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1.
Lake sediment, glacier extent and tree rings were used to reconstruct Holocene climate changes from Goat Lake at 550 m asl in the Kenai Mountains, south‐central Alaska. Radiocarbon‐dated sediment cores taken at 55 m water depth show glacial‐lacustrine conditions until about 9500 cal. yr BP, followed by organic‐rich sedimentation with an overall increasing trend in organic matter and biogenic silica content leading up to the Little Ice Age (LIA). Through most of the Holocene, the northern outlet of the Harding Icefield remained below the drainage divide that currently separates it from Goat Lake. A sharp transition from gyttja to inorganic mud about AD 1660 signifies the reappearance of glacier meltwater into Goat Lake during the LIA, marking the maximum Holocene (postglacial) extent. Meltwater continued to discharge into the lake until about AD 1900. A 207 yr tree‐ring series from 25 mountain hemlocks growing in the Goat Lake watershed correlates with other regional tree‐ring series that indicate an average summer temperature reduction of about 1°C during the 19th century compared with the early–mid 20th century. Cirque glaciers around Goat Lake reached their maximum LIA extent in the late 19th century. Assuming that glacier equilibrium‐line altitudes (ELA) are controlled solely by summer temperature, then the cooling of 1°C combined with the local environmental lapse rate would indicate an ELA lowering of 170 m. In contrast, reconstructed ELAs of 12 cirque glaciers near Goat Lake average only 34 ± 18 m lower during the LIA. The restricted ELA lowering can be explained by a reduction in accumulation‐season precipitation caused by a weakening of the Aleutian low‐pressure system during the late LIA. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

2.
Two glaciers at Eyjafjallajökull, south Iceland, provide a record of multiple episodes of glacier advance since the Sub-Atlantic period, ca. 2000 yr ago. A combination of tephrochronology and lichenometry was applied to date ice-marginal moraines, tills and meltwater deposits. Two glacier advances occurred before the 3rd century AD, others in the 9th and 12th centuries bracketing the Medieval Warm Period, and five groups of advances occurred between AD 1700 and 1930, within the Little Ice Age. The advances of Eyjafjallajökull before the Norse settlement (ca. AD 870) were synchronous with other glacier advances identified in Iceland. In contrast, medieval glacier advances between the 9th and 13th centuries are firmly identified for the first time in Iceland. This challenges the view of a prolonged Medieval Warm Period and supports fragmentary historical data that indicate significant medieval episodes of cooler and wetter conditions in Iceland. An extended and more detailed glacier chronology of the mid- and late Little Ice Age is established, which demonstrates that some small outlet glaciers achieved their Little Ice Age maxima around AD 1700. While Little Ice Age advances across Iceland appear to synchronous, the timing of the maximum differs between glacier type and region.  相似文献   

3.
Holocene glacier variations pre‐dating the Little Ice Age are poorly known in the western Alps. Studied for two centuries, the Miage morainic amphitheatre (MMA) is composed of three subconcentric sets of c. 25 moraines. Because of its location and of a dominant mode of morainic accretion, the MMA is a well‐preserved marker of the glacier dynamics during the Neoglacial. Radiocarbon dates were obtained by digging and coring in inter‐ morainic depressions of the MMA and through a deep core drilling in a dammed‐lake infill (Combal); complementary data for the inner MMA were obtained by lichenometry and dendrochronology. Radiocarbon chronology shows that (i) the MMA not only pre‐dates the Little Ice Age (LIA), but was built at least since 5029–4648 cal. yr BP (beginning of the Neoglacial); (ii) outer sets of moraines pre‐date 2748–2362 cal. yr BP; (iii) the MMA dammed the Lake Combal from 4.8 to 1.5 cal. kyr BP, while lakes/ponds formed inside the moraines (e.g. from 2147–1928 to 1506–1295 cal. yr BP). The ‘Neoglacial model’ proposed here considers that the MMA formed during the whole Neoglacial by a succession of glacier advances at 4.8–4.6 cal. ky BP (early Neoglacial), around 2.5 cal. ky BP (end of Göschener I), at AD 600–900 (end of Göschener II) and during the LIA, separated by raising phases of the right‐lateral moraine by active dumping because of the Miage debris cover.  相似文献   

4.
Ma, L., Wu, J., Yu, H., Zeng, H. & Abuduwaili, J. 2011: The Medieval Warm Period and the Little Ice Age from a sediment record of Lake Ebinur, northwest China. Boreas, Vol. 40, pp. 518–524. 10.1111/j.1502‐3885.2010.00200.x. ISSN 0300‐9483. Lake Ebinur, Xinjiang, northwest China, is a closed‐basin, shallow lake that responds rapidly to changes in the ratio of precipitation to evaporation (P/E). A sediment record spanning the last 1500 years was obtained from the lake. We used δ18O and δ13C in bulk carbonate, and δ13C of organic matter in the lake sediments to infer environmental changes in the Ebinur region during the Medieval Warm Period (MWP) and the Little Ice Age (LIA). Decreased δ18O values of carbonate largely reflect an enhanced P/E ratio within the basin and a higher lake level. Bulk carbonates with higher δ13C values are deposited during periods when lake‐water pH is high, while lower δ13C values reflect a lower pH in the water column. δ13C in organic matter is associated with the amount of precipitation. The results indicate that the Ebinur region experienced a dry MWP and a wet LIA, although the MWP and LIA were warm and cold periods, respectively, as expected. Furthermore, the MWP and LIA were hydrologically complex and cannot be characterized as uniformly wet or dry. Peak wet periods are recorded in the sediment core around AD 1000, 1400 and 1700, and a dry event also occurred in the period of temperature change within the LIA (cold to warm around AD 1500). A comparison of the Lake Ebinur data with proxy records for the strength of the Siberian High and climate proxy indicators suggests that precipitation in the Ebinur region was a consequence, in part, of an enhanced Siberian High during the LIA.  相似文献   

5.
Swift, D. A., Sanderson, D. C. W., Nienow, P. W., Bingham, R. G. & Cochrane, I. C. 2010: Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla, Switzerland – a consequence of resetting at the glacier bed? Boreas, Vol. 40, pp. 446–458. 10.1111/j.1502‐3885.2010.00196.x. ISSN 0300‐9483. Luminescence has the potential to elucidate glacial geomorphic processes because primary glacial sediment sources and transport pathways are associated with contrasting degrees of exposure to light. Most notably, sediment entrained from extraglacial sources should be at least partially reset, whereas sediment produced by glacial erosion of subglacial bedrock should retain substantial luminescence commensurate with a geological irradiation history. We set out to test the validity of this assumption at Haut Glacier d'Arolla, Switzerland using sediment sampled extraglacially and from the glacier bed. Contrary to our expectations, the subglacial samples exhibited natural signals that were substantially lower than those of other sample groups, and further (albeit limited) analyses have indicated no obvious differences in sample‐group luminescence characteristics or behaviour that could account for this observation. For glaciological reasons, we can eliminate the possibilities that the subglacial sediment has been extraglacially reset or exposed in situ to heat or light. We therefore advocate investigation of possible resetting processes related to subglacial crushing and grinding, and speculate that such processes, if more generally present, may enable the dating of subglacially deposited tills using luminescence‐based techniques.  相似文献   

6.
Samples of glacial till deposited since the Little Ice Age (LIA) maximum by two glaciers, North Bogbre at Svartisen and Corneliussen-breen at Okstindan, northern Norway, were obtained from transects running from the current glacier snout to the LIA (c. AD 1750) limit. The samples were analysed to determine their sediment magnetic properties, which display considerable variability. Significant trends in some magnetic parameters are evident with distance from the glacier margin and hence length of subaerial exposure. Magnetic susceptibility (χ) decreases away from the contemporary snout, perhaps due to the weathering of ferrimagnetic minerals into antiferromagnetic forms, although this trend is generally not statistically significant. Trends in the ratios of soft IRM/hard IRM which are statistically significant support this hypothesis, suggesting that antiferromagnetic minerals are increasing relative to ferrimagnetic minerals towards the LIA maximum. Backfield ratios (IRM -100 mT/SIRM) also display a significant and strong trend towards magnetically harder behaviour with proximity to the LIA maximum. Thus, by employing a chronosequence approach, it may be possible to use sediment magnetics data as a tool for reconstructing glacier retreat in areas where more traditional techniques, such as lichenometry, are not applicable.  相似文献   

7.
Understanding Arctic glacier sensitivity is key to predicting future response to air temperature rise. Previous studies have used proglacial lake sediment records to reconstruct Holocene glacier advance–retreat patterns in South and West Greenland, but high‐resolution glacier records from High Arctic Greenland are scarce, despite the sensitivity of this region to future climate change. Detailed geochemical analysis of proglacial lake sediments close to Zackenberg, northeast Greenland, provides the first high‐resolution record of Late Holocene High Arctic glacier behaviour. Three phases of glacier advance have occurred in the last 2000 years. The first two phases (c. 1320–800 cal. a BP) occurred prior to the Little Ice Age (LIA), and correspond to the Dark Ages Cold Period and the Medieval Climate Anomaly. The third phase (c. 700 cal. a BP), representing a smaller scale glacier oscillation, is associated with the onset of the LIA. Our results are consistent with recent evidence of pre‐LIA glacier advance in other parts of the Arctic, including South and West Greenland, Svalbard, and Canada. The sub‐millennial glacier fluctuations identified in the Madsen Lake succession are not preserved in the moraine record. Importantly, coupled XRF and XRD analysis has effectively identified a phase of ice advance that is not visible by sedimentology alone. This highlights the value of high‐resolution geochemical analysis of lake sediments to establish rapid glacier advance–retreat patterns in regions where chronological and morphostratigraphical control is limited.  相似文献   

8.
Little Ice Age (LIA) fluctuations of Glaciar Río Manso, north Patagonian Andes, Argentina are studied using information from previous work and dendrogeomorphological analyses of living and subfossil wood. The most extensive LIA expansion occurred between the late 1700s and the 1830-1840s. Except for a massive older frontal moraine system apparently predating ca. 2240 14C yr BP and a small section of a south lateral moraine ridge that is at least 300 yr old, the early nineteenth century advance overrode surficial evidence of any earlier LIA glacier events. Over the past 150 yr the gently sloping, heavily debris-covered lower glacier tongue has thinned significantly, but several short periods of readvance or stasis have been identified and tree-ring dated to the mid-1870s, 1890s, 1900s, 1920s, 1950s, and the mid-1970s. Ice mass loss has increased in recent years due to calving into a rapidly growing proglacial lake. The neighboring debris-free and land-based Glaciar Frías has also retreated markedly in recent years but shows substantial differences in the timing of the peak LIA advance (early 1600s). This indicates that site-specific factors can have a significant impact on the resulting glacier records and should thus be considered carefully in the development and assessment of regional glacier chronologies.  相似文献   

9.
Ice‐cored lateral and frontal moraine complexes, formed at the margin of the small, land‐based Rieperbreen glacier, central Svalbard, have been investigated through field observations and interpretations of aerial photographs (1936, 1961 and 1990). The main focus has been on the stratigraphical and dynamic development of these moraines as well as the disintegration processes. The glacier has been wasting down since the ‘Little Ice Age’ (LIA) maximum, and between 1936 and 1990 the glacier surface was lowered by 50–60 m and the front retreated by approximately 900 m. As the glacier wasted, three moraine ridges developed at the front, mainly as melting out of sediments from debris‐rich foliation and debris‐bands formed when the glacier was polythermal, probably during the LIA maximum. The disintegration of the moraines is dominated by wastage of buried ice, sediment gravity‐flows, meltwater activity and some frost weathering. A transverse glacier profile with a northward sloping surface has developed owing to the higher insolation along the south‐facing ice margin. This asymmetric geometry also strongly affects the supraglacial drainage pattern. Lateral moraines have formed along both sides of the glacier, although the insolation aspect of the glacier has resulted in the development of a moraine 60 m high along its northern margin. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

10.
Understanding the processes that deposit till below modern glaciers provides fundamental information for interpreting ancient subglacial deposits. A process‐deposit‐landform model is developed for the till bed of Saskatchewan Glacier in the Canadian Rocky Mountains. The glacier is predominantly hard bedded in its upper reaches and flows through a deep valley carved into resistant Palaeozoic carbonates but the ice margin rests on a thick (<6 m) soft bed of silt‐rich deformation till that has been exposed as the glacier retreats from its Little Ice Age limit reached in 1854. In situ tree stumps rooted in a palaeosol under the till are dated between ca 2900 and 2700 yr bp and record initial glacier expansion during the Neoglacial. Sedimentological and stratigraphic observations underscore the importance of subglacial deformation of glaciofluvial outwash deposited in front of the advancing glacier and mixing with glaciolacustrine carbonate‐rich silt to form a soft bed. The exposed till plain has a rolling drumlinoid topography inherited from overridden end moraines and is corrugated by more than 400 longitudinal flute ridges which record deformation of the soft bed and fall into three genetically related types: those developed in propagating incipient cavities in the lee of large subglacial boulders embedded in deformation till, and those lacking any originating boulder and formed by pressing of wet till up into radial crevasses under stagnant ice. A third type consists of U‐shaped flutes akin to barchan dunes; these wrap around large boulders at the downglacier ends of longitudinal scours formed by the bulldozing of boulders by the ice front during brief winter readvances across soft till. Pervasive subglacial deformation during glacier expansion was probably facilitated by large boulders rotating within the soft bed (‘glacioturbation’).  相似文献   

11.
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   

12.
Complex moraine-ridge sequences in front of seven outlet glaciers of the Jostedalsbreen ice-cap (Austerdalsbreen, Bergsetbreen, Fåbergstølsbreen, Lodalsbreen, Stegaholbreen, Tuftebreen, Bødalsbreen) are dated using families of lichenometric dating curves established previously at an eighth outlet (Nigardsbreen). Applicability of the Nigardsbreen curves at the regional level is tested using independent historical evidence: moraines deposited during the present century are dated to an accuracy of ± 9.4 yr (16.0%), and most of them are dated to an accuracy of ± 5.5 yr (9.4%). Results from the moraine sequences are combined to form a composite ‘Jostedalsbreen’ record. Median predicted dates for moraine ridges cluster around AD 1939 ± 2 yr, 1929 ± 3, 1908 ± 3, 1886 ± 5, 1875 ± 2, 1867 ± 4, 1855 ± 3, 1842 ± 5, 1822 ± 5, 1807 ± 4 and 1785 ± 5. At least four glaciers reached their ‘Little ice age’ maxima prior to AD 1780, two (Nigardsbreen and Bødalsbreen) at ca. 1750, one (Fåbergstølsbreen) at ca. 1705. Stegaholbreen attained its maximum ca. 1863. Since the ‘Little ice age’ maximum, and despite large differences in glacier size, frontal variations of the various outlets have exhibited a high degree of synchroneity, which suggests that the moraine sequences contain a sensitive record of high-frequency climatic variations over the last ca. 250 yr. During the early twentieth century, measured readvances of the order of 5–150 m over 1–10 yr led to moraine formation. Dendroclimatic evidence indicates that since the late eighteenth century, moraine ridges formed about 5 yr after summer temperature minima and correlate with runs of cool summers (temperature depression of 0.5–1.0°C below the AD 1700–1950 average). Almost simultaneous glacier advances appear to have been caused by reduced ablation. This near-immediate response to climatic variation, by glacier tongues that descend to relative low altitudes, is superimposed upon the longer-term dynamic response of the ice cap to climate.  相似文献   

13.
Moraine sequences in front of seven relatively low‐altitude glaciers in the Breheimen region of central southern Norway are described and dated using a ‘multi‐proxy’ approach to moraine stratigraphy. Lichenometric dating, based on the Rhizocarpon subgenus, is used to construct a composite moraine chronology, which indicates eight phases of synchronous moraine formation: AD 1793–1799, 1807–1813, 1845–1852, 1859–1862, 1879–1885, 1897–1898, 1906–1908 and 1931–1933. Although the existence of a few cases of older moraines, possibly dating from earlier in the eighteenth or late in the seventeenth centuries cannot be ruled out by lichenometry, Schmidt hammer R‐values from boulders on outermost moraine ridges suggest an absence of Holocene moraines older than the Little Ice Age. Twenty‐three radiocarbon dates from buried soils and peat associated with outermost moraines at three glaciers—Tverreggibreen, Storegrovbreen and Greinbreen—also indicate that the ‘Little Ice Age’ glacier maximum was the Neoglacial maximum at most if not all glaciers. Several maximum age estimates for the Little Ice Age glacier maximum range between the fifteenth and seventeenth centuries, with the youngest from a buried soil being AD 1693. A pre‐Little Ice Age maximum cannot be ruled out at Greinbreen, however, where the age of buried peat suggests the outermost moraine dates from AD 981–1399 (at variance with the lichenometric evidence). Glaciofluvial stratigraphy at Tverreggibreen provides evidence for minor glacier advances about AD 655–963 and AD 1277–1396, respectively. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

14.
The Holocene evolution of Rhone River clastic sediment supply in Lake Le Bourget is documented by sub-bottom seismic profiling and multidisciplinary analysis of well-dated sediment cores. Six high-amplitude reflectors within the lacustrine drape can be correlated to periods of enhanced inter- and underflow deposition in sediment cores. Based on the synthesis of major environmental changes in the NW Alps and on the age-depth model covering the past 7500 years in Lake Le Bourget, periods of enhanced Rhone River flood events in the lake can be related to abrupt climate changes and/or to increasing land use since c. 2700 cal. yr BP. For example, significant land use under rather stable climate conditions during the Roman Empire may be responsible for large flood deposits in the northern part of Lake Le Bourget between AD 966 and 1093. However, during the Little Ice Age (LIA), well-documented major environmental changes in the catchment area essentially resulted from climate change and formed basin-wide major flood deposits in Lake Le Bourget. Up to five 'LIA-like' Holocene cold periods developing enhanced Rhone River flooding activity in Lake Le Bourget are documented at c. 7200, 5200, 2800, 1600 and 200 cal. yr BP. These abrupt climate changes were associated in the NW Alps with Mont Blanc glacier advances, enhanced glaciofluvial regimes and high lake levels. Correlations with European lake level fluctuations and winter precipitation regimes inferred from glacier fluctuations in western Norway suggest that these five Holocene cooling events at 45°N were associated with enhanced westerlies, possibly resulting from a persistent negative mode of the North Atlantic Oscillation.  相似文献   

15.
The Gschnitz stadial was a period of regionally extensive glacier advance in the European Alps that lies temporally between the breakdown of the Last Glacial Maximum piedmont lobes and the beginning of the Bølling warm interval. Moraines of the Gschnitz stadial are found in medium to small catchments, are steep‐walled and blocky, and reflect a snowline lowering of 650–700 m in comparison to the Little Ice Age reference snowline. 10Be surface exposure dating of boulders from the moraine at the type locality at Trins (Gschnitz valley, Tyrol, Austria) shows that it stabilised no later than 15 400 ± 1400 yr ago. The overall morphological situation and the long reaction time of the glacier suggest that the climatic downturn lasted about 500 ± 300 yr, indicating that the Gschnitz cold period began approximately 15 900 ± 1400 yr ago, if not somewhat earlier. This is consistent with published radiocarbon dates that imply that the stadial occurred sometime between 15 400 14C yr BP (18 020–19 100 cal. yr) and 13 250 14C yr BP (15 360–16 015 cal. yr). A palaeoclimatic interpretation of the Gschnitz glacier based on a simple glacier flow model and statistical glacier‐climate models shows that precipitation was about one‐third of modern‐day precipitation and summer temperatures were about 10 K lower than today. In comparison, during the Younger Dryas, precipitation in this area was only about 10% less and Ts (summer temperature) was only 3.5–4 K lower than modern values. Based on the age of the moraine and the cold and dry climate at that time, we suggest that the Gschnitz stadial was the response of Alpine glaciers to cooling of the North Atlantic Ocean associated with Heinrich Event 1. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

16.
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   

17.
This article examines the link between late Holocene fluctuations of Lambatungnajökull, an outlet glacier of the Vatnajökull ice cap in Iceland, and variations in climate. Geomorphological evidence is used to reconstruct the pattern of glacier fluctuations, while lichenometry and tephrostratigraphy are used to date glacial landforms deposited over the past ˜400 years. Moraines dated using two different lichenometric techniques indicate that the most extensive period of glacier expansion occurred shortly before c . AD 1795, probably during the 1780s. Recession over the last 200 years was punctuated by re-advances in the 1810s, 1850s, 1870s, 1890s and c . 1920, 1930 and 1965. Lambatungnajökull receded more rapidly in the 1930s and 1940s than at any other time during the last 200 years. The rate and style of glacier retreat since 1930 compare well with other similar-sized, non-surging, glaciers in southeast Iceland, suggesting that the terminus fluctuations are climatically driven. Furthermore, the pattern of glacier fluctuations over the 20th century broadly reflects the temperature oscillations recorded at nearby meteorological stations. Much of the climatic variation experienced in southern Iceland, and the glacier fluctuations that result, can be explained by secular changes in the North Atlantic Oscillation (NAO) Advances of Lambatungnajökull generally occur during prolonged periods of negative NAO index. The main implication of this work relates to the exact timing of the Little Ice Age in the Northeast Atlantic. Mounting evidence now suggests that the period between AD 1750 and 1800, rather than the late 19th century, represented the culmination of the Little Ice Age in Iceland.  相似文献   

18.
Detailed 10Be and 14C dating and supporting pollen analysis of Alpine Lateglacial glacial and landslide deposits in the Hohen Tauern Mountains (Austria) constrain a sequence‐based stratigraphy comprising a major landslide (13.0±1.1 ka) overlain by till and termino‐lateral moraines of an advancing (12.6±1.0 ka) and retreating (11.3±0.8 ka) glacier in turn overlain by a minor landslide (10.8±1.1 ka). These results define glacier activity during the Younger Dryas age Egesen stadial bracketed by landslide activities during the Bølling‐Allerød interstadial and the Preboreal. In contrast to recent studies on Holocene glaciation in the Alps, no traces of any Holocene glacier advance bigger than during the Little Ice Age are documented. Furthermore, this study demonstrates the advantages of using an allostratigraphical approach based on unconformity‐bounded sedimentary units as a tool for glacial stratigraphy in formerly glaciated mountain regions, rather than a stratigraphy based on either isolated morphological features or lithostratigraphical characteristics.  相似文献   

19.
Surging outlet glaciers are important in draining large ice caps, but the mechanisms controlling surge periodicities are poorly known. We investigated a sediment sequence from the glacier‐fed Lake Lögurinn in eastern Iceland, and our unique annually resolved data, based on sedimentary varves, imply that Eyjabakkajökull, an outlet glacier of Vatnajökull, began surging about 2200 cal a BP (before 1950 AD). Approximately 1700 cal a BP, the glacier started to surge at a uniform 34‐ to 38‐year periodicity that prevailed until the coldest part of the Little Ice Age when the periodicity almost halved to 21–23 years. Since the late 1800 s the surge periodicity of Eyjabakkajökull has returned to a longer period of 35–40 years. We suggest that surge periodicities of Eyjabakkajökull are forced by climatically driven mass balance changes, which may be a common forcing factor for similar surge‐type outlet glaciers. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

20.
Changes in the supply of water and sediment to high‐latitude rivers related to contemporary climate change and glacier fluctuations largely determine the activity of fluvial processes. This study reconstructs fluvial dynamics since the end of the Little Ice Age (LIA) in two small, partially glaciated basins in the southern part of Spitsbergen, Svalbard Archipelago. We use a combination of aerial photograph interpretation, field mapping and dendrochronological analysis. Sequences of abandoned channels and glacifluvial terraces are distinctly visible in middle and lower parts of the Brattegg and Arie basins in this area. The advance of glaciers during the LIA in the upper part of the basins led to the development of a braiding pattern and to channel aggradation corresponding to the highest glacifluvial levels. The decreasing activity of these braidplains occurred at the turn of the 19th and 20th centuries, immediately prior to a significant incision period. A second generation of braided channels developed during the first half of the 20th century. Ice‐marginal lake development, less input of fine‐grained sediment to the river channel, and fast incision began from the second half of the 20th century onward. During the last two decades, the main fluvial response to the climatic warming has been contraction of flow within a narrower channel and the abandonment of braidplains. The increased lateral erosion and rate of downcutting and the formation of the most downstream reaches of the modern valley bottom occurred in the 1980s and 1990s. This process was intensified under flood conditions generated by extreme rainfall events. These micro‐scale (small partially glaciated basins) observations concerning the changes of the activity of fluvial processes since the end of LIA may be helpful for the reconstruction of past fluvial changes over longer time scales.  相似文献   

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