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Antony J. Long Sarah A. Woodroffe Sue Dawson David H. Roberts Charlotte L. Bryant 《第四纪科学杂志》2009,24(4):345-359
In West Greenland, early and mid Holocene relative sea level (RSL) fall was replaced by late Holocene RSL rise during the Neoglacial, after 4–3 cal. ka BP (thousand calibrated years before present). Here we present the results of an isolation basin RSL study completed near to the coastal town of Sisimiut, in central West Greenland. RSL fell from 14 m above sea level at 5.7 cal. ka BP to reach a lowstand of ?4.0 m at 2.3–1.2 cal. ka BP, before rising by an equivalent amount to present. Differences in the timing and magnitude of the RSL lowstand between this and other sites in West and South Greenland record the varied interplay of local and non‐Greenland RSL processes, notably the reloading of the Earth's crust caused by a Neoglacial expansion of the Greenland Ice Sheet (GIS) and the subsidence associated with the collapse of the Laurentide Ice Sheet forebulge. This means that the timing of the sea level lowstand cannot be used to infer directly when the GIS advanced during the Neoglacial. The rise in Late Holocene RSL is contrary to recently reported bedrock uplift in the Sisimiut area, based on repeat GPS surveys. This indicates that a belt of peripheral subsidence around the current ice sheet margin was more extensive in the late Holocene, and that there has been a switch from subsidence to uplift at some point in the last thousand years or so. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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Relative sea level (RSL) data derived from isolation basins at Innaarsuit, a site on the south shores of the large marine embayment of Disko Bugt, West Greenland, record rapid RSL fall from the marine limit (ca. 108 m) at 10,300-9900 cal yr B.P. to reach the present sea level at 3500 cal yr B.P. Since 2000 cal yr B.P., RSL rose ca. 3 m to the present. When compared with data from elsewhere in Disko Bugt, our results suggest that the embayment was deglaciated later and more quickly than previously thought, at or slightly before 10,300 cal yr B.P. The northern part of Disko Bugt experienced less rebound (ca. 10 m at 6000 cal yr B.P.) compared with areas to the south. Submergence during the late Holocene supports a model of crustal down-warping as a result of renewed ice-sheet growth during the neoglacial. There is little evidence for west to east differences in crustal rebound across the southern shores of Disko Bugt. 相似文献
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Radiocarbon dating of thin palaeopodsols buried beneath turf-banked gelifluction lobes at four localities in the low alpine mountain zone in the Jostedalsbreen region, western Norway, show that gelifluction processes were initiated subsequent to the late Subboreal Chronozone. Although large age-depth gradients have been demonstrated from buried palaeosols in southern Norway, evidence is presented that the palaeosols in this study show only moderate age-depth gradients. The age estimates from these buried palaeosols give maximum dates of burial, but the error is not thought to be large. Gelifluction processes were probably initiated close to the time of the climatic deterioration, which led to the formation of the present glaciers during the Subatlantic Chronozone. The processes may have been most active during the peak of the Little Ice Age, during which a periglacial climate was established to low levels in this mountainous region. 相似文献
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Neoglacial (<3000 years) till and flutes at Saskatchewan Glacier,Canadian Rocky Mountains,formed by subglacial deformation of a soft bed 
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下载免费PDF全文 Understanding the processes that deposit till below modern glaciers provides fundamental information for interpreting ancient subglacial deposits. A process‐deposit‐landform model is developed for the till bed of Saskatchewan Glacier in the Canadian Rocky Mountains. The glacier is predominantly hard bedded in its upper reaches and flows through a deep valley carved into resistant Palaeozoic carbonates but the ice margin rests on a thick (<6 m) soft bed of silt‐rich deformation till that has been exposed as the glacier retreats from its Little Ice Age limit reached in 1854. In situ tree stumps rooted in a palaeosol under the till are dated between ca 2900 and 2700 yr bp and record initial glacier expansion during the Neoglacial. Sedimentological and stratigraphic observations underscore the importance of subglacial deformation of glaciofluvial outwash deposited in front of the advancing glacier and mixing with glaciolacustrine carbonate‐rich silt to form a soft bed. The exposed till plain has a rolling drumlinoid topography inherited from overridden end moraines and is corrugated by more than 400 longitudinal flute ridges which record deformation of the soft bed and fall into three genetically related types: those developed in propagating incipient cavities in the lee of large subglacial boulders embedded in deformation till, and those lacking any originating boulder and formed by pressing of wet till up into radial crevasses under stagnant ice. A third type consists of U‐shaped flutes akin to barchan dunes; these wrap around large boulders at the downglacier ends of longitudinal scours formed by the bulldozing of boulders by the ice front during brief winter readvances across soft till. Pervasive subglacial deformation during glacier expansion was probably facilitated by large boulders rotating within the soft bed (‘glacioturbation’). 相似文献
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At least three sets of moraines mark distinct glacial stands since the last glacial maximum (LGM) in the Three Sisters region of the Oregon Cascade Range. The oldest stand predates 8.1 ka (defined here as post-LGM), followed by a second between ∼ 2 and 8 ka (Neoglacial) and a third from the Little Ice Age (LIA) advance of the last 300 years. The post-LGM equilibrium line altitudes were 260 ± 100 m lower than that of modern glaciers, requiring 23 ± 9% increased winter snowfall and 1.4 ± 0.5°C cooler summer temperatures than at present. The LIA advance had equilibrium line altitudes 110 ± 40 m lower than at present, implying 10 ± 4% greater winter snowfall and 0.6 ± 0.2°C cooler summer temperatures. 相似文献
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Adrian G. Parker Julia Lee-Thorp Peter J. Mitchell 《Proceedings of the Geologists' Association. Geologists' Association》2011,122(1):201-211
The late Holocene environmental history of the Lesotho highlands, southern Africa, is poorly understood with few detailed studies to date. At Likoaeng, Senqu Valley, Lesotho, a 3 m stratified sedimentary sequence from an open-air archaeological site records vegetation development for the period 3400-1070 cal. BP. Phytolith analyses and bulk sediment organic matter δ13C indicate that C4 grassland dominated the lower part of the sequence until approximately 2960 cal. BP when there was a switch to C3 Pooid grassland (2960-2160 cal. BP). Also noted was a change from hunting mainly bovids to a dominance of fishing at the site. The change in grassland type and archaeological subsistence strategies corresponds with an episode of neoglacial cooling and the expansion of Alpine sourgrasses into lower altitudes. From 2160 to 1600 cal. BP grassland became a mix of C3 and C4 types and by 1600-1070 cal. BP there was a return to C4 dominated grassland. During this latter phase there was a reversal from fishing to hunting again (and eventually some keeping of domestic livestock) at the site. These data outline the vegetation response to latitudinal shifts of frontal systems, and relatively strong atmospheric circulation variability, perhaps underpinned by variations of polar water into the Benguela Current during the late Holocene. 相似文献
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Late Glacial and Neoglacial (Little Ice Age) deposits on the Humboldt Massif were analyzed for relative-age dating parameters, including geomorphic and weathering characteristics, geochemical and soil properties. The soil chronosequence, formed in chemically uniform parent materials, provides an important database to study soil evolution in a tropical alpine environment. Extractable and total Fe and Al concentrations, examined to assess their use in relative-age determination, and as paleoenvironmental indicators, provide an important measure of the accumulation and downward profile movement over time of organically-bound Al, ferrihydrite and other crystalline forms (hematite and goethite) of extractable Fe. Ferrihydrite is particularly useful in determining former perched water levels in soils with relation to paleoclimate. The ratios of most Fe extracts are time dependent. The Fed/Fet ratio, within statistical limits, shows a slow increase from LIA (Little Ice Age) to Late Glacial soils, which closely correlates with other alpine soil studies in the middle latitudes and other tropical alpine locales. Values of Ald (dithionite) and Alo (oxalate extractable) generally do not correlate with time; however, Alp (pyrophosphate extractable) measured against Alt (total) provides insight on the downward translocation over time of organically-bound Al. Low leaching rates in this chronosequence are further supported by clay mineralogy trends and the geochemical data. 相似文献
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Diatom-based Holocene paleoenvironmental records from continental sites on northeastern Ellesmere Island, high Arctic, Canada 总被引：2，自引：0，他引：2
I. Rod Smith 《Journal of Paleolimnology》2002,27(1):9-28
Stratigraphic changes in diatom assemblages from four small lakes on northeastern Ellesmere Island, high Arctic, Canada, provide a proxy lake-ice cover and paleoenvironmental record. Low absolute diatom abundances and a benthic Fragilaria (sensu lata) dominated assemblage during the postglacial (< 7.6 ka B.P.) to mid-Holocene record the moderating effects of locally retreating glaciers. Around 5.5 ka B.P. diatom concentrations begin to rise, reaching their highest levels (109 valves per g dry sediment) between 4.2 and 3 ka B.P., interpreted to be the warmest period in this region. Topoclimatic differences between lakes on Hazen Plateau and those lower in Lake Hazen Basin account for the initial decline in diatom abundances in the upper lakes after 3 ka B.P. This change is thought to reflect a lowering of the regional snowline, accordant with widely recognized Neoglacial advances on Ellesmere Island and Greenland. Lakes in lower Lake Hazen Basin maintained extensive summer ice free conditions until ~ 1.9 ka B.P., after which diatom abundances declined, suggesting prolonged summer lake-ice cover through the remainder of the recovered Holocene record. Differences between the records presented here and those from coastal areas of the Canadian high Arctic highlight the unique topoclimatic characteristics and continentality of the Lake Hazen region, and possible effects that local marine environments may have had on coastal records. Such differences serve to demonstrate the inherent geographic variability of paleoenvironmental records from the high Arctic. 相似文献
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The relative ages of Neogfacial deposits above 2900m near Cloudveil Dome, Middle Teton, and Teepe glaciers, central Teton Range, western Wyoming, were determined using topographic position, weathering features, lichenometry, vegetation characteristics, and soils. A three-fold deposit sequence is identified and correlated with the Gannett Peak, Audubon and Indian Basin Neoglacial advances described elsewhere in the Teton and nearby Wind River ranges. While soil profile morphology proved very useful in distinguishing deposits of different age, other age indicators such as clay mineralogy and soil chemistry, proved of only marginal value in age discrimination. This chronosequence, emplaced over the last ～5000yr, provides an excellent setting upon which to test changes in plant species composition and its use as a relative dating method. Vascular plant species composition cannot be used to discriminate deposits of the different advances, possibly because of random colonisation and establishment combined with extensive mass wasting during the Neoglacial. However, certain vegetation features, such as total cover and species densities, appear useful as relative dating methods. 相似文献