首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 531 毫秒
1.
  相似文献   

2.
The extent of Late Quaternary glaciation in the northwest Nelson region of New Zealand has traditionally been regarded as minor, with small‐scale valley glaciation in confined upland reaches. New geomorphological evidence, including moraines, kame terraces, till‐mantled bedrock and outwash terraces, indicate that greatly expanded valley glaciers flowed into the lowland valley system at the mouths of the Cobb‐Takaka and Anatoki drainages. The timing for this ice advance into lowland valleys is constrained by lowland landform characteristics and a single cosmogenic exposure age, suggesting Late and Middle Pleistocene ice expansion, respectively. Evidence for expanded upland ice on the Mount Arthur Tableland and adjacent areas includes trimlines, boulder trains and roche moutonées. Two cosmogenic exposure ages on upland bedrock surfaces suggest that major ice expansion occurred during MIS 3 and/or 4, while previously published exposure dating from Cobb Valley suggests large MIS 2 ice expansion as well. The inferred, markedly expanded ice left little or no clear geomorphic imprint on the Cobb–Takaka Gorge, and required temperature depression of 4–6°C with near‐modern precipitation levels.  相似文献   

3.
We present a glaciological and climatic reconstruction of a former glacier in Coire Breac, an isolated cirque within the Eastern Grampian plateau of Scotland, 5 km from the Highland edge. Published glacier reconstructions of presumed Younger Dryas‐age glaciers in this area show that equilibrium line altitudes decreased steeply towards the east coast, implying a arctic maritime glacial environment. Extrapolation of the ELA trend surface implies that glaciers should have existed in suitable locations on the plateau, a landscape little modified by glaciation. In Coire Breac, a 0.35 km2 cirque glacier existed with an equilibrium line altitude of 487 ± 15 m above present sea level. The equilibrium line altitude matches closely the extrapolated regional equilibrium line altitude trend surface for Younger Dryas Stadial glaciers. The mean glacier thickness of 24 m gives an ice volume of 7.8 × 106 m3, and a maximum basal shear stress of c. 100 kPa?1. Ablation gradient was c. –0.0055 m m?1, with a mean July temperature at the equilibrium line altitude of c. 5.1°C. The reconstruction implies an arctic maritime climate of low precipitation with local accumulation enhanced by blown snow, which may explain the absence of other contemporary glaciers nearby. Reconstructed ice flow lines show zones of flow concentration around the lower ice margin which help to explain the distribution of depositional facies associated with a former debris cover which may have delayed eventual glacier retreat. No moraines in the area have been dated, so palaeoclimatic interpretations remain provisional, and a pre‐Lateglacial Interstadial age cannot be ruled out.  相似文献   

4.
Jansson and Glasser (Jansson, K.N., Glasser, N.F., 2008. Modification of peripheral mountain ranges by former ice sheets: the Brecon Beacons, southern UK. Geomorphology 97, 178–189.) have recently provided unconventional interpretations of selected glacial erosional and depositional landforms in the Brecon Beacons, UK, based on remotely sensed imagery. These new interpretations contradict well-established and reliable evidence for the origins and ages of certain glacial landforms of this upland area and elsewhere. They suggest that during a post-Last Glacial Maximum (LGM) ice-sheet event ice flowed up supposed, essentially “fluvial” valleys producing “glacial lineations” and depositing marginal moraines at the valley heads and on cirque floors. We argue that their interpretations of some key landforms are incorrect and that they have ignored much of the previous dating and field geomorphological evidence. Sedimentary and morphological evidence (e.g., lack of erratic content; convex planform with respect to the headwall; relatively large height range of moraines; and close association with headwall extent, height, and steepness) all indicate that higher level cirque-floor and valley-head moraines in the Brecon Beacons (> c. 400 m) were formed by cirque glaciers. Available dating evidence indicates a Younger Dryas age. We demonstrate that the supposed “fluvial” valleys, comprising trough heads with steep headwalls, have more nearly parabolic than V-shaped cross profiles indicating substantial glacial modification. Field evidence shows that proposed key exemplar post-LGM glacial lineations are in fact debris flow deposits. We conclude that whilst the adoption of a macroscale approach can shed new light on large-scale, ice-sheet movements, this approach should not be undertaken without consideration of the associated field evidence.  相似文献   

5.
Relict marginal moraines are commonly used landforms in palaeoglaciological reconstructions. In the Swedish mountains, a large number of relict marginal moraines of variable morphology and origin occur. In this study, we have mapped 234 relict marginal moraines distributed all along the Swedish mountains and classified them into four morphological classes: cirque‐and‐valley moraines, valley‐side moraines, complex moraines and cross‐valley moraines. Of these, 46 moraines have been reclassified or are here mapped for the first time. A vast majority of the relict moraines are shown to have formed during deglaciation of an ice‐sheet, rather than by local mountain glaciers as suggested in earlier studies. The relict marginal moraines generally indicate that deglaciation throughout the mountains was characterised by a retreating ice‐sheet, successively damming glacial lakes, and downwasting around mountains. The general lack of moraines indicating valley and cirque glaciers during deglaciation suggests that climatic conditions were unfavourable for local glaciation during the last phase of the Weichselian. This interpretation contrasts with some earlier studies that have reconstructed the formation of local glaciers in the higher parts of the Swedish mountains during deglaciation.  相似文献   

6.
A new lichen dating method and new moraine observations enabled us to improve the chronology of glacier advances in the Cordillera Blanca (Peru) during the Little Ice Age (LIA). Our results reveal that an early LIA glacial advance occurred around AD 1330 ± 29. However, a second major glacial advance at the beginning of the 17th century overlapped the earlier stage for most glaciers. Hence, this second glacial stage, dated from AD 1630 ± 27, is considered as the LIA maximum glacial advance in the Cordillera Blanca. During the 17th–18th centuries, at least three glacial advances were recorded synchronously for the different glaciers (AD 1670 ± 24, 1730 ± 21, and 1760 ± 19). The moraines corresponding to the two first stages are close to the one in 1630 suggesting a slow recession of about 18% in the total length of the glacier. From the LIA maximum extent to the beginning of the 20th century, the 24 glaciers have retreated a distance of about 1000 m, corresponding to a reduction of 30% in their length. This rate is comparable to that observed during the 20th century. Estimates of palaeo-Equilibrium Line Altitudes show an increase in altitude of about 100 m from the LIA maximum glacial extension at the beginning of the 17th century to the beginning of the 20th century. Because long time series are not available for precipitation and temperature, this glacial retreat is difficult to explain by past climate changes. However, there is a fair correspondence between changes in glacier length and the δ18O recorded in the Quelccaya ice core at a century timescale. Our current knowledge of tropical glaciers and isotope variations leads us to suggest that this common tropical signal reflects a change from a wet LIA to the drier conditions of today. Finally, a remarkable synchronicity is observed with glacial variations in Bolivia, suggesting a common regional climatic pattern during the LIA.  相似文献   

7.
The arctic islands of the Lofoten-Vesterålen archipelago in northern Norway have a wide distribution of weathered land surfaces commonly located above 250 m with several apparent similarities. In order to investigate the characteristics of (deep) weathering in this region, northern Langøya and Hadseløya were chosen for in-depth analyses. Eight weathering profiles were excavated from various surfaces, and the stratigraphies were logged in detail. Material was collected throughout the weathering horizons, and all samples were subsequently analysed for clay mineralogy (< 63 μm fraction) and grain size distribution. The sampling strategy was complemented by samples from additional saprolites and other landforms such as moraines and rock glaciers. The XRD results indicate that the presence of secondary minerals, such as gibbsite (Al(OH)3) and kaolinite (Al2Si2O5(OH)4), are very common throughout the profiles. Gibbsite is an extreme end product of silicate weathering and usually associated with a warmer and more humid climate, as found in Scandinavia during the Tertiary. The grain size analyses (< 63 μm) show that the finer silt fractions (< 8 μm) tend to be high in the profiles (20–40%), with significant amounts of clay (5–15%) demonstrating that the regolith itself is susceptible to frost sorting mechanisms.10Be exposure dates from in situ quartz knobs on tors and boulders of local origin suggest > 40,000 years of subaerial conditions. Considering the steady surface erosion, this figure should be viewed as an absolute minimum age estimate. Mapping of the superficial sediments and geomorphological features of the study areas has revealed several common morphological features, which indicate dominance of glacial and periglacial processes in the areas lying below the lower boundary of blockfields (c. 250 m). The weathering mantles are not a periglacial end product, but rather a relict tertiary landform that were modulated by permafrost processes as well as biological processes at later stages. The regolith cover constrain the vertical extension of warm-based Quaternary ice sheets challenging the notion of a parabolic ice mass consuming every mountain top of Lofoten and Vesterålen.  相似文献   

8.
Widespread till and moraines record excursions of middle-Pleistocene ice that flowed up-slope into several watersheds of the Valley and Ridge Province along the West Branch of the Susquehanna River. A unique landform assemblage was created by ice-damming and jökulhlaups emanating from high gradient mountain watersheds. This combination of topography formed by multiple eastward-plunging anticlinal ridges, and the upvalley advance of glaciers resulted in an ideal geomorphic condition for the formation of temporary ice-dammed lakes. Extensive low gradient (1°–2° slope) gravel surfaces dominate the mountain front geomorphology in this region and defy simple explanation. The geomorphic circumstances that occurred in tributaries to the West Branch Susquehanna River during middle Pleistocene glaciation are extremely rare and may be unique in the world. Failure of ice dams released sediment-rich water from lakes, entraining cobbles and boulders, and depositing them in elongated debris fans extending up to 9 km downstream from their mountain-front breakout points. Poorly developed imbrication is rare, but occasionally present in matrix-supported sediments resembling debris flow deposits. Clast weathering and soils are consistent with a middle Pleistocene age for the most recent flows, circa the 880-ka paleomagnetic date for glacial lake sediments north of the region on the West Branch Susquehanna River. Post-glacial stream incision has focused along the margins of fan surfaces, resulting in topographic inversion, leaving bouldery jökulhlaup surfaces up to 15 m above Holocene channels. Because of their coarse nature and high water tables, jökulhlaup surfaces are generally forested in contrast to agricultural land use in the valleys and, thus, are readily apparent from orbital imagery.  相似文献   

9.
The easternmost extremity of the ice cap that developed in the Tasmanian Central Highlands during the time of most extensive Late Cainozoic glaciation lay on the doleritecapped Central Plateau east and north-east of Lake St Clair. During the Last Glacial Maximum (LGM), the more restricted ice cover included a small discrete ice cap (probably less than 250-300 m thick) that formed on the Central Plateau. The LGM ice limits on the southern part of the Central Plateau, including all five southern outlet valleys, are reported here. Earlier ice limits have been identified in two of these valleys, but on the plateau proper earlier glacial deposits have been generally extensively reworked beyond the LGM limit, such that confirmation of a glacial origin for diamictons on slopes is difficult. South of the plateau, the oldest deposits flooring lower reaches of two outlet valleys indicate that ice flowed southwards directly from the plateau, but later deposits indicate diffluent flow from the Derwent Glacier.  相似文献   

10.
The geomorphology of Heard Island-McDonald Island is primarily the product of close interplay between volcanism, glaciation, and vigorous marine processes in a stormy sub-Antarctic environment. The dominant landform is the strato-volcano Big Ben (2745m), which is the highest mountain on Australian territory outside Antarctica. Other volcanic landforms include scoria cones, domes, open vertical volcanic conduits, lava flows and lava tubes. Volcanic activity is ongoing from the summit of Big Ben, and from Samarang Hill on McDonald Island. Early, but unproven, glacial sediments may exist within the Late Miocene - Early Pliocene Drygalski Formation, which forms a 300m high plateau along the northern coast of Heard Island. Growth of the present glaciers, some of which reach sea level, has been a response to progressive growth of the volcanoes. A variety of erosional and depositional glacial landforms is present, including major lateral moraines and extensive hummocky moraines. Vigorous longshore drift and an abundant sediment supply have produced a large spit at the downdrift end of the island, and formed bars from reworked glacigenic sediment that now impound proglacial estuarine lagoons, some of which have grown rapidly over recent decades as tidewater glaciers have retreated. Integrated study of the volcanic, glacial and coastal sequences offers the possibility of constructing a well-dated record of climate change. Research into the geomorphology, surficial sediments, and contemporary geomorphological processes, including glaciofluvial sediment flux, is also important as an aid to environmental management on land, and to management of the adjacent marine environment.  相似文献   

11.
The Marinoan glaciation (Neoproterozoic) in northeast Svalbard   总被引：4，自引：0，他引：4  
Two separate and distinct diamictite‐rich units occur in the mixed carbonate‐siliciclastic Polarisbreen Group, which comprises the top kilometer of >7 km of Neoproterozoic strata in the northeast of the Svalbard archipelago. The platformal succession accumulated on the windward, tropical to subtropical margin of Laurentia. The older Petrovbreen Member is a thin glacimarine diamictite that lacks a cap carbonate. It contains locally derived clasts and overlies a regional karstic disconformity that was directly preceded by a large (>10‰) negative δ13C anomaly in the underlying shallow‐marine carbonates. This anomaly is homologous to anomalies in Australia, Canada and Namibia that precede the Marinoan glaciation. The younger and thicker Wilsonbreen Formation comprises terrestrial ice‐contact deposits. It contains abundant extrabasinal clasts and is draped by a transgressive cap dolostone 3–18 m thick. The cap dolostone is replete with sedimentary features strongly associated with post‐Marinoan caps globally, and its isotopic profile is virtually identical to that of other Marinoan cap dolostones. From the inter‐regional perspective, the two diamictite‐rich units in the Polarisbreen Group should represent the first and final phases of the Marinoan glaciation. Above the Petrovbreen diamictite are ～200 m of finely laminated, dark olive‐coloured rhythmites (MacDonaldryggen Member) interpreted here to represent suspension deposits beneath shorefast, multi‐annual sea ice (sikussak). Above the suspension deposits and below the Wilsonbreen diamictites is a <30‐m‐thick regressive sequence (Slangen Member) composed of dolomite grainstone and evaporitic supratidal microbialaminite. We interpret this sabkha‐like lagoonal sequence as an oasis deposit that precipitated when local marine ice melted away under greenhouse forcing, but while the tropical ocean remained covered due to inflow of sea glaciers from higher latitudes. It appears that the Polarisbreen Group presents an unusually complete record of the Marinoan snowball glaciation.  相似文献   

12.
  相似文献   

13.
Effects of sample size on the accuracy of geomorphological models   总被引：1，自引：1，他引：0  
Commonly, the most costly part of geomorphological distribution modelling studies is gathering the data. Thus, guidance for researchers concerning the quantity of field data needed would be extremely practical. This paper scrutinises the relationship between the sample size (the number of observations varied from 20 to 600) and the predictive ability of the generalized linear model (GLM), generalized additive model (GAM), generalized boosting method (GBM) and artificial neural network (ANN) in two data settings, i.e., independent and split-sample approaches. The study was performed using empirical data of periglacial processes from an area of 600 km2 in northernmost Finland at grid resolutions of 1 ha (100 × 100 m) and 25 ha (500 × 500 m). A rather sharp increase in the predictive ability of the models was observed when the number of observations increased from 20 to 100, and the level of robust predictions was reached with 200 observations. The result indicates that no more than a few hundred observations are needed in geomorphological distribution modelling at a medium scale resolution (ca. 0.01–1 km2).  相似文献   

14.
Glaciers are the most important fresh-water resources in arid and semi-arid regions of western China. According to the Second Chinese Glacier Inventory (SCGI), primarily compiled from Landsat TM/ETM+ images, the Qilian Mountains had 2684 glaciers covering an area of 1597.81±70.30 km2 and an ice volume of ~84.48 km3 from 2005 to 2010. While most glaciers are small (85.66% are <1.0 km2), some larger ones (12.74% in the range 1.0–5.0 km2) cover 42.44% of the total glacier area. The Laohugou Glacier No.12 (20.42 km2) located on the north slope of the Daxue Range is the only glacier >20 km2 in the Qilian Mountains. Median glacier elevation was 4972.7 m and gradually increased from east to west. Glaciers in the Qilian Mountains are distributed in Gansu and Qinghai provinces, which have 1492 glaciers (760.96 km2) and 1192 glaciers (836.85 km2), respectively. The Shule River basin contains the most glaciers in both area and volume. However, the Heihe River, the second largest inland river in China, has the minimum average glacier area. A comparison of glaciers from the SCGI and revised glacier inventory based on topographic maps and aerial photos taken from 1956 to 1983 indicate that all glaciers have receded, which is consistent with other mountain and plateau areas in western China. In the past half-century, the area and volume of glaciers decreased by 420.81 km2 (–20.88%) and 21.63 km3 (–20.26%), respectively. Glaciers with areas <1.0 km2 decreased the most in number and area recession. Due to glacier shrinkage, glaciers below 4000 m completely disappeared. Glacier changes in the Qilian Mountains presented a clear longitudinal zonality, i.e., the glaciers rapidly shrank in the east but slowly in the central-west. The primary cause of glacier recession was warming temperatures, which was slightly mitigated with increased precipitation.  相似文献   

15.
Glacial geomorphologic features composed of (or cut into) Llanquihue drift delineate former Andean piedmont glaciers in the region of the southern Chilean Lake District, Seno Reloncav', Golfo de Ancud, and northern Golfo Corcovado during the last glaciation. These landforms include extensive moraine belts, main and subsidiary outwash plains, kame terraces, and meltwater spillways. Numerous radiocarbon dates document Andean ice advances into the moraine belts during the last glacial maximum (LGM) at 29,363–29,385 14C yr BP , 26,797 14C yr BP , 22,295–22,570 14C yr BP , and 14,805–14,869 14C yr BP . Advances may also have culminated at close to 21,000 14C yr BP , shortly before 17,800 14C yr BP , and shortly before 15,730 14C yr BP . The maximum at 22,295–22,567 14C yr BP was probably the most extensive of the LGM in the northern part of the field area, whereas that at 14,805–14,869 14C yr BP was the most extensive in the southern part. Snowline depression during these maxima was about 1000 m. Andean piedmont glaciers did not advance into the outer Llanquihue moraine belts during the portion of middle Llanquihue time between 29,385 14C yr BP and more than 39,660 14C yr BP . In the southern part of the field area, the Golfo de Ancud lobe, as well as the Golfo Corcovado lobe, achieved a maximum at the outermost Llanquihue moraine prior to 49,892 14C yr BP . Pollen analysis of the Taiquemmire, which is located on this moraine, suggests that the old Llanquihue advance probably corresponds to the time of marine isotope stage 4. The implication is that the Andean snowline was then depressed as much as during the LGM. A Llanquihue-age glacier expansion into the outer moraine belts also occurred more than about 40,000 14C yr BP for the Lago Llanquihue piedmont glacier.  相似文献   

16.
The Basin of Ubaté–Chichinquirá (5°28′N, 73°45′ W, c. 2580 m altitude) includes the Fúquene Valley and is located in the central part of the Eastern Cordillera of Colombia. Rocks and sediments were folded and faulted during the Miocene, uplifted during the (late) Pliocene, and affected by glaciers during the Pleistocene. Successive glacial and interglacial periods left significant marks in the landscape which were used to reconstruct six stages in the development of the landscape along a relative chronology. During early Pleistocene episode 1 glaciers formed U-shape valleys. Evidence of the impact of ice sheets has been found as far downslope as ca. 2900 m elevation. During episode 2 moraines developed which were cut by the present San José River. During episode 3 abundant sediment was produced by glacial erosion. It accentuated the sculpturing of hard rock and deepening of the drainage basin. The ancestral Ubaté–Suarez River constituted a dynamic erosive system that gave rise to deep V-shaped valleys and progressively formed a set of intricate valleys with a high sediment production. Finally, intense glacial and fluvio-glacial erosion led to a geomorphological system with high energy levels and intensive sediment transport leading to wide valleys. During episode 4 the Ubaté–Suarez River eroded and deepened its valley until it captured the old El Hato–San José Valley. It caused intense erosion of the moraine and the fluvio-glacial gravels. Deep V-shaped valleys stabilized in the high areas of the main drainage system and these valleys form the present-day fluvial sub-basins. During episode 5 the deep valley in the northern part of the Basin of Ubaté–Chichinquirá developed. During middle Pleistocene episode 6 colluvial sediments formed the Saboya dam and a lake was formed in the river valley of which the present Lake Fúquene is only a small remnant. Lithological changes indicate fluctuating water levels and Lake Fúquene must have expanded periodically up to an area 5 to 10 times the present-day surface.  相似文献   

17.
Little Ice Age (LIA) moraines along the margins of Skálafellsjökull and Heinabergsjökull, two neighbouring outlet glaciers flowing from the Vatnajökull ice‐cap, have been re‐dated to test the reliability of different lichenometric approaches. During 2003, 12 000 lichens were measured on 40 moraine fragments at Skálafellsjökull and Heinabergsjökull to provide surface age proxies. The results are revealing. Depending on the chosen method of analysis, Skálafellsjökull either reached its LIA maximum in the early 19th century (population gradient) or the late 19th century (average of five largest lichens), whereas the LIA maximum of Heinabergsjökull occurred by the mid‐19th century (population gradient) or late‐19th century (average of 5 largest lichens). Discrepancies (c. 80 years for Skálafellsjökull and c. 40 years for Heinabergsjökull) suggest that the previously cited AD 1887 LIA maxima for both glaciers should be reassessed. Dates predicted by the lichen population gradient method appear to be the most appropriate, as mounting evidence from other geochronological reconstructions and sea‐ice records throughout Iceland tends to support an earlier LIA glacier maximum (late 18th to mid‐19th century) and probably reflects changes in the North Atlantic Oscillation. These revised chronologies shed further light on the precise timing of the Icelandic LIA glacier maximum, whilst improving our understanding of glacier‐climate interactions in the North Atlantic.  相似文献   

18.
Landslides in blanket peat on Cuilcagh Mountain, northwest Ireland   总被引：1，自引：0，他引：1  
The northern and eastern sides of the Cuilcagh Mountain upland, in northwest Ireland, are mantled with over 50 km2 of blanket bog that has experienced an unusually high spatial and temporal frequency of peat mass movements. In all, 29 peaty-debris slides, nine bog slides, two peat slides and five more peat landslides of uncertain type have been recorded within this study area. More than 27 km2 of this peatland has been afforded several levels of statutory protection as well as international recognition of its geo-environmental importance. Field and laboratory investigations of the peat at several of the more recent failure sites showed it to be typical of Irish and Pennine (northern England) blanket bogs in most physical and hydrological respects. Field geomorphological evidence and modelling of stability thresholds indicate that the particular susceptibility of the Cuilcagh Mountain blanket bog to failure arises from two local factors: (i) the attainment of threshold maximum peat depths on the East Cuilcagh plateau, and (ii) the unconformable deposition of thin layers of glacial till (in places) and blanket peat over the pre-existing topographic surface formed from the major shale formations that underlie the northern slopes. With two exceptions, there is no conclusive evidence that human activities and management strategies for the area have had any significant influence on the occurrence of the peat landslides. The high frequency of large rainfall events since 1961 that did not trigger landslides suggests that failures are unlikely to become more frequent in response to climate change effects because they are controlled by slowly changing internal thresholds.  相似文献   

19.
Large debris-flow units commonly occur on the distal sides of subaqueous end moraines deposited by surges of Svalbard tidewater glaciers, but have rarely been described in terrestrial settings. Some researchers have argued that these kinds of debris flows reflect processes unique to the subaqueous environment, such as the extrusion of subglacial deforming layers or extensive failure of oversteepened moraine fronts. In this paper, we describe terrestrial and subaqueous parts of a single late Holocene moraine system deposited by a major surge of the tidewater glacier Paulabreen in west Spitsbergen. The ice-marginal landforms on land closely resemble the corresponding landforms on the seabed as evidenced by geomorphic mapping and geophysical profiles from both environments. Both onland and offshore, extensive areas of hummocky moraine occur on the proximal side of the maximum glacier position, and large mud aprons (interpreted as debris flows) occur on the distal side. We show that the debris-flow sediments were pushed in front of the advancing glacier as a continuously failing, mobile push moraine. We propose that the mud aprons are end members of a proglacial landforms continuum that has thrust-block moraines as the opposite end member. Two clusters of dates (~ 8000 YBP and ~ 700 YBP) have previously been interpreted to indicate two separate surges responsible for the moraine formation. New dates suggest that the early cluster indicates a local extinction of the abounded species Chlamys islandica. Other changes corresponding to the widespread 8.2 ka event within the fjord, may suggest that the extinction of the C. islandica corresponds to that time.  相似文献   

20.
In southern South America, the maximum areal extent of ice during the Quaternary Period, the Greatest Patagonian Glaciation (GPG, [Mercer, J.H., 1983. Cenozoic glaciation in the southern hemisphere. Annual Reviews of Earth and Planetary Science 11, 99–132.]), occurred at 1.1 Ma and subsequent glaciations were overall less extensive. The GPG preceded global minimum temperatures and maximal volume of ice, which occurred in the last ~ 800 kyr, as recorded in the marine δ18O record. Significant modification of the drainage morphology of the southern Andes from a non-glaciated to glaciated landscape occurred throughout the Quaternary Period. We infer a non-climatic relationship between glacial modification of the mountains and the decreasing extent of ice and we discuss processes of landscape development that could have caused the trend. Specifically, these include modification of valleys, such as development from a V- to a U-shape, and lowering of mass-accumulation areas. Such changes would strongly affect glacial dynamics, the mass balance profile and mass-flux during succeeding glaciations, especially for low-gradient outlet glaciers occupying low areas. Other areas around Earth (at least where ice has been warm-based) also may exhibit a non-random trend of decreasing extent of ice with time, ultimately because of glacial erosion in the Quaternary Period.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号