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1.
Melting glaciers and ice caps on Baffin Island contribute roughly half of the sea-level rise from all ice in Arctic Canada, although they comprise only one-fourth of the total ice in the region. The uncertain future response of arctic glaciers and ice caps to climate change motivates the use of paleodata to evaluate the sensitivity of glaciers to past warm intervals and to constrain mechanisms that drive glacier change. We review the key patterns and chronologies of latest Pleistocene and Holocene glaciation on Baffin Island. The deglaciation by the Laurentide Ice Sheet occurred generally slowly and steadily throughout the Holocene to its present margin (Barnes Ice Cap) except for two periods of rapid retreat: An early interval 12 to 10 ka when outlet glaciers retreated rapidly through deep fiords and sounds, and a later interval 7 ka when ice over Foxe Basin collapsed. In coastal settings, alpine glaciers were smaller during the Younger Dryas period than during the Little Ice Age. At least some alpine glaciers apparently survived the early Holocene thermal maximum, which was several degrees warmer than today, although data on glacier extent during the early Holocene is extremely sparse. Following the early Holocene thermal maximum, glaciers advanced during Neoglaciation, beginning in some places as early as 6 ka, although most sites do not record near-Little Ice Age positions until 3.5 to 2.5 ka. Alpine glaciers reached their largest Holocene extents during the Little Ice Age, when temperatures were 1–1.5 °C cooler than during the late 20th century. Synchronous advances across Baffin Island throughout Neoglaciation indicate sub-Milankovitch controls on glaciation that could involve major volcanic eruptions and solar variability. Future work should further elucidate the state of glaciers and ice caps during the early Holocene thermal maximum and glacier response to climate forcing mechanisms.  相似文献   

2.
Alpine glacier fluctuations provide important paleoclimate proxies where other records such as ice cores, tree rings, and speleothems are not available. About 20 years have passed since a special issue of Quaternary Science Reviews was published to review the worldwide evidence for Holocene glacier fluctuations. Since that time, numerous sites have been discovered, new dating techniques have been developed, and refined climatic hypotheses have been proposed that contribute to a better understanding of Earth's climate system. This special volume includes 12 papers on Holocene and latest Pleistocene alpine glacier fluctuations that update the seven review papers from 1988.Major findings of these 12 papers include the following: many, but certainly not all, alpine areas record glacier advances during the Younger Dryas cold interval. Most areas in the Northern Hemisphere witnessed maximum glacier recession during the early Holocene, with some glaciers disappearing, although a few sites yield possible evidence for advances during the 8.2 ka cooling event. In contrast, some alpine areas in the Southern Hemisphere saw glaciers reach their maximum post-glacial extents during the early to middle Holocene. In many parts of the globe, glaciers reformed and/or advanced during Neoglaciation, beginning as early as 6.5 ka. Neoglacial advances commonly occurred with millennial-scale oscillations, with many alpine glaciers reaching their maximum Holocene extents during the Little Ice Age of the last few centuries. Although the pattern and rhythm of these glacier fluctuations remain uncertain, improved spatial coverage coupled with tighter age control for many events will provide a means to assess forcing mechanisms for Holocene and latest Pleistocene glacial activity and perhaps predict glacier response to future impacts from human-induced climate change.  相似文献   

3.
《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   

4.
Atle Nesje   《Quaternary Science Reviews》2009,28(21-22):2119-2136
During the early Holocene abrupt, decadal to centennial-scale climate variations caused significant glacier variations in Norway. Increased freshwater inflow to the North Atlantic and Arctic Oceans has been suggested as one of the most likely mechanisms to explain the abrupt and significant Lateglacial and early Holocene climatic events in NW Europe. The largest early Holocene glacier readvances occurred 11,200, 10,500, 10,100, 9700, 9200 and 8400–8000 cal. yr BP. The studied Norwegian glaciers apparently melted away at least once during the early/mid-Holocene. The period with the most contracted glaciers in Scandinavia was between 6600 and 6000 cal. yr BP. Subsequent to 6000 cal. yr BP the glaciers started to advance and the most extensive glaciers existed at about 5600, 4400, 3300, 2300, 1600 cal. yr BP, and during the ‘Little Ice Age’. Times with overall less glacier activity were apparently around 5000, 4000, 3000, 2000, and 1200 cal. yr BP. It has been proposed that several glacier advances occurred in Scandinavia (including northern Sweden) at 8500–7900, 7400–7200, 6300–6100, 5900–5800, 5600–5300, 5100–4800, 4600–4200, 3400–3200, 3000–2800, 2700–2000, 1900–1600, 1200–1000, and 700–200 cal. yr BP. Glaciers in northern Sweden probably reached their greatest ‘Little Ice Age’ extent between the 17th and the beginning of the 18th centuries. Evidence for early Holocene glacier advances in northern Scandinavia, however, has been questioned by more recent, multi-disciplinary studies. The early to mid-Holocene glacier episodes in northern Sweden may therefore be questioned.Most Norwegian glaciers attained their maximum ‘Little Ice Age’ extent during the mid-18th century. Cumulative glacier length variations in southern Norway, based on marginal moraines dated by lichenometry and historic evidence, show an overall retreat from the mid-18th century until the 1930s–40s. Subsequently, most Norwegian glaciers retreated significantly. Maritime outlet glaciers with short frontal time lags (<10–15 years) started to advance in the mid-1950s, whereas long outlet glaciers with longer frontal time lags (>15–20 years) continued their retreat to the 1970s and 1980s. However, maritime glaciers started to advance as a response to higher winter accumulation during the first part of the 1990s. After 2000 several of the observed glaciers have retreated remarkably fast (annual frontal retreat > 100 m) mainly due to high summer temperatures. The general glacier retreat during the early Holocene and the Neoglacial advances after 6000 cal. yr BP are in line with orbital forcing, due to the decrease of Northern Hemisphere summer solar insolation and the increase in winter insolation. In addition, regional weather modes, such as the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO), play a significant role with respect to decadal and multi-decadal climate variability.  相似文献   

5.
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   

6.
With accelerated melting of alpine glaciers, understanding the future state of the cryosphere is critical. Because the observational record of glacier response to climate change is short, palaeo‐records of glacier change are needed. Using proglacial lake sediments, which contain continuous and datable records of past glacier activity, we investigate Holocene glacier fluctuations on northeastern Baffin Island. Basal radiocarbon ages from three lakes constrain Laurentide Ice Sheet retreat by ca. 10.5 ka. High sedimentation rates (0.03 cm a?1) and continuous minerogenic sedimentation throughout the Holocene in proglacial lakes, in contrast to organic‐rich sediments and low sedimentation rates (0.005 cm a?1) in neighbouring non‐glacial lakes, suggest that glaciers may have persisted in proglacial lake catchments since regional deglaciation. The presence of varves and relatively high magnetic susceptibility from 10 to 6 ka and since 2 ka in one proglacial lake suggest minimum Holocene glacier extent ca. 6–2 ka. Moraine evidence and proglacial and threshold lake sediments indicate that the maximum Holocene glacier extent occurred during the Little Ice Age. The finding that glaciers likely persisted through the Holocene is surprising, given that regional proxy records reveal summer temperatures several degrees warmer than today, and may be due to shorter ablation seasons and greater accumulation‐season precipitation. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

7.
Ice-divide migration may explain the pattern of Holocene glacier fluctuations around the Mýrdalsjökull ice cap in southern Iceland. On at least three occasions Sölheimajokull, the principal outlet glacier on the southwest flank of the ice cap, has exceeded the Little Ice Age limits of recent centuries that mark the maximum extent of neighbouring glaciers in the Holocene. Bedrock divides beneath the Mýrdalsjökull ice cap do not coincide with present ice divides. It is suggested that the ice divide migrated during the course of ice-cap growth. At various stages during the Holocene (7000-4500, c. 3100, 1400-1200 BP) Sólheimajokull could have drained more of the ice cap than today, so becoming more advanced than neighbouring glaciers. In the Little Ice Age ( c. AD 1600–1900) the glacier could have had a smaller catchment as a result of ice-divide migration, resulting in a more inhibited advance compared with neighbouring glaciers which reached their Holocene maximum at that time. Identification of ice-divide migration is important for palaeoclimatic reconstructions because of the need to recognize different responses of glaciers to climate if one is to use their fluctuations as indicators of change.  相似文献   

8.
Our 12.81 m long record from Lake Ximencuo provides insight into the Lateglacial and Holocene evolution of the lake and the Nianbaoyeze glacier in its catchment, and the regional climate history since 19 kyr. Lake Ximencuo was a permanent deep lake throughout its history. In contrast to numerous lakes on the Tibetan Plateau which experienced shallow lake levels or even desiccation during the Lateglacial, Lake Ximencuo was fed by large quantities of meltwater in the Lateglacial. The existence of glaciated upper catchment areas is apparently a prerequisite for lakes on the Tibetan Plateau which maintained relatively high water levels during the generally cold and dry periods following the global LGM (21 kyr). A minor re-advance of the Nianbaoyeze glacier was recorded coeval with the Greenland Stadial 2a (16.4 and 14.5 kyr), followed by rapid warming apparently synchronous with the Greenland Interstadial 1. Warmest conditions were recorded at Lake Ximencuo during the early Holocene which was punctured by a remarkable pulse of climate deterioration around 8.3 kyr. This spell represents probably the 8.2 kyr event of the North Atlantic region, suggesting that it had a significant impact on the Tibetan Plateau. Colder conditions of longer duration occurred from 4.7 to 3.7 kyr, apparently in phase with numerous records signalling colder and drier conditions on the Tibetan Plateau. Two minor spells of colder conditions and probably catchment erosion were recorded in the late Holocene between 2.0 and 1.4 kyr and between 0.5 and 0.1 kyr with the latter representing the Little Ice Age. It is unclear, whether human activities may have accelerated or even solely triggered the late Holocene erosion events.  相似文献   

9.
Local glaciers and ice caps (GICs) comprise only ~5.4% of the total ice volume, but account for ~14–20% of the current ice loss in Greenland. The glacial history of GICs is not well constrained, however, and little is known about how they reacted to Holocene climate changes. Specifically, in North Greenland, there is limited knowledge about past GIC fluctuations and whether they survived the Holocene Thermal Maximum (HTM, ~8 to 5 ka). In this study, we use proglacial lake records to constrain the ice‐marginal fluctuations of three local ice caps in North Greenland including Flade Isblink, the largest ice cap in Greenland. Additionally, we have radiocarbon dated reworked marine molluscs in Little Ice Age (LIA) moraines adjacent to the Flade Isblink, which reveal when the ice cap was smaller than present. We found that outlet glaciers from Flade Isblink retreated inland of their present extent from ~9.4 to 0.2 cal. ka BP. The proglacial lake records, however, demonstrate that the lakes continued to receive glacial meltwater throughout the entire Holocene. This implies that GICs in Finderup Land survived the HTM. Our results are consistent with other observations from North Greenland but differ from locations in southern Greenland where all records show that the local ice caps at low and intermediate elevations disappeared completely during the HTM. We explain the north–south gradient in glacier response as a result of sensitivity to increased temperature and precipitation. While the increased temperatures during the HTM led to a complete melting of GICs in southern Greenland, GICs remained in North Greenland probably because the melting was counterbalanced by increased precipitation due to a reduction in Arctic sea‐ice extent and/or increased poleward moisture transport.  相似文献   

10.
In the southern Argentine Andes, ten advances of valley glaciers were used to reconstruct the late-glacial and Holocene glacier history. The accumulation areas of these glaciers lie in the Precordillera and are thus independent of fluctuations of the South Patagonian Icefield. Like the Viedma outlet glacier, the valley glaciers advanced three times during late-glacial time (14,000–10,000 yr B.P.). The youngest advance correlates with the Younger Dryas Stade, based on two minimum AMS14C dates of 9588 and 9482 yr B.P. The second oldest advance occurred before 11,800 yr B.P. During the first half of the Holocene, (ca. 10,000–5000 yr B.P.), advances culminated about 8500, 8000–7500, and 5800–5500 yr B.P. During the second half of the Holocene, advances occurred between ca. 4500 and 4200 yr B.P., as well as between 3600 and 3300 yr B.P. In the Río Cóndor valley three subsequent advances have been identified.  相似文献   

11.
Holocene glacial variations in Sarek National Park, northern Sweden   总被引：3，自引：0，他引：3  
Detailed mapping of well-preserved moraine systems fronting 17 small alpine glaciers in Sarek National Park in Swedish Lapland reveals two Holocene intervals of prolonged glacier expansion, each involving a complex of minor fluctuations. The younger interval, which corresponds to the Little Ice Age, experienced advances that culminated about A.D. 1916–1920, 1880–1890, 1850–1860, 1800–1810, 1780, 1700–1720, 1680, 1650, and 1590–1620. The older expansion interval, which probably centered around 2500 14C yr B.P., experienced several minor fluctuations spread through about 600 years.
Lichen data collected on moraine systems in Sarek are internally consistent from glacier to glacier. Lichen measurements on surfaces of known age in Sarek and nearby Kebnekaise match closely, allowing moraine correlations between these areas. Several older expansion intervals are recorded in the Kebnekaise Mountains. Taken together, the two sequences suggest that a series of prolonged expansion intervals, each similar to the Little Ice Age, has characterized the Holocene in Lapland. Fluctuations of the Scandinavian Ice Sheet in Sweden suggest that this series of Little-Ice-Age events extends back into the late Weichsel in the form of the Younger Dryas and Oldest Dryas stadials.  相似文献   

12.
Large outlet glaciers draining the Greenland Ice Sheet significantly influence overall ice sheet mass balance. Considerable short term (years to decades) retreat and fluctuations in velocity of Jakobshavn Isbræ, western Greenland, illustrate the complex nature by which large outlet glaciers respond to climate change, making predictions of future ice sheet change challenging. To provide a longer-term view (centuries), we investigate the geological record of Jakobshavn Isbræ change. We use continuous sediment records from lakes that were influenced by the recent advance of Jakobshavn Isbræ, which took place during the Little Ice Age. In particular, we explore the use of annually laminated lake sediments (varves) to precisely constrain the advance of the ice margin as it approached its late Holocene maximum extent. We find that the ice margin advanced recently, at least after ～1650 to ～1700 AD, and more likely ～1800 AD. We suggest that during this period Jakobshavn Isbræ advanced at a rate that was similar to its historically documented average retreat since ～1850 AD. Our results indicate that Jakobshavn Isbræ, and presumably other large marine calving glaciers, have the ability to advance quickly in response to climate forcing.  相似文献   

13.
Reconstructions of past environmental changes are critical for understanding the natural variability of Earth's climate system and for providing a context for present and future global change. Radiocarbon-dated lake sediments from Lake CF3, northeastern Baffin Island, Arctic Canada, are used to reconstruct past environmental conditions over the last 11,200 years. Numerous proxies, including chironomid-inferred July air temperatures, diatom-inferred lakewater pH, and sediment organic matter, reveal a pronounced Holocene thermal maximum as much as 5°C warmer than historic summer temperatures from 10,000 to 8500 cal yr B.P. Following rapid cooling 8500 cal yr B.P., Lake CF3 proxies indicate cooling through the late Holocene. At many sites in northeastern Canada, the Holocene thermal maximum occurred later than at Lake CF3; this late onset of Holocene warmth is generally attributed to the impacts of the decaying Laurentide Ice Sheet on early Holocene temperatures in northeastern Canada. However, the lacustrine proxies in Lake CF3 apparently responded to insolation-driven warmth, despite the proximity of Lake CF3 to the Laurentide Ice Sheet and its meltwater. The magnitude and timing of the Holocene thermal maximum at Lake CF3 indicate that temperatures and environmental conditions at this site are highly sensitive to changes in radiative forcing.  相似文献   

14.
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   

15.
During the last glacial stage, Washington Land in western North Greenland was probably completely inundated by the Greenland Ice Sheet. The oldest shell dates from raised marine deposits that provide minimum ages for the last deglaciation are 9300 cal. yr BP (northern Washington Land) and 7600 cal. yr BP (SW Washington Land). These dates indicate that Washington Land, which borders the central part of Nares Strait separating Greenland from Ellesmere Island in Canada, did not become free of glacier ice until well into the Holocene. The elevation of the marine limit falls from 110 m a.s.l. in the north to 60 m a.s.l. in the southwest. The recession was followed by readvance of glaciers in the late Holocene, and the youngest shell date from Neoglacial lateral moraines north of Humboldt Gletscher is 600 cal. yr BP. Since the Neoglacial maximum, probably around 100 years ago, glaciers have receded. The Holocene marine assemblages comprise a few southern extralimital records, notably of Chlamys islandica dated to 7300 cal. yr BP. Musk ox and reindeer disappeared from Washington Land recently, perhaps in connection with the cold period that culminated about 100 years ago.  相似文献   

16.
Radiocarbon age determination of a Greenland whale (Balaena mysticetus) vertebra from Melville Bugt in northwestern Greenland yields an age of 9259-8989 cal yr BP. The margin of the Greenland Ice Sheet in Melville Bugt was situated behind its AD 1950-2000 position in the early Holocene, at a similar position to that being reached following rapid retreat in recent years. Such an early deglaciation of areas close to the Greenland Ice Sheet is unusual. This probably reflects the unique glaciological setting resulting from the narrow fringe of ice-free islands and peninsulas and offshore waters with deep areas that characterize this part of Greenland. The timing of Greenland Ice Sheet retreat to its present margin varies significantly around Greenland.  相似文献   

17.
Holocene climatic and paleoceanographic development of the SE Greenland Shelf is studied from cores MD99-2317 and MD99-2322, at sites north and south of the Denmark Strait, respectively. Lithofacies, IRD counts, calcium carbonate percentages, benthic and planktic foraminiferal assemblages and oxygen isotope analyses, and summer SSTs reveal significant climate variations in the Holocene driven by declining solar insolation and its interaction with waning continental ice sheets, and changing atmospheric pressure patterns. Large changes in the East Greenland and Irminger Currents and the Greenland Ice Sheet are manifested as a 4-part division of the Holocene. An early Holocene cold interval dominated by melting of the Greenland Ice Sheet and Polar Front retreat extends from 11.8 to 9.5 cal kyr BP. A cold interval from 9.5 to 8.1 cal kyr BP involved episodic cooling of the Irminger Current resulting from the last phases of Laurentide Ice Sheet deglaciation and delayed the Holocene optimum off East Greenland by 3 kyr relative to peak summer solar insolation, which likely helped to limit the early Holocene melting of the Greenland Ice Sheet. The period 8.1–3.5 cal kyr BP represents a climatic optimum interval of maximum Greenland Ice Sheet retreat and strong Irminger Current inflow to the Denmark Strait. Between 6.8 and 3.5 cal kyr BP, the Irminger Current penetrated further North into the Nordic Seas than has been observed in recent decades. This signal is consistent with diminished northerly winds, a weaker Greenland High and contracted subpolar gyre. By 5 cal kyr BP, periods of increased Polar Water and decreasing salinity in the Irminger Current suggest a transition toward expansion of the subpolar gyre and increased Polar Water in the EGC. The Neoglacial interval from 3.5 to 0.2 cal kyr BP was cold and variable with increased freshwater forcing from the Arctic Ocean, advance of the Greenland Ice Sheet and southward advance of the Polar Front. Enhanced northerly winds and a strengthened Greenland High are consistent with thicker and more extensive Polar Water and greatly diminished northward advection of Irminger Current in the Denmark Strait.  相似文献   

18.
To develop a more precise understanding of Alpine glacier fluctuations during the Holocene, the glacier forefields of the Triftjegletscher and the Oberseegletscher east of Zermatt in the Valais Alps, Switzerland, were investigated. A multidisciplinary approach of detailed geological and geomorphological field mapping combined with 10Be exposure and radiocarbon dating was applied. A total of twelve samples of boulders and bedrock were taken from both Little Ice Age (LIA) landforms, as documented by the Dufour map published in 1862, and from landforms outside of the LIA. The resulting 10Be ages range between 12590 ± 350 a and 420 ± 170 a. A piece of wood found embedded in the Little Ice Age moraine gave radiocarbon ages that range between 293 cal years BP up to modern (356–63 cal years before 2013). Based on these results, four tentative steps of the Holocene evolution could be distinguished. An early Holocene stage, which documents the decay of the Egesen stadial glaciers when the first parts of the study area became ice free. This was followed by a phase with no evidence of glacier advance. Then in the late Holocene, the glaciers advanced (at least) twice. An advance around 1200 a, as shown by several moraine ages, coincides with the Göschenen II cold phase. A more extensive readvance occurred during the LIA as shown on the historical maps and underpinned by one 10Be exposure age and the radiocarbon age. This later advance destroyed or overprinted the earlier landforms in most parts of the area.  相似文献   

19.
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   

20.
The Greenland Ice Sheet is thinning at an accelerating pace and the ice sheet's contribution to sea-level rise has doubled in less than a decade. New data show rapid and widespread changes in the behaviour of the ice sheet, particularly along the coastal margin. These changes coincide with a decade of sustained Arctic warming of up to 3 °C. Decay of the Greenland Ice Sheet in response to global warming will not only be governed by increased surface melting during longer and warmer summers but also by a speed-up of coastal glaciers that drain the interior ice sheet. A precise estimate of sea-level rise in the twenty-first century relies on improved theoretical treatment of these glaciers in computer models.  相似文献   

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