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1.
Here we combine 10Be depth profile techniques applied to late glacial ice‐contact marine and lacustrine deltas, as well as boulder exposure dating of associated features in the Scoresby Sound region, east Greenland, to determine both the surface age and the magnitude of cosmogenic nuclide inheritance. Boulder ages from an ice‐contact delta in northern Scoresby Sund show scatter typical of polar regions and yield an average age of 12.8 ± 0.5 ka – about 2 ka older than both our average profile surface age of 10.9 ± 0.7 ka from three depth profiles and a radiocarbon‐based estimate. On the other hand, boulder exposure ages from a set of moraines in southern Scoresby Sund show excellent internal consistency for polar regions and yield an average age of 11.6 ± 0.2 ka. The profile surface age from a corresponding ice‐contact delta is 8.1 ± 0.9 ka, while a second delta yields an age of 10.0 ± 0.4 ka. Measured 10Be inheritance concentrations from all depth profiles are internally consistent and are between 10% and 20% of the surface concentrations, suggesting a regional cosmogenic inheritance signal for the Scoresby Sound landscape. Based on the profile inheritance concentrations, we explore the first‐order catchment‐averaged bedrock erosion under the Greenland ice sheet, yielding estimates of total erosion during the last glacial cycle of the order of 2–30 m. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

2.
We constrain a three-dimensional thermomechanical model of Greenland ice sheet (GrIS) evolution from the Last Glacial Maximum (LGM, 21 ka BP) to the present-day using, primarily, observations of relative sea level (RSL) as well as field data on past ice extent. Our new model (Huy2) fits a majority of the observations and is characterised by a number of key features: (i) the ice sheet had an excess volume (relative to present) of 4.1 m ice-equivalent sea level at the LGM, which increased to reach a maximum value of 4.6 m at 16.5 ka BP; (ii) retreat from the continental shelf was not continuous around the entire margin, as there was a Younger Dryas readvance in some areas. The final episode of marine retreat was rapid and relatively late (c. 12 ka BP), leaving the ice sheet land based by 10 ka BP; (iii) in response to the Holocene Thermal Maximum (HTM) the ice margin retreated behind its present-day position by up to 80 km in the southwest, 20 km in the south and 80 km in a small area of the northeast. As a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka BP, which corresponds to a deficit volume (relative to present) of 0.17 m ice-equivalent sea level. Our results suggest that remaining discrepancies between the model and the observations are likely associated with non-Greenland ice load, differences between modelled and observed present-day ice elevation around the margin, lateral variations in Earth structure and/or the pattern of ice margin retreat.  相似文献   

3.
Along the northeast Greenland continental margin, bedrock on interfjord plateaus is highly weathered, whereas rock surfaces in fjord troughs are characterized by glacial scour. Based on the intense bedrock weathering and lack of glacial deposits from the last glaciation, interfjord plateaus have long been thought to be ice-free throughout the last glacial maximum (LGM). In recent years there is growing evidence from shelf and fjord settings that the northeast Greenland continental margin was more extensively glaciated during the LGM than previously thought. However, little is still known from interfjord settings. We present cosmogenic 10Be data from meltwater channels and weathered sandstone outcrops on Jameson Land, an interfjord highland north of Scoresby Sund. The mean exposure age of samples from channel beds (n = 3) constrains on the onset of deglaciation on interior Jameson Land to 18.5 ± 1.3–21.4 ± 1.9 ka (for erosion conditions of 0–10 mm/ka, respectively). This finding adds to growing evidence that the northeast Greenland continental margin was more heavily glaciated during the LGM than previously thought.  相似文献   

4.
Fourteen samples obtained from Torridon sandstone boulders on four moraines marking the limit of the Wester Ross Readvance (WRR) in NW Scotland yielded tightly clustered 10Be exposure ages confirming contemporaneous or penecontemporaneous moraine deposition. Collectively, the 14 samples yield mean ages of 13.5 ± 1.2 ka to 14.0 ± 1.7 ka, depending on choice of geomagnetic scaling and sampling surface erosion rates. All fourteen moraine ages are significantly younger than an age of ca 16.3 ka previously proposed for the WRR, and also younger than most samples obtained from rock outcrops within the WRR limits. The ages obtained for the WRR moraines appear to confirm that a substantial cover of glacier ice persisted over low ground in NW Scotland during at least the early part of the Lateglacial Interstade (≈Greenland Interstade 1). We infer that the WRR probably occurred in response to rapid short-lived cooling during the Older Dryas climatic reversal (≈Greenland Interstade 1d), though the possibilities that the WRR represents ice-margin response to a later climatic reversal during the Lateglacial Interstade or stabilization and readvance of the ice margin following rapid offshore calving cannot be discounted.  相似文献   

5.
《Quaternary Science Reviews》2007,26(3-4):494-499
Cosmogenic surface-exposure ages from boulders on a terminal moraine complex establish the timing of the local last glacial maximum (LGM) in the Taylor River drainage basin, central Colorado. Five zero-erosion 10Be ages have a mean of 19.5±1.8 ka while that for three 36Cl ages is 20.7±2.3 ka. Corrections for modest rates (∼1 mm ka−1) of boulder surface erosion result in individual and mean ages that are generally within 2% of their zero-erosion values. Both the means and the range in ages of individual boulders are consistent with those reported for late Pleistocene moraines elsewhere in the southern and middle Rocky Mountains, and thus suggest local LGM glacier activity was regionally synchronous. Two anomalously young (?) zero-erosion 10Be ages (mean 14.4±0.8 ka) from a second terminal moraine are tentatively attributed to the boulders having been melted out during a late phase of ice stagnation.  相似文献   

6.
The offshore and coastal geomorphology of southwest Greenland records evidence for the advance and decay of the Greenland Ice Sheet during the Last Glacial Maximum. Regional ice flow patterns in the vicinity of Sisimiut show an enlarged ice sheet that extended southwestwards on to the shelf, with an ice stream centred over Holsteinsborg dyb. High level periglacial terrain composed of blockfield and tors is dated to between 101 and 142 ka using 26Al and 10Be cosmogenic exposure ages. These limit the maximum surface elevation of the Last Glacial Maximum ice sheet in this part of southwest Greenland to ca 750–810 m asl, and demonstrate that terrain above this level has been ice free since MIS 6. Last Glacial Maximum ice thickness on the coast of ca 700 m implies that the ice sheet reached the mid to outer continental shelf edge to form the Outer Hellefisk moraines. Exposure dates record ice surface thinning from 21.0 to 9.8 ka, with downwasting rates varying from 0.06 to 0.12 m yr−1. This reflects strong surface ablation associated with increased air temperatures running up to the Bølling Interstadial (GIS1e) at ca 14 ka, and later marine calving under high sea levels. The relatively late retreat of the Itilleq ice stream inland of the present coastline is similar to the pattern observed at Jakobshavn Isbræ, located 250 km north in Disko Bugt, which also retreated from the continental shelf after ca 10 ka. We hypothesise that the ice streams of West Greenland persisted on the inner shelf until the early Holocene because of their considerable ice thickness and greater ice discharge compared with the adjacent ice sheet.  相似文献   

7.
We report cosmogenic surface exposure 10Be ages of 21 boulders on moraines in the Jeullesh and Tuco Valleys, Cordillera Blanca, Peru (～10°S at altitudes above 4200 m). Ages are based on the sea-level at high-latitude reference production rate and scaling system of Lifton et al. (2005. Addressing solar modulation and long-term uncertainties in scaling secondary cosmic rays for in situ cosmogenic nuclide applications. Earth and Planetary Science Letters 239, 140–161) in the CRONUS-Earth online calculator of Balco et al. (2008. A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10Be and 26Al measurements. Quaternary Geochronology 3, 174–195). Using the Lifton system, large outer lateral moraines in the Jeullesh Valley have a 10Be exposure age of 12.4 ka, inside of which are smaller moraine systems dated to 10.8, 9.7 and 7.6 ka. Large outer lateral moraines in the Tuco Valley have a 10Be exposure age of 12.5 ka, with inner moraines dated to 11.3 and 10.7 ka. Collectively, these data indicate that glacier recession from the Last Glacial Maximum (LGM) in the Cordillera Blanca was punctuated by three to four stillstands or minor advances during the period 12.5–7.6 ka, spanning the Younger Dryas Chronozone (YDC; ～12.9–11.6 ka) and the cold event identified in Greenland ice cores and many other parts of the world at 8.2 ka. The inferred fluctuations of tropical glaciers at these times, well after their withdrawal from the LGM, indicate an increase in precipitation or a decrease in temperature in this region. Although palaeoenvironmental records show regional and temporal variability, comparison with proxy records (lacustrine sediments and ice cores) indicate that regionally this was a cold, dry period so we ascribe these glacier advances to reduced atmospheric temperature rather than increased precipitation.  相似文献   

8.
The sedimentary record around outer Scorcsby Sund begins with the Scorcsby Sund glaciation (≅ isotope stage 6), but is incomplete. Both at Kap Hope, headward of the fjord mouth, and at Kikiakajik on the outer coast. there are shallow marine sediments, correlated with the Langelandselv interglaciation (≅ isotope substagc 5e) on the basis of molluse assemblages and luminescence dates. Abundant Balanus crenatus , and several bivalves. show that thc adveetion of warm Atlantic water to the East Greenland coast was higher during that interglacial than during the Holocenc. Glacial striae at Kap Brewster (facing the open ocean) and till on top of the interglacial beds at Kikiakajik show that both an outlet from the Greenland Iee Sheet, and more local glaciers reached the continental shelf during the Weichselian. This glacial event is poorly dated. but tentatively correlated with the Flakkerhuk stade (≅ 19 15 ka BP) when, from marine geological data, it is suggested that thc Scoresby Sund glacier terminated c . 30 km east of Kap Brewster. During the Milne Land stade ( c . 10 ka BP) there was a resurgence of local ice caps in the mountains both north and south of the fjord mouth, but Scoresby Sund and Hall Bredning probably remained free of glaciers. Dating of these events was achieved through Iuminescence- (TL and OSL) and the 14C-method. and biostratigraphical and amino acid correliition Interglacial shells on thc outer coast show much lower amino acid D/L ratios than shells of the same age within the Scoresby Sund area. This may indicate that the outer coast remained free of ice cover and marine inundation much longer, arid suffered colder temperatures than areas along the fjord.  相似文献   

9.
Almost 90% of 39 m of core material recovered from Scoresby Sund and the adjacent East Greenland shelf is massive diamicton, interpreted to be formed predominantly by the release of iceberg rafted debris and reworking by iceberg scouring. There is also likely to be a contribution from suspension settling of fines derived from glaciofluvial sources. Model calculations suggest that the 14C derived Holocene sedimentation rate of 0.1-0.3 m 1000 yr−1 in Scoresby Sund can be accounted for mainly by iceberg rafting of debris. A further 4% of core material is of gravel or coarse sand lenses, interpreted to reflect iceberg dumping of debris. Intensive iceberg scouring, which reworks sea floor sediments, is observed on acoustic records from over 30 000 km2 of the Scoresby Sund fiord system and the adjacent East Greenland shelf (69-72°N and 75°N). The rate of iceberg production from Greenland Ice Sheet outlet glaciers, and iceberg drift tracks on the shelf, suggests that iceberg rafting and scouring may be important over a significant proportion of the 500 000 km2 area above the shelf break. The relatively extensive modern occurrence of massive diamicton, formed by iceberg rafting and scouring, together with suspension settling of fines, suggests that it may also be a significant facies in the glacier-influenced geological record. The recognition in the geological record of the massive diamicton facies described above may also indicate the former presence of fast flowing ice sheet outlet glaciers.  相似文献   

10.
The now acknowledged thinning of the Greenland Ice Sheet raises concerns about its potential contribution to future sea level rise. In order to appreciate the full extent of its contribution to sea level rise, reconstruction of the ice sheet's most recent last deglaciation could provide key information on the timing and the height of the ice sheet at a time of rapid climate readjustment. We measured 10Be concentrations in 12 samples collected along longitudinal and altitudinal transects from Sisimiut to within 10 km of the Isunguata Sermia Glacier ice margin on the western coast of Greenland. Along the longitudinal transect, we collected three perched boulders and two bedrocks. In addition, we sampled seven perched boulders along a vertical transect in a valley within 10 km of the Isunguata Sermia Glacier ice margin. Our pilot dataset constrains the height of the ice sheet during the Last Glacial Maximum (LGM) between 500 m and 840 m (including the 120 m relative sea level depression at the time of the LGM, 21 ka BP). From the transect we estimate the thinning of the ice sheet at the end of the deglaciation between 12.3 ± 1.5 10Be ka (n = 2) and 8.3 ± 1.2 10Be ka (n = 3) to be ～6 cm a?1 over this time period. Direct dating of the retreat of the western margin of the Greenland Ice Sheet has the potential to better constrain the retreat rate of the ice margin, the thickness of the former ice sheet as well as its response to climate change. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

11.
Digital elevation models of the area around the Solway Lowlands reveal complex subglacial bedform imprints relating the central sector of the LGM British and Irish Ice Sheet. Drumlin and lineation mapping in four case studies show that glacier flow directions switched significantly through time. These are summarised in four major flow phases in the region: Phase I flow was from a dominant Scottish dispersal centre, which transported Criffel granite erratics to the Eden Valley and forced Lake District ice eastwards over the Pennines at Stainmore; Phase II involved easterly flow of Lake District and Scottish ice through the Tyne Gap and Stainmore Gap with an ice divide located over the Solway Firth; Phase III was a dominant westerly flow from upland dispersal centres into the Solway lowlands and along the Solway Firth due to draw down of ice into the Irish Sea basin; Phase IV was characterised by unconstrained advance of Scottish ice across the Solway Firth. Forcing of a numerical model of ice sheet inception and decay by the Greenland ice core record facilitates an assessment of the potential for rapid ice flow directional switching during one glacial cycle. The model indicates that, after fluctuations of smaller radially flowing ice caps prior to 30 ka BP, the ice sheet grows to produce an elongate, triangular-shaped dome over NW England and SW Scotland at the LGM at 19.5 ka BP. Recession after 18.5 ka BP displays a complex pattern of significant ice flow directional switches over relatively short timescales, complementing the geomorphologically-based assessments of palaeo-ice dynamics. The palaeoglaciological implications of this combined geomorphic and modelling approach are that: (a) the central sector of the BIIS was as a major dispersal centre for only ca 2.5 ka after the LGM; (b) the ice sheet had no real steady state and comprised constantly migrating dispersal centres and ice divides; (c) subglacial streamlining of flow sets was completed over short phases of fast flow activity, with some flow reversals taking place in less than 300 years.  相似文献   

12.
Recent changes along the margins of the Antarctic Peninsula, such as the collapse of the Wilkins Ice Shelf, have highlighted the effects of climatic warming on the Antarctic Peninsula Ice Sheet (APIS). However, such changes must be viewed in a long-term (millennial-scale) context if we are to understand their significance for future stability of the Antarctic ice sheets. To address this, we present nine new cosmogenic 10Be exposure ages from sites on NW Alexander Island and Rothschild Island (adjacent to the Wilkins Ice Shelf) that provide constraints on the timing of thinning of the Alexander Island ice cap since the last glacial maximum. All but one of the 10Be ages are in the range 10.2–21.7 ka, showing a general trend of progressive ice-sheet thinning since at least 22 ka until 10 ka. The data also provide a minimum estimate (490 m) for ice-cap thickness on NW Alexander Island at the last glacial maximum. Cosmogenic 3He ages from a rare occurrence of mantle xenoliths on Rothschild Island yield variable ages up to 46 ka, probably reflecting exhumation by periglacial processes.  相似文献   

13.
Isla de los Estados is a mountainous island southeast of Tierra del Fuego, in southernmost South America. Its central and eastern parts have an alpine topography, transected by U-shaped valleys, small, partly over-deepened fjords, and a multitude of abandoned cirques, all associated with extensive former local glaciations. Traces of glacial erosion generally reach 400–450 m a.s.l., and above that trimline a distinct sharp-edged nunatak derived landscape is present. The westernmost part of the island has a lower, more subdued topography, reflecting its “softer” geology but possibly also over-running and erosion by mainland-derived ice streams. The present study concentrated on glacigenic sediment sequences exposed along coastal erosional cliffs. A combination of OSL and 14C datings show that these sediments mostly date from the latest (Wisconsinan/Weichselian) glacial cycle, i.e. from the last ca. 100 ka with the oldest (glaciolacustrine) deposits possibly as old as 90–80 ka. The upper parts of overlying tills, with associated lateral and terminal moraines from glaciers that expanded onto an eustatically exposed dry shelf north of the island, date from the last global glacial maximum (LGM). Radiocarbon ages of peat and lake sediments indicate that deglaciation began 17–16 cal ka BP.  相似文献   

14.
Exposure dating using cosmogenic 36Cl demonstrates that the summit plateau of Scafell Pike (978 m) in the SW Lake District escaped erosion by glacier ice during the last glacial maximum (LGM; c. 26–21 kyr) and probably throughout the Devensian Glacial Stage (MIS 5d-2). Exposure ages obtained for ice-moulded bedrock on an adjacent col at 750–765 m confirm over-riding and erosion of bedrock by warm-based glacier ice during the LGM. The contrast between the two sites is interpreted in terms of preservation of tors, frost-shattered outcrops and blockfields on terrain above 840–870 m under cold-based ice. An exposure age of 17.3 ± 1.1 kyr for the col at 750–765 m suggests that substantial downwastage of the last ice sheet had occurred by c. 17 kyr, consistent with deglacial exposure ages obtained for other high-level sites in the British Isles. An exposure age of 12.5 ± 0.8 kyr obtained for a glacially transported rockfall boulder within the limits of later corrie glaciation confirms that the final episode of local glaciation in the Lake District occurred during the Loch Lomond Stade (c. 12.9–11.7 kyr). This research also demonstrated the difficulties of obtaining reliable exposure ages from rhyolite and andesite bedrock that has proved resistant to glacial abrasion.  相似文献   

15.
Until recently, little was known about the Quaternary marine sedimentary record in East Greenland. Geophysical and geological investigations in Scoresby Sund were undertaken to characterize the nature and chronology of this record. Seismic records show that almost 70% of the outer fjord system is covered by about 10 m of unlithified sediments, making direct correlation with the Quaternary records on land and the adjacent continental margin difficult. These acoustically unstratified sediments are scoured by icebergs above 550 m water depth. Almost 90% of core material is massive diamicton of Holocene age, deposited mainly from iceberg rafting and turbid meltwater. Sedimentation rates are 0.1 -0.3 m 1000 yr-1. Thicker accumulations of unlithified Quaternary sediments in Scoresby Sund occur as sediment ridges and in two other major depocentres. A low sediment ridge runs across the mouth of Scoresby Sund, and is interpreted as an end moraine of Late Weichselian Flakkerhuk stadial age. The very restricted sediment thickness suggests that grounded ice filled the fjord during the Flakkerhuk and an ice shelf was not present. High inputs of ice rafted debris to the continental margin at about 18 000 BP indicate this as a probable age for the moraine. During the Allerød Interstadial, ice probably retreated from the outer fjord system, since massive diamictons similar to those of Holocene age are present at the base of most cores. A major depocentre of acoustically stratified sediments at the head of Hall Bredning is interpreted to represent ice proximal deposits from a glacier margin extending across the fjord. It is adjacent to dated moraines on land and is inferred to be of Milne Land stadial age (about 10 000 BP). A similar age is interpreted for acoustically laminated sediments and a moraine at the entrance of Vikingebugt, on the south side of Scoresby Sund. Dated kame terraces in the inner fjord system indicate that ice retreated to its present position 6–7000 years ago.  相似文献   

16.
Marine, fluvial and glacigene sediments exposed in coastal cliffs and stream-cut sections in East Greenland between latitudes 69° and 78° N display a record of Quaternary climatic and environmental change going back to pre-Saalian times (> 240 ka), but with main emphasis on the last interglacial/glacial cycle. The stratigraphical scheme is based on studies on the Jameson Land peninsula, and contains five glacial stages and stades with the Greenland ice sheet or its outlets reaching the outer coasts. Individual sites are correlated and dated by a combination of biostratigraphy, luminescence dating, amino acid analyses, as well as 14C- and uranium series dating. The pre-Weichselian Lollandselv and Scoresby Sund glaciations were the most extensive. During the Weichselian the Inland Ice margin in this part of East Greenland was apparently very stable. The Aucellaelv, Jyllandselv and Flakkerhuk stades mark the advance and subsequent retreat of outlet glaciers from the Inland Ice which advanced through the wide Scoresby Sund basin and reached the inner shelf. In-between the glacier advances, three interglacial or interstadial periods have been recognized. During the Langelandselv interglacia-tion (≅ Eemian) the advection of warm Atlantic water was higher than during the Holocene, and the terrestrial flora and insect faunas show that summer temperatures were 3–4°C higher than during the Holocene optimum. There is no unambiguous evidence for cooling in the sediments from this interval. Later, in isotope stage 5, there were apparently two ice-free periods. During the Hugin Sø interstade, stable Polar water dominated Scoresby Sund, and the terrestrial flora suggests summer temperatures 2° -3° lower than the present. The marine and fluvial sediments from the second ice-free period, the Mønselv interstade, are devoid of organic remains.  相似文献   

17.
This study seeks to quantify the rate and timing of regolith generation in the Critical Zone at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Meteoric 10Be depth profiles were determined using measurements from 30 hillslope soil and bedrock core samples in an effort to constrain 10Be inventories. The SSHO is located in the temperate climate zone of central Pennsylvania and comprises a first-order watershed developed entirely on a Fe-rich, organic-poor, Silurian-aged shale. Two major perturbations to the landscape have occurred at SSHO in the geologically recent past, including significant and sustained periglacial activity until after the retreat of the Laurentide ice sheet (～21 ka) and deforestation during early colonial land-use. Bulk soil samples (n = 16) were collected at three locations along a planar hillslope on the southern ridge of the catchment, representing the ridge top, mid-slope and valley floor. Rock chip samples (n = 14) were also collected from a 24 m deep core drilled into the northern ridge top. All meteoric 10Be concentration profiles show a declining trend with depth, with most of the 10Be retained in the uppermost decimeters of the soil. Meteoric 10Be inventories are higher at the mid-slope and valley floor sample sites, at 3.71 ± 0.02 × 1010 at/cm2 and 3.69 ± 0.02 × 1010 at/cm2, than at the ridge top site (1.90 ± 0.01 × 1010 at/cm2). The 10Be inventory at the convex ridge top site implies a minimum residence time of ～10.6 ka, or if erosion is steady, an erosion rate of 19.4 ± 0.2 m/My.  相似文献   

18.
《Quaternary Science Reviews》2005,24(10-11):1223-1241
The late Quaternary ice sheet/ice shelf extent in the George V Basin (East Antarctica) has been reconstructed through analyses of Chirp sub-bottom profiles, integrated with multi-channel seismic data and sediment cores. Four glacial facies, related to the advance and retreat history of the glaciated margin, have been distinguished: Facies 1 represents outcrop of crystalline and sedimentary rocks along the steep inner shelf and comprises canyons once carved by glaciers; Facies 2 represents moraines and morainal banks and ridges with a depositional origin along the middle-inner shelf; Facies 3 represents glacial flutes along the middle-outer shelf; Facies 4 is related to ice-keel turbation at water depths <500 m along the outer shelf. A sediment drift deposit, located in the NW sector of the study area, partly overlies facies 2 and 3 and its ground-truthing provides clues to understanding their age. We have distinguished: (a) an undisturbed sediment drift deposit at water depth >775 m, with drape/sheet and mound characters and numerous undisturbed sub-bottom sub-parallel reflectors (Facies MD1); (b) a fluted sediment drift deposit at water depth <775 m, showing disrupted reflectors and a hummocky upper surface (Facies MD2). Radiocarbon ages of sediment cores indicate that the glacial advance producing facies MD2 corresponds to the Last Glacial Maximum (LGM) and that during the LGM the ice shelf was floating over the deep sector of the basin, leaving the sediment drift deposit undisturbed at major depths (Facies MD1). This observation further implies that: (a) glacial facies underneath the sediment drift were the result of a grounding event older than the LGM, (b) this sector of the East Antarctic fringe was sensitive to sea-level rise at the end of the LGM; thus potentially contributing to meltwater discharge during the last deglaciation.  相似文献   

19.
The Yulong Mountain massif is tectonically active during Quaternary and contains the southernmost glacierized mountains in China, and all of Eurasia. Past glacial remnants remain preserved on the east and west sides of the Yulong Mountains. A ridge of moraine protruded into the Jinsha River at the Daju Basin, damming the river, and forming a lake at the head of the Jinsha River. Cosmogenic 10Be and 26Al provide exposure age dates for the moraine-based fluvial terraces left behind after the dam breached, and for moraine boulders on both the eastern and western sides of the Yulong Mountains. Our results yield exposure ages for the terraces that range from 29 ka to 8 ka, and a downcutting rate of 7.6 m/ka. The preservation of the remaining dam for over 10,000 years suggests stability of the moraine dam and gradual erosion of the dam during drainage of the dammed lake. From the relationship between exposure ages and elevations of the fluvial terraces located in different walls of the Daju fault, we obtain a late Quaternary dip-slip rate of about 5.6 m/ka for the Daju fault. The exposure ages of 10.2 ka and 47 ka for moraine boulders located in the east and west sides of the Yulong Mountains, respectively, coincide with warm periods in the late Quaternary. This implies that precipitation provides the major control for glaciations on the Yulong Mountains, a domain of the southwest Asian monsoon.  相似文献   

20.
Uummannaq Fjord, West Greenland, held the Uummannaq Ice Stream system that drained an estimated ~6% of the Greenland Ice Sheet (GrIS) during the Last Glacial Maximum. Published ages for the final deglaciation in Uummannaq Fjord vary from as early as c. 9.8 ka to as late as c. 5.3 ka. Assessing this variability requires additional chronological controls to improve the deglaciation history of central West Greenland. Here, we combine 14C dating of lake sediment cores with cosmogenic 10Be exposure dating at sites adjacent to the present GrIS margin in the central‐inland sector of the Uummannaq Fjord system. We find that ice retreated to or within the present GrIS margin at 10.8±0.2 ka (n = 6). Although this ‘final deglaciation’ to or within the present GrIS margin across the Uummannaq Fjord system varies from c. 10.8 to 5.3 ka, all chronologies indicate collapse from the continental shelf to the inner fjords at c. 11.0 ka, which occurred at a net retreat rate of 300–1100 m a−1. The Uummannaq Fjord system deglaciated c. 1000 years earlier than the major fjord system to the south, Disko Bugt. However, similarly rapid retreat rates of the two palaeo‐ice stream systems suggest that their collapse may have been aided by high calving rates. The asynchronous deglaciation of the GrIS throughout the Uummannaq Fjord system probably relates to the influence of varying fjord geometry on marine glacier behaviour.  相似文献   

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