only factor that produced global deglaciation at this time. Late Pleistocene glaciation in the Cordillera Real culminated when precipitation was 200 mm yr−1higher and temperatures were 3.5° ±1.6°C lower than today; this produced an equilibrium‐line altitude depression of about 300 ± 100 m on the western side of the cordillera. Prior to this, conditions were drier and probably at least as cold. However, the lack of moraines in the Cordillera Real dated to the Last Glacial Maximum (ca. 18000 yr BP) precludes using the equilibrium‐line altitude method to quantitatively evaluate the discrepancy between warm sea‐surface temperatures and cold terrestrial conditions reconstructed with other proxies for this

ISSN:0267-8179    

DOI:10.1002/jqs.3390070202

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe results of a thermoluminescence (TL) dating and geochemical survey of loessic deposits in southeast England, involving sampling at 26 localities, demonstrate that the loessic deposits appear to be the product of at least three depositional phases of aeolian silt during the Late Pleistocene, namely 10‐25 ka BP, 50–125 ka BP and>170 ka BP. The majority of localities sampled date to the Late Devensian, but isolated pockets of older material were found throughout the area. The majority of the samples analysed were of decalcified material. The results of geochemical analyses of major oxide and trace element concentrations indicate the uniformity of material and suggest a common provena

ISSN:0267-8179    

DOI:10.1002/jqs.3390070203

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe influence of glacier hydrology on the time‐dependent morphology and flow behaviour of the late Weichselian Scandinavian ice sheet is explored using a simple one‐dimensional ice sheet model. The model is driven by orbitally induced radiation variations, ice‐albedo feedback and eustatic sea‐level change. The influence of hydrology is most marked during deglaciation and on the southern side of the ice sheet, where a marginal zone of rapid sliding, thin ice and low surface slopes develops. Such a zone is absent when hydrology is omitted from the model, and its formation results in earlier and more rapid deglaciation than occurs in the no‐hydrology model. The final advance to the glacial maximum position results from an increase in the rate of basal sliding as climate warms after 23000 yr BP. Channelised subglacial drainage develops only episodically, and is associated with relatively low meltwater discharges and high hydraulic gradients. The predominance of iceberg calving as an ablation mechanism on the northern side of the ice sheet restricts the occurrence of surface melting. Lack of meltwater penetration to the glacier bed in this area means that ice flow is predominantly by internal deformation and the ice sheet adopts a classical parabolic surfac

ISSN:0267-8179    

DOI:10.1002/jqs.3390070204

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe deglaciation of Skye at the close of the Loch Lomond Stadial is assessed on the basis of detailed geomorphological mapping and pollen‐stratigraphic correlations. It is concluded that deglaciation proceeded in two distinct stages. The first was marked by numerous glacier stillstands and readvances, while uninterrupted retreat and local glacier stagnation occurred during the second and final stage. The pollen evidence indicates that the first stage was well advanced before the marked thermal improvement at the start of the Flandrian, and it is inferred that initial glacier retreat occurred in response to a decline in precipitation in the later part of the Loch Lomond Stadial. The first stage of glacier retreat continued into the early Flandrian, during which climatic amelioration was interrupted briefly. Final deglaciation appears to have occurred rapidly in response to sustained temperature increases. The collective evidence also indicates spatial variations in the timing of deglaciation, which appear to reflect differences in glacier morpholog

ISSN:0267-8179    

DOI:10.1002/jqs.3390070205

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractExamination of two radiocarbon‐dated vibrocores taken from south of St Kilda at a water depth of about 155 m, a short distance within the maximum position of the Late Devensian (Dimlington Stadial) ice sheet, suggests that the St Kilda Basin became free of glacier ice after 15250 yr BP. Sedimentation in a shallow, low energy, high arctic, muddy environment continued until after 13500 yr BP. There followed a higher energy temperate episode during which water depths were roughly about 40 m: this is correlated with the latter part of the Windermere Interstadial and with the warmer interval known in shallow Scottish seas about or a little before 11 000 yr BP. The Loch Lomond (Younger Dryas) Stadial is marked in the vibrocores by the return of muddy sediments and a cold‐water fauna. Relatively shallow water conditions seem to have persisted into the earliest Flandrian, when the water depth was still roughly 60 m, corresponding to a sea‐level in the area 90–100 m below present. It is suggested that pack ice was widespread in the northeast Atlantic before the Windermere Interstadial and also during the Loch Lomond Stadial, when it transported shards of Icelandic volcanic ash into the St Kilda basin. Estimates of sea‐surface temperature for the last part of the Windermere Interstadial are close to those derived from the deep‐sea record for the

ISSN:0267-8179    

DOI:10.1002/jqs.3390070206

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractRadiocarbon‐dated pollen diagrams from four Danish sequences are presented in their sedimentological context. The pollen diagrams indicate the former presence of stands ofBetulaandPinusand of Ericales, Poaceae, Cyperaceae and other herbs. From a straightforward comparison with other diagrams, one of the present series is tentatively correlated with part of the Early Weichselian Brørup Interstadial and two with parts of the Odderade Interstadial, which has not been demonstrated previously in Denmark. One diagram and part of another do not seem to match previously published records in northwest Europe. Yet, the possibility that the sediments date to hitherto unrecorded periods with environmental conditions similar to those reflected in known Weichselian deposits is not excluded. In two of the sequences high percentages of charcoal particles suggest the former occurrence of fires, which influenced the palaeovegetation and soil surface stability. It is suggested that counting of charcoal particles is included in future investigations in order to establish whether periods with fires can be identified in the sedimentary sequenc

ISSN:0267-8179    

DOI:10.1002/jqs.3390070207

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThis paper tests two assumptions fundamental to the use of fine‐grained silicic tephras from Iceland for creating chronological frameworks in northwest Europe. It is shown firstly, that glass shards can retain their overall chemical integrity on at least a four millennial time‐scale in contrasting depositional environments in Iceland and Scotland, and secondly, that an acid digestion process, the most practical method for extracting tephra from peat, does not significantly bias the results of major element analysis by electron microprobe. The implication is that there is great potential and an appropriate method for developing both the resolution and the spatial coverage of chronologies based on silicic Icelandic tephras in northwest Eur

ISSN:0267-8179    

DOI:10.1002/jqs.3390070208

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390070210

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390070201

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY