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1.
Northern Folgefonna (c. 23 km2), is a nearly circular maritime ice cap located on the Folgefonna Peninsula in Hardanger, western Norway. By combining the position of marginal moraines with AMS radiocarbon dated glacier‐meltwater induced sediments in proglacial lakes draining northern Folgefonna, a continuous high‐resolution record of variations in glacier size and equilibrium‐line altitudes (ELAs) during the Lateglacial and early Holocene has been obtained. After the termination of the Younger Dryas (c. 11 500 cal. yr BP), a short‐lived (100–150 years) climatically induced glacier readvance termed the ‘Jondal Event 1’ occurred within the ‘Preboreal Oscillation’ (PBO) c. 11 100 cal. yr BP. Bracketed to 10 550–10 450 cal. yr BP, a second glacier readvance is named the ‘Jondal Event 2’. A third readvance occurred about 10 000 cal. yr BP and corresponds with the ‘Erdalen Event 1’ recorded at Jostedalsbreen. An exponential relationship between mean solid winter precipitation and ablation‐season temperature at the ELA of Norwegian glaciers is used to reconstruct former variations in winter precipitation based on the corresponding ELA and an independent proxy for summer temperature. Compared to the present, the Younger Dryas was much colder and drier, the ‘Jondal Event 1’/PBO was colder and somewhat drier, and the ‘Jondal Event 2’ was much wetter. The ‘Erdalen Event 1’ started as rather dry and terminated as somewhat wetter. Variations in glacier magnitude/ELAs and corresponding palaeoclimatic reconstructions at northern Folgefonna suggest that low‐altitude cirque glaciers (lowest altitude of marginal moraines 290 m) in the area existed for the last time during the Younger Dryas. These low‐altitude cirque glaciers of suggested Younger Dryas age do not fit into the previous reconstructions of the Younger Dryas ice sheet in Hardanger. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

2.
Twenty-two new radiocarbon ages from Skagit valley provide a detailed chronology of alpine glaciation during the Evans Creek stade of the Fraser Glaciation (early marine oxygen isotope stage (MIS) 2) in the Cascade Range, Washington State. Sediments at sites near Concrete, Washington, record two advances of the Baker valley glacier between ca. 30.3 and 19.5 cal ka BP, with an intervening period of glacier recession about 24.9 cal ka BP. The Baker valley glacier dammed lower Skagit valley, creating glacial Lake Concrete, which discharged around the ice dam along Finney Creek, or south into the Sauk valley. Sediments along the shores of Ross Lake in upper Skagit valley accumulated in glacial Lake Skymo after ca. 28.7 cal ka BP behind a glacier flowing out of Big Beaver valley. Horizontally laminated silt and bedded sand and gravel up to 20 m thick record as much as 8000 yr of deposition in these glacially dammed lakes. The data indicate that alpine glaciers in Skagit valley were far less extensive than previously thought. Alpine glaciers remained in advanced positions for much of the Evans Creek stade, which may have ended as early as 20.8 cal ka BP.  相似文献   

3.
Mapping along a transect from the southeastern margin of the South Patagonian Ice-field in Torres del Paine National Park (Chile) to the limits of fresh moraines of the last glacial cycle indentified eight glacier advances. The four younger ones have been dated by dendrochronology, tephrochronology and radiocarbon dating. Although the bases of 10 m deep bogs were sampled, close limiting radiocarbon dates were not obtained because bog formation in this rain-shadow area appears not to have commenced until ca.12000 yr ago. The outermost Little Ice Age moraine formed during the seventeenth century and three inner ones were deposited around ad 1805, 1845 and after 1890. Densely vegetated older moraines contiguous with Little Ice Age deposits are possibly of late Holocene age. Tephra from the eruption of Reclus volcano at ca. 11 880 yr BP was incorporated by a readvance that deposited large multiple moraines 10–16 km from the modern ice-front; the oldest basal peat found inside the moraine has been dated to ca. 9200 yr BP. These bracketing dates indicate that some eastern outlet glaciers of the ice-field advanced at a time when some western tidewater outlet glaciers terminated inside their modern limits. This questions the view of J. H. Mercer and other that Patagonian glaciers did not readvance during the late-glacial interval. A stadial event also occurred when the glaciers were some 18–20 km from their modern positions and is closely dated to ca. 11880 yr BP because Reclus pumice flushed down-glacier forms thick upper beds in outwash deltas deposited in proglacial lakes. The four older moraines pre-date the late-glacial eruption of Reclus but are not dated closely. Comparison of their spatial extent with well-dated moraines in the Chilean Lakes Region suggests that they may mark advances culminating at ca. 14000 yr BP, ca. 20000 yr BP and earlier.  相似文献   

4.
We summarize evidence of the latest Pleistocene and Holocene glacier fluctuations in the Canadian Cordillera. Our review focuses primarily on studies completed after 1988, when the first comprehensive review of such evidence was published. The Cordilleran ice sheet reached its maximum extent about 16 ka and then rapidly decayed. Some lobes of the ice sheet, valley glaciers, and cirque glaciers advanced one or more times between 15 and 11 ka. By 11 ka, or soon thereafter, glacier cover in the Cordillera was no more extensive than at the end of the 20th century. Glaciers were least extensive between 11 and 7 ka. A general expansion of glaciers began as early as 8.4 ka when glaciers overrode forests in the southern Coast Mountains; it culminated with the climactic advances of the Little Ice Age. Holocene glacier expansion was not continuous, but rather was punctuated by advances and retreats on a variety of timescales. Radiocarbon ages of wood collected from glacier forefields reveal six major periods of glacier advance: 8.59–8.18, 7.36–6.45, 4.40–3.97, 3.54–2.77, 1.71–1.30 ka, and the past millennium. Tree-ring and lichenometric dating shows that glaciers began their Little Ice Age advances as early as the 11th century and reached their maximum Holocene positions during the early 18th or mid-19th century. Our data confirm a previously suggested pattern of episodic but successively greater Holocene glacier expansion from the early Holocene to the climactic advances of the Little Ice Age, presumably driven by decreasing summer insolation throughout the Holocene. Proxy climate records indicate that glaciers advanced during the Little Ice Age in response to cold conditions that coincided with times of sunspot minima. Priority research required to further advance our understanding of late Pleistocene and Holocene glaciation in western Canada includes constraining the age of late Pleistocene moraines in northern British Columbia and Yukon Territory, expanding the use of cosmogenic surface exposure dating techniques, using multi-proxy paleoclimate approaches, and directing more of the research effort to the northern Canadian Cordillera.  相似文献   

5.
We report radiocarbon dates that constrain the timing of the deposition of the late-glacial Puerto Bandera moraine system alongside the western reaches of Lago Argentino adjacent to the Southern Patagonian Icefield. Close maximum-limiting radiocarbon ages (n = 11) for glacier advance into the outer moraines, with a mean value of 11,100 ± 60 14C yrs BP (12,990 ± 80 cal yrs BP), were obtained from wood in deformation (soft) till exposed beneath flow and lodgment till in Bahía del Quemado on the northeast side of Brazo Norte (North Branch) of western Lago Argentino. Other exposures of this basal deformation till in Bahía del Quemado reveal incorporated clasts of peat, along with larger inclusions of deformed glaciofluvial and lacustrine deposits. Radiocarbon dates of wood included in these reworked peat clasts range from 11,450 ± 45 14C yrs BP to 13,450 ± 150 14C yrs BP (13,315 ± 60 to 16,440 ± 340 cal yrs BP). The implication is that, during this interval, glacier fronts were situated inboard of the Puerto Bandera moraines, with the peat clasts and larger proglacial deposits being eroded and then included in the basal till during the Puerto Bandera advance.Minimum-limiting radiocarbon ages for ice retreat come from basal peat in cores sampled in spillways and depressions generated during abandonment of the Puerto Bandera moraines. Glacier recession and subsequent plant colonization were initiated close behind different frontal sectors of these moraines prior to: 10,750 ± 75 14C yrs BP (12,660 ± 70 cal yrs BP) east of Brazo Rico, 10,550 ± 55 14C yrs BP (12,490 ± 80 cal yrs BP) in Peninsula Avellaneda, and 10,400 ± 50 14C yrs BP (12,280 ± 110 cal yrs BP) in Bahía Catalana. In addition, a radiocarbon date indicates that by 10,350 ± 45 14C yrs BP (12,220 ± 110 cal yrs BP), the Brazo Norte lobe (or former Upsala Glacier) had receded well up the northern branch of Lago Argentino, to a position behind the Herminita moraines. Furthermore, glacier termini had receded to just outboard of the outer Holocene moraines at Lago Frías and Lago Pearson (Anita) prior to 10,400 ± 40 14C yrs BP (12,270 ± 100 cal yrs BP) and 9040 ± 45 14C yrs BP (10,210 ± 50 cal yrs BP), respectively. The most extensive recession registered during the early Holocene was in Agassiz Este Valley, where the Upsala Glacier had pulled back behind the outer Holocene moraine, reaching close to the present-day glacier terminus before 8290 ± 40 14C yrs BP (9300 ± 80 cal yrs BP).The radiocarbon-dated fluctuations of the Lago Argentino glacier in late-glacial time, given here, are in accord with changes in ocean mixed layer properties, predominately temperature, derived from the isotopic record given here of ODP Core 1233, taken a short distance off shore of the Chilean Lake District. It also matches recently published chronologies of late-glacial moraines in the Southern Alps of New Zealand on the opposite side of the Pacific Ocean from Lago Argentino. Finally, the timing of the late-glacial reversal of the Lago Argentino glacier fits the most recent chronology for the culmination of the Antarctic Cold Reversal (ACR) in the deuterium record of the EPICA Dome C ice core from high on the East Antarctic Plateau. Therefore, we conclude that the climate signature of the ACR was widespread in both the ocean and the atmosphere over at least the southern quarter of the globe.  相似文献   

6.
Radiocarbon dates on molluses in marine facies associated with glacial deposits in northern Cumberland Peninsula indicate both main fiord (Laurentide) ice and local glaciers remained at their late Wisconsin maxima until ca. 8000 BP. Essentially continuous deglaciation followed; local corrie glaciers melted out by 7100 BP and by 5500 BP fiord glaciers had receded behind the present margin of the Penny Ice Cap. The Hypsithermal warm interval probably lasted from ca. 8000 to 5000 BP. Lichenometry and radiocarbon dates on peat and buried organic horizons delimit a detailed Neoglacial chronology. Of 46 outlet and corrie glaciers investigated, the oldest Neoglacial moraines are dated lichenometrically at 3200 ± 600 BP. Subsequent advances terminated immediately prior to ca. 1650, 780, 350, and 65 yr BP, the most recent of which marked the most extensive ice coverage during the Neoglacial. The highest occurrence of lateral moraines from late Wisconsin advances of local and Laurentide ice suggest that at the late Wisconsin glacial maximum, depression of snowline varied from 450 m below present at the coast to 350 m below present level in the vicinity of the Penny Ice Cap. Moraines, surrounded by glacial ice and lying above the present steady-state ELA, suggest that during the Hypsithermal snowline was up to ca. 200 m above its present elevation. A radiometrically controlled reconstruction of relative summer paleotemperatures for the postglacial derived independently of lichenometry agrees well with the lichenometric age dating of moraines. The data suggest that between ca. 1650 and 900 BP climatic conditions were unfavorable for glacier growth, whereas the period ca. 800-65 yr BP was one of general glacial activity. During the last decade permanent snow cover has been increasing in the area. Previously reported data on climatic trends in the Canadian Arctic based on palynological analyses are similar to the chronology reported here.  相似文献   

7.
The rock glacier Innere Ölgrube, located in a small side valley of the Kauner Valley (Ötztal Alps, Austria), consists of two separate, tongue-shaped rock glaciers lying next to each other. Investigations indicate that both rock glaciers contain a core of massive ice. During winter, the temperature at the base of the snow cover (BTS) is significantly lower at the active rock glacier than on permafrost-free ground adjacent to the rock glacier. Discharge is characterized by strong seasonal and diurnal variations, and is strongly controlled by the local weather conditions. Water temperature of the rock glacier springs remains constantly low, mostly below 1°C during the whole melt season. The morphology of the rock glaciers and the presence of meltwater lakes in their rooting zones as well as the high surface flow velocities of >1 m/yr point to a glacial origin. The northern rock glacier, which is bounded by lateral moraines, evolved from the debris-covered tongue of a small glacier of the Little Ice Age with its last highstand around A.D. 1850. Due to the global warming in the following decades, the upper parts of the steep and debris-free ice glacier melted, whereas the debris-covered glacier tongue transformed into an active rock glacier. Due to this evolution and due to the downslope movement, the northern rock glacier, although still active, at present is cut off from its ice and debris supply. The southern rock glacier has developed approximately during the same period from a debris-covered cirque glacier at the foot of the Wannetspitze massif.  相似文献   

8.
Chronology of cirque glaciation,Colorado front range   总被引：2，自引：0，他引：2  
Moraines and rock glaciers in Front Range cirques record at least four, and possibly five, intervals of Holocene glacier expansion. The earliest and most extensive was the Satanta Peak advance, which deposited multiple terminal moraines near present timberline shortly before 9915 ± 165 BP. By 9200 ± 135 BP, timberline had risen to at least its modern elevation; by 8460 ± 140 BP, patterned ground on Satanta Peak moraines had become inactive. Although a minor ice advance may have occurred just prior to 7900 ± 130 BP, there is no evidence that glaciers or perennial snowbanks survived in the Front Range during the “Altithermal” maximum (ca. 6000–7500 BP), or during a subsequent interval of alpine soil formation (ca. 5000–6000 BP).Glaciers were larger during the Triple Lakes advance (3000–5000 BP) than at any other time during Neoglaciation. Minimum ages of 4485 ± 100 BP, 3865 ± 100 BP, and ca. 3150 BP apply to a threefold sequence of Triple Lakes deposits in Arapaho Cirque. After an important interval of soil formation and cavernous weathering, glaciers and rock glaciers of the Audubon advance (950–1850 BP) reoccupied many cirques, and perennial snowbanks blanketed much of the area above present timberline; although the general Audubon snow cover had begun to melt from valley floors by 1505 ± 95 BP, expanded snowbanks lingered on tundra ridge crests until 1050–1150 BP, and glaciers persisted is sheltered cirques until at least 955 ± 95 BP. Following a minor interval of ice retreat, glaciers of the Arapaho Peak advance (100–300 BP) deposited multiple moraines in favorably oriented cirques.Interpretation of Holocene glacial deposits in the Southern and Central Rocky Mountains has been hampered by (1) a heavy reliance upon relative-dating criteria, many of which are influenced by factors other than age; (2) the assumption that glacial advances in high-altitude cirques can be correlated directly with alluvial deposition in far-distant lowlands; and (3) the assumption that glacial advances have necessarily been synchronous throughout the Rocky Mountain region and the world. Although Holocene glacier fluctuations in the Front Range are believed to reflect changes in regional climate, the Front Range chronology does not have particularly close analogs in other parts of North America. Better-dated local sequences are needed before the hypothesis of global synchroneity can be adequately evaluated; until synchroneity has been proven, long-distance correlations and worldwide cycles of recurring glaciation will remain unconvincing.  相似文献   

9.
《Quaternary Science Reviews》2007,26(3-4):479-493
Evidence from glacier forefields and lakes is used to reconstruct Holocene glacier fluctuations in the Spearhead and Fitzsimmons ranges in southwest British Columbia. Radiocarbon ages on detrital wood and trees killed by advancing ice and changes in sediment delivery to downstream proglacial lakes indicate that glaciers expanded from minimum extents in the early Holocene to their maximum extents about two to three centuries ago during the Little Ice Age. The data indicate that glaciers advanced 8630–8020, 6950–6750, 3580–2990, and probably 4530–4090 cal yr BP, and repeatedly during the past millennium. Little Ice Age moraines dated using dendrochronology and lichenometry date to early in the 18th century and in the 1830s and 1890s. Limitations inherent in lacustrine and terrestrial-based methods of documenting Holocene glacier fluctuations are minimized by using the two records together.  相似文献   

10.
Radiocarbon dates from critical stratigraphic localities in southern British Columbia indicate that the growth history of the late Wisconsin Cordilleran Ice Sheet was different from that of most of the Laurentide Ice Sheet to the east. Much of southern British Columbia remained free of ice until after about 19,000 to 20,000 yr ago; only adjacent to the Coast Mountains is there a record of lowland glacier tongues in the interval 22,000 to 20,000 yr B.P. A major advance to the climax of late Wisconsin Cordilleran glacier ice in the northern States was not begun until after about 18,000 yr B.P. in the southwest of British Columbia and after about 17,500 yr B.P. in the southeast. The rate of glacier growth must have been very rapid in the two to three millennia prior to the climax, which has been dated in western Washington at shortly after 15,000 yr B.P.  相似文献   

11.
Sediment cores from two bedrock-dammed lakes in North Fork Big Pine Creek, Sierra Nevada, California, preserve the most detailed and complete record of Holocene glaciation yet recovered in the region. The lakes are fed by outwash from the Palisade Glacier, the largest (～1.3 km2) and presumably longest-lived glacier in the range, and capture essentially all of the rock flour it produces. Distinct late-Holocene (Matthes) and late-Pleistocene (Recess Peak) moraines lie between the modern glacier and the lakes. The lakes have therefore received continuous sedimentation from the basin since the retreat of the Tioga glacier (Last Glacial Maximum) and capture rock flour related to all post-LGM advances. A total of eight long cores (up to 5.5 m sediment depth) and one short surface sediment short core preserve a coherent record of fluctuating rock flour flux to the lakes through the Holocene. Age constraints on rock flour spikes in First and Second lakes based on 31 14C-dated macrofossils indicate Holocene glaciation began ～3200 cal yr B P, followed by a possible glacier maximum at ～2800 cal yr B P and four distinct glacier maxima at ～2200, ～1600, ～700 and ～250-170 cal yr. B.P., the most recent maximum being the largest.Reconstruction of the equilibrium-line altitudes (ELA) associated with each distinct advance recorded in the moraines (Recess Peak, Matthes, and modern) indicates ELA depressions (relative to modern) of ～250 m and 90 m for Recess Peak and Matthes advances, respectively. These differences represent decreases in summer temperatures of 1.7–2.8 °C (Recess Peak) and 0.2–2° (Matthes), and increases in winter precipitation of 22-34 cm snow water equivalent (s.w.e.) (Recess Peak) and 3-26 cm s.w.e. (Matthes) compared to modern conditions. Although small, these changes are significant and similar to those noted in the Cascade Range to the north, and represent a significant departure from historical climate trends in the region.  相似文献   

12.
In the Schiantala Valley of the Maritime Alps, the relationship between a till-like body and a contiguous rock glacier has been analyzed using geomorphologic, geoelectric and ice-petrographic methodologies. DC resistivity tomographies undertaken in the till and in the rock glacier show the presence of buried massive ice and ice-rich sediments, respectively. Ice samples from a massive ice outcrop show spherical gas inclusions and equidimensional ice crystals that are randomly orientated, confirming the typical petrographic characteristics of sedimentary ice. The rock glacier formation began after a phase of glacier expansion about 2550 ± 50 14C yr BP. Further ice advance during the Little Ice Age (LIA) overrode the rock glacier root and caused partial shrinkage of the pre-existing permafrost. Finally, during the 19th and 20th centuries, the glacial surface became totally debris covered. Geomorphological and geophysical methods combined with analyses of ice structure and fabric can effectively interpret the genesis of landforms in an environment where glaciers and permafrost interact. Ice petrography proved especially useful for differentiating ice of past glaciers versus ice formed under permafrost conditions. These two mechanisms of ice formation are common in the Maritime Alps where many sites of modern rock glaciers were formerly occupied by LIA glaciers.  相似文献   

13.
Randomisation tests on boulder weathering data distinguish moraines of four different ages in the Rongbuk Valley, all deposited by valley glaciers flowing northward into Tibet from the Himalaya. Lichenometry utilising subgenus Rhizocarpon distinguishes two groups of moraines, those <100 yr old and those older than several thousand years. The degree of soil development has a similar, limited utility in relative-age dating these moraines. The radiocarbon ages of calcium carbonate coatings in the lower horizons of moraine soils provide minimum-limiting ages of 1900 yr BP for the penultimate advance of the Rongbuk glacier (Samdopo moraine) and 9500 yr BP for the Rongbuk moraine, the moraine suggested by previous workers to represent the last glacial maximum. Equilibrium-line depression associated with the Rongbuk moraine probably was slight, <200 m. The small magnitude of this depression relative to glaciers in other mountain ranges could relate to a weakening of the monsoon in full glacial times, recent tectonic uplift, and/or to the insensitivity of these high-altitude glaciers to lowering temperatures in the rain shadow of Mount Everest.  相似文献   

14.
Clague, J. J., Mathewes, R. W., Guilbault, J.-P., Hutchinson, I. & Ricketts, B. D. 1997 (September): Pre-Younger Dryas resurgence of the southwestern margin of the Cordilleran ice sheet, British Columbia, Canada. Boreas , Vol. 26, pp. 261–278. Oslo. ISSN 0300–9483.
A lobe of the Cordilleran ice sheet readvanced into the central Fvaser Lowland, southwestern British Columbia, Canada, on at least two occasions near the end of the last glaciation. This ice also flowed into the previously deglaciated, lower reaches of mountain valleys adjacent to the Fraser Lowland and into Washington state. The first of these advances occurred before about 11900 BP and ended with glacier retreat and the establishment of lodgepole pine forest on newly deglaciated terrain. Parts of this forest were overridden by ice during a second advance, shortly after 11300 BP. The younger advance is most likely older than the Younger Dryas Chronozone (11000–10000 BP) and may correlate with an intra-Allerad cooling event (the Killarney-Gerzensee oscillation). The older advance may have occurred during the Oldest Dryas or Older Dryas cold period. Non-climatic factors could also be involved, as emergence of the Fraser Lowland before the older advance greatly reduced or eliminated calving at the glacier margin and thus altered the mass balance of the ice lobe.  相似文献   

15.
During the Itkillik Glaciation the Brooks Range supported an extensive mountain-glacier complex that extended for 750 km between 141° and 158°W longitude. Individual ice streams and piedmont lobes flowed as much as 50 km beyond the north and south margins of the range. Glaciers in the southern Brooks Range were longer than those farther north because of a southerly precipitation source, whereas those in the central and eastern part of the range were larger than glaciers at the extremities of the mountain system because of higher and more-extensive accumulation areas. Glacier equilibrium-line altitudes (ELAs) at the time of greatest advance were depressed 600 ± 100 m below present levels, whereas during a less-extensive late-glacial readvance (Alapah Mountain) ELA depression was about 300 ± 30 m. Radiocarbon dates indicate that Itkillik drift correlates with Late Wisconsin drift along the southern margin of the Laurentide Ice Sheet and with drift of Cordilleran glaciers in southern Alaska and the western conterminous United States deposited during the last glaciation. Itkillik I moraines represent the maximum ice advance under cold full-glacial conditions between about 24,000 and 17,000 14C y. a. Itkillik II sediments, probably deposited close to 14,000 y. a., are characterized by abundant outwash and ice-contact stratified drift implying a milder climate than that of the Itkillik I phase. Alapah Mountain moraines at the heads of valleys draining high-altitude (≥1800 m) source areas record a possible late Itkillik readvance that is not yet closely dated. Itkillik glaciers may have largely disappeared from Brooks Range valleys by the beginning of the Holocene.  相似文献   

16.
Large and complete glaciotectonic sequences formed by marine‐terminating glaciers are rarely observed on land, hampering our understanding of the behaviour of such glaciers and the processes operating at their margins. During the Late Weichselian in western Iceland, an actively retreating marine‐terminating glacier resulted in the large‐scale deformation of a sequence of glaciomarine sediments. Due to isostatic rebound since the deglaciation, these formations are now exposed in the coastal cliffs of Belgsholt and Melabakkar‐Ásbakkar in the Melasveit district, and provide a detailed record of past glacier dynamics and the inter‐relationships between glaciotectonic and sedimentary processes at the margin of this marine‐terminating glacier. A comprehensive study of the sedimentology and glaciotectonic architecture of the coastal cliffs reveals a series of subaquatic moraines formed by a glacier advancing from Borgarfjörður to the north of the study area. Analyses of the style of deformation within each of the moraines demonstrate that they were primarily built up by ice‐marginal/proglacial thrusting and folding of marine sediments, as well as deposition and subsequent deformation of ice‐marginal subaquatic fans. The largest of the moraines exposed in the Melabakkar‐Ásbakkar section is over 1.5 km wide and 30 m high and indicates the maximum extent of the Borgarfjörður glacier. Generally, the other moraines in the series become progressively younger towards the north, each designating an advance or stillstand position as the glacier oscillated during its overall northward retreat. During this active retreat, glaciomarine sediments rapidly accumulated in front of the glacier providing material for new moraines. As the glacier finally receded from the area, the depressions between the moraines were infilled by continued glaciomarine sedimentation. This study highlights the dynamics of marine‐terminating glaciers and may have implications for the interpretation of their sedimentological and geomorphological records.  相似文献   

17.
The North Atlantic Younger Dryas climatic reversal did not cause a glacier advance on Mount Rainier. The glaciers on Mount Rainier seem to have advanced in response to regional or local shifts in climate. However, the Younger Dryas climatic reversal may have affected the Mount Rainier area, causing a cold, but dry, climate unfavorable to glacier advances. Glaciers in the vicinity of Mount Rainier advanced twice during late glacial/early Holocene time. Radiocarbon dates obtained from lake sediments adjacent to the corresponding moraines are concordant, indicating that the ages for the advances are closely limiting. The first advance occurred before 11,300 14C yr BP (13,200 cal yr BP). During the North Atlantic Younger Dryas event, between 11,000 and 10,000 14C yr BP (12,900 and 11,600 cal yr BP), glaciers retreated on Mount Rainier, probably due to a lack of available moisture, but conditions may have remained cold. The onset of warmer conditions on Mount Rainier occurred around 10,000 14C yr BP (11,600 cal yr BP). Organic sedimentation lasted for at least 700 years before glaciers readvanced between 9800 and 8950 14C yr BP (10,900 and 9950 cal yr BP).  相似文献   

18.
Lateral moraines constructed along west to east sloping outlet glaciers from mountain centred, pre-last glacial maximum (LGM) ice fields of limited extent remain largely preserved in the northern Swedish landscape despite overriding by continental ice sheets, most recently during the last glacial. From field evidence, including geomorphological relationships and a detailed weathering profile including a buried soil, we have identified seven such lateral moraines that were overridden by the expansion and growth of the Fennoscandian ice sheet. Cosmogenic 10Be and 26Al exposure ages of 19 boulders from the crests of these moraines, combined with the field evidence, are correlated to episodes of moraine stabilisation, Pleistocene surface weathering, and glacial overriding. The last deglaciation event dominates the exposure ages, with 10Be and 26Al data derived from 15 moraine boulders indicating regional deglaciation 9600 ± 200 yr ago. This is the most robust numerical age for the final deglaciation of the Fennoscandian ice sheet. The older apparent exposure ages of the remaining boulders (14,600-26,400 yr) can be explained by cosmogenic nuclide inheritance from previous exposure of the moraine crests during the last glacial cycle. Their potential exposure history, based on local glacial chronologies, indicates that the current moraine morphologies formed at the latest during marine oxygen isotope stage 5. Although numerous deglaciation ages were obtained, this study demonstrates that numerical ages need to be treated with caution and assessed in light of the geomorphological evidence indicating moraines are not necessarily formed by the event that dominates the cosmogenic nuclide data.  相似文献   

19.
Younger Dryas cirque glaciers are known to have existed beyond the Scandinavian Ice Sheet in parts of western Norway. At Kråkenes, on the outermost coast, a cirque glacier formed and subsequently wasted away during the Younger Dryas. No glacier existed there during the Allerød. Large cirque moraines, some with marine deltas and associated fans, extend into the western part of Sykkylvsfjorden. Comparison with existing late-glacial sea-level curves shows that the uppermost marine sediment in these features was deposited well above Younger Dryas sea-level, demonstrating that the cirques were occupied by glaciers before the Younger Dryas. During the Younger Dryas the cirque glaciers expanded, and some advanced across the deltas, depositing till and supplying the sediment to form lower-level fans and deltas controlled by Younger Dryas sea level. The extent of the Younger Dryas advance of some of the glaciers was, at least in part, controlled by grounding on material deposited before the Younger Dryas. The depositional history of the glacial–marine deposits in the Sykkylven area indicates that cirque glaciers existed throughout Late-glacial time and only expanded during the Younger Dryas. The sediment sequence in glacial lakes beyond cirque moraines and reconstructions of glacier equilibrium lines indicate that this was true for most cirques in western Norway. Only on the outermost coast were new glaciers formed in response to Younger Dryas climate cooling. © 1998 John Wiley & Sons Ltd.  相似文献   

20.
The outermost moraines in front of the Scottbreen glacier in Spitsbergen date from c . AD 1900. These moraines rest on top of a marine shoreline radiocarbon-dated to about 11 200 14C yr BP and demonstrate that the AD-1900 moraines show the maximum glacier extent since late Allerød time. This means that Scottbreen was smaller during the Younger Dryas than at AD 1900, in contrast with glaciers on mainland western Europe, which were all much larger during the Younger Dryas. The explanation is probably starvation of precipitation on western Spitsbergen during the Younger Dryas. In contrast, ice sheets and glaciers in Spitsbergen reacted more or less in concert with glaciers in western Europe, during the global Last Glacial Maximum and the Little Ice Age.  相似文献   

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