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 46 毫秒
1.
Glacier and permafrost hazards such as glacial‐lake outburst floods and rock–ice avalanches cause significant socio‐economic damages worldwide, and these processes may increase in frequency and magnitude if the atmospheric temperature rises. In the extratropical Andes nearly 200 human deaths were linked to these processes during the twentieth century. We analysed bibliographical sources and satellite images to document the glacier and permafrost dynamics that have caused socio‐economic damages in this region in historic time (including glacial lake outburst floods, ice and rock–ice avalanches and lahars) to unravel their causes and geomorphological impacts. In the extratropical Andes, at least 15 ice‐dammed lakes and 16 moraine‐dammed lakes have failed since the eighteenth century, causing dozens of floods. Some floods rank amongst the largest events ever recorded (5000 × 106 m3 and 229 × 106 m3, respectively). Outburst flood frequency has increased in the last three decades, partially as a consequence of long‐term (decades to centuries) climatic changes, glaciers shrinkage, and lake growth. Short‐term (days to weeks) meteorological conditions (i.e. intense and/or prolonged rainfall and high temperature that increased meltwater production) have also triggered outburst floods and mass movements. Enormous mass failures of glaciers and permafrost (> 10 × 106 m3) have impacted lakes, glaciers, and snow‐covered valleys, initiating chain reactions that have ultimately resulted in lake tsunamis and far‐reaching (> 50 km) flows. The eruption of ice‐covered volcanoes has also caused dozens of damaging lahars with volumes up to 45 × 106 m3. Despite the importance of these events, basic information about their occurrence (e.g. date, causes, and geomorphological impact), which is well established in other mountain ranges, is absent in the extratropical Andes. A better knowledge of the processes involved can help to forecast and mitigate these events. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

2.
Bulk runoff and meteorological data suggest the occurrence of two meltwater outburst events at Finsterwalderbreen, Svalbard, during the 1995 and 1999 melt seasons. Increased bulk meltwater concentrations of Cl? during the outbursts indicate the release of snowmelt from storage. Bulk meltwater hydrochemical data and suspended sediment concentrations suggest that this snowmelt accessed a chemical weathering environment characterized by high rock:water ratios and long rock–water contact times. This is consistent with a subglacial origin. The trigger for both the 1995 and 1999 outbursts is believed to be high rates of surface meltwater production and the oversupply of meltwater to areas of the glacier bed that were at the pressure melting point, but which were unconnected to the main subglacial drainage network. An increase in subglacial water pressure to above the overburden pressure lead to the forcing of a hydrological connection between the expanding subglacial reservoir and the ice‐marginal channelized system. The purging of ice blocks from the glacier during the outbursts may indicate the breach of an ice dam during connection. Although subglacial meltwater issued continually from the glacier terminus via a subglacial upwelling during both melt seasons, field observations showed outburst meltwaters were released solely via an ice‐marginal channel. It is possible that outburst events are a seasonal phenomenon at this glacier and reflect the periodic drainage of meltwaters from the same subglacial reservoir from year to year. However, the location of this reservoir is uncertain. A 100 m high bedrock ridge traverses the glacier 6·5 km from its terminus. The overdeepened area up‐glacier from this is the most probable site for subglacial meltwater accumulation. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

3.
The ongoing debate over the effects of global environmental change on Earth's cryosphere calls for detailed knowledge about process rates and their variability in cold environments. In this context, appraisals of the coupling between glacier dynamics and para‐glacial erosion rates in tectonically active mountains remain rare. We contribute to filling this knowledge gap and present an unprecedented regional‐scale inventory of supra‐glacial sediment flux and hillslope erosion rates inferred from an analysis of 123 large (> 0·1 km2) catastrophic bedrock landslides that fell onto glaciers in the Chugach Mountains, Alaska, as documented by satellite images obtained between 1972 to 2008. Assuming these supra‐glacial landslide deposits to be passive strain markers we infer minimum decadal‐scale sediment yields of 190 to 7400 t km–2 yr–1 for a given glacier‐surface cross‐section impacted by episodic rock–slope failure. These rates compare to reported fluvial sediment yields in many mountain rivers, but are an order of magnitude below the extreme sediment yields measured at the snouts of Alaskan glaciers, indicating that the bulk of debris discharged derives from en‐glacial, sub‐glacial or ice‐proximal sources. We estimate an average minimum para‐glacial erosion rate by large, episodic rock–slope failures at 0·5–0·7 mm yr–1 in the Chugach Mountains over a 50‐yr period, with earthquakes likely being responsible for up to 73% of this rate. Though ranking amongst the highest decadal landslide erosion rates for this size of study area worldwide, our inferred rates of hillslope erosion in the Chugach Mountains remain an order of magnitude below the pace of extremely rapid glacial sediment export and glacio‐isostatic surface uplift previously reported from the region. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

4.
Glacier recession and landform development in a debris‐charged glacial landsystem characterized by an overdeepening is quantified using digital photogrammetry, digital elevation model (DEM) construction and mapping of the Icelandic glacier Kvíárjökull for the period 1945–2003. Melting of ice‐cores is recorded by surface lowering rates of 0·8 m yr–1 (1945–1964), 0·3 m yr–1 (1964–1980), 0·015 m yr–1 (1980–1998) and 0·044 m yr–1 (1998–2003). The distribution/preservation of pushed and stacked ice‐cored moraine complexes are determined by the location of the long‐term glacial drainage network in combination with retreat from the overdeepening, into which glacifluvial sediment is being directed and where debris‐rich ice masses are being reworked and replaced by esker networks produced in englacial meltwater pathways that bypassed the overdeepening and connected to outwash fans prograding over the snout. Recent accelerated retreat of Kvíárjökull, potentially due to increased mass balance sensitivity, has made the snout highly unstable, especially now that the overdeepening is being uncovered and the snout flooded by an expanding pro‐glacial, and partially supraglacial, lake. This case study indicates that thick sequences of debris‐charged basal ice/controlled moraine have a very low preservation potential but ice‐cored moraine complexes can develop into hummocky moraine belts in de‐glaciated terrains because they are related to the process of incremental stagnation, which at Kvíárjökull has involved periodic switches from transport‐dominant to ablation‐dominant conditions. Glacier recession is therefore recorded temporally and spatially by two suites of landforms relating to two phases of landform production which are likely typical for glaciers occupying overdeepenings: an early phase of active, temperate recession recorded by push moraines and lateral moraines and unconfined pro‐glacial meltwater drainage; and a later phase of incremental stagnation and pitted outwash head development initiated by the increasing topographic constraints of the latero‐frontal moraine arc and the increasing importance of the overdeepening as a depo‐centre. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

5.
Glacial erosion rates are estimated to be among the highest in the world. Few studies have attempted, however, to quantify the flux of sediment from the periglacial landscape to a glacier. Here, erosion rates from the nonglacial landscape above the Matanuska Glacier, Alaska are presented and compare with an 8‐yr record of proglacial suspended sediment yield. Non‐glacial lowering rates range from 1·8 ± 0·5 mm yr?1 to 8·5 ± 3·4 mm yr?1 from estimates of rock fall and debris‐flow fan volumes. An average erosion rate of 0·08 ± 0·04 mm yr?1 from eight convex‐up ridge crests was determined using in situ produced cosmogenic 10Be. Extrapolating these rates, based on landscape morphometry, to the Matanuska basin (58% ice‐cover), it was found that nonglacial processes account for an annual sediment flux of 2·3 ± 1·0 × 106 t. Suspended sediment data for 8 years and an assumed bedload to estimate the annual sediment yield at the Matanuska terminus to be 2·9 ± 1·0 × 106 t, corresponding to an erosion rate of 1·8 ± 0·6 mm yr?1: nonglacial sources therefore account for 80 ± 45% of the proglacial yield. A similar set of analyses were used for a small tributary sub‐basin (32% ice‐cover) to determine an erosion rate of 12·1 ± 6·9 mm yr?1, based on proglacial sediment yield, with the nonglacial sediment flux equal to 10 ± 7% of the proglacial yield. It is suggested that erosion rates by nonglacial processes are similar to inferred subglacial rates, such that the ice‐free regions of a glaciated landscape contribute significantly to the glacial sediment budget. The similar magnitude of nonglacial and glacial rates implies that partially glaciated landscapes will respond rapidly to changes in climate and base level through a rapid nonglacial response to glacially driven incision. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

6.
Pro‐glacial landscapes are some of the most active on Earth. Previous studies of pro‐glacial landscape change have often been restricted to considering either sedimentological, geomorphological or topographic parameters in isolation and are often mono‐dimensional. This study utilized field surveys and digital elevation model (DEM) analyses to quantify planform, elevation and volumetric pro‐glacial landscape change at Sólheimajökull in southern Iceland for multiple time periods spanning from 1960 to 2010. As expected, the most intense geomorphological changes persistently occurred in the ice‐proximal area. During 1960 to 1996 the pro‐glacial river was relatively stable. However, after 2001 braiding intensity was higher, channel slope shallower and there was a shift from overall incision to aggradation. Attributing these pro‐glacial river channel changes to the 1999 jökulhlaup is ambiguous because it coincided with a switch from a period of glacier advance to that of glacier retreat. Furthermore, glacier retreat (of ~40 m yr?1) coincided with ice‐marginal lake development and these two factors have both altered the pro‐glacial river channel head elevation. From 2001 to 2010 progressive increase in channel braiding and progressive downstream incision occurred; these together probably reflecting stream power due to increased glacier ablation and reduced sediment supply due to trapping of sediment by the developing ice‐marginal lake. Overall, this study highlights rapid spatiotemporal pro‐glacial landscape reactions to changes in glacial meltwater runoff regimes, glacier terminus position, sediment supply and episodic events such as jökuhlaups. Recognizing the interplay of these controlling factors on pro‐glacial landscapes will be important for understanding the geological record and for landscape stability assessments. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

7.
There has been little work to date into the controls on slope‐to‐channel fine sediment connectivity in alpine environments largely ice‐free for most of the Holocene. Characterization of these controls can be expected to result in better understanding of how landscapes “relax” from such perturbations as climate shock. We monitored fine sediment mobilization on a slope segment hydrologically connected to a stream in the largely ice‐free 8·3 km2 Hoophorn Valley, New Zealand. Gerlach traps were installed in ephemeral slope channels to trap surficial material mobilized during rainfall events. Channel sediment flux was measured using turbidimeters above and below the connected slope, and hysteresis patterns in discharge‐suspended sediment concentrations were used to determine sediment sources. Over the 96 day measurement period, sediment mobilization from the slope segment was limited to rainfall events, with increasingly larger particles trapped as event magnitude increased. Less than 1% of the mass of particles collected during these events was fine sediment. During this period, 714 t of suspended sediment was transported through the lower gauging station, 60% of it during rainfall events. Channel sediment transfer patterns during these events were dominated by clockwise hysteresis, interpreted as remobilization of nearby in‐channel sources, further suggesting limited input of fine sediment from slopes in the lower valley. Strong counterclockwise hysteresis, representing input of fine sediment from slope segments, was restricted to the largest storm event (JD2 2009) when surfaces in the upper basin were activated. The results indicate that the slopes of the lower Hoophorn catchment are no longer functioning as sources of fine sediment, but rather as sources of coarse material, with flux rates controlled by the intensity and duration of rainfall events. Although speculative, these findings suggest a shift to a coarse sediment dominated slope‐to‐channel transfer system as the influence of pre‐Holocene glacial erosion declines. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

8.
There are still relatively few hydrochemical studies of glacial runoff and meltwater routing from the high latitudes, where non-temperate glacier ice is frequently encountered. Representative samples of glacier meltwater were obtained from Scott Turnerbreen, a ‘cold-based’ glacier at 78° N in the Norwegian high Arctic archipelago of Svalbard, during the 1993 melt season and analysed for major ion chemistry. Laboratory dissolution experiments were also conducted, using suspended sediment from the runoff. Significant concentrations of crustal weathering derived SO2−4 are present in the runoff, which is characterized by high ratios of SO2−4: (SO2−4+HCO−3) and high p(CO2). Meltwater is not routed subglacially, but flows to the glacier terminus through subaerial, ice marginal channels, and partly flows through a proglacial icing, containing highly concentrated interstitial waters, immediately afront the terminus. The hydrochemistry of the runoff is controlled by: (1) seasonal variations in the input of solutes from snow- and icemelt; (2) proglacial solute acquisition from the icing; and (3) subaerial chemical weathering within saturated, ice-cored lateral moraine adjoining drainage channels at the glacier margins, sediment and concentrated pore water from which is entrained by flowing meltwater. Diurnal variations in solute concentration arise from the net effects of variable sediment pore water entrainment and dilution in the ice marginal streams. Explanation of the hydrochemistry of Scott Turnerbreen requires only one major subaerial flow path, the ice marginal channel system, in which seasonally varying inputs of concentrated snowmelt and dilute icemelt are modified by seepage or entrainment of concentrated pore waters from sediment in lateral moraine, and by concentrated interstitial waters from the proglacial icing, supplied by leaching, slow drainage at grain intersections or simple melting of the icing itself. The ice marginal channels are analogous neither to dilute supra/englacial nor to concentrated subglacial flow components. © 1998 John Wiley & Sons, Ltd.  相似文献   

9.
Inner gorges often result from the propagation of erosional waves related to glacial/interglacial climate shifts. However, only few studies have quantified the modern erosional response to this glacial conditioning. Here, we report in situ 10Be data from the 64 km2 Entlen catchment (Swiss Alps). This basin hosts a 7 km long central inner gorge with two tributaries that are >100 m‐deeply incised into thick glacial till and bedrock. The 10Be concentrations measured at the downstream end of the gorge yield a catchment‐wide erosion rate of 0.42 ± 0.04 mm yr‐1, while erosion rates are consistently lower upstream of the inner gorge, ranging from 0.14 ± 0.01 mm yr‐1 to 0.23 ± 0.02 mm yr‐1. However, 10Be‐based sediment budget calculations yield rates of ~1.3 mm yr‐1 for the inner gorge of the trunk stream. Likewise, in the two incised tributary reaches, erosion rates are ~2.0 mm yr‐1 and ~1.9 mm yr‐1. Moreover, at the erosional front of the gorge, we measured bedrock incision rates ranging from ~2.5 mm yr‐1 to ~3.8 mm yr‐1. These rates, however, are too low to infer a post‐glacial age (15–20 ka) for the gorge initiation. This would require erosion rates that are between 2 and 6 times higher than present‐day estimates. However, the downcutting into unconsolidated glacial till favored high erosion rates through knickzone propagation immediately after the retreat of the LGM glaciers, and subsequent hillslope relaxation led to a progressive decrease in erosion rates. This hypothesis of a two‐ to sixfold decrease in erosion rates does not conflict with the 10Be‐based erosion rate budgets, because the modern erosional time scale recorded by 10Be cover the past 2–3 ka only. These results point to the acceleration of Holocene erosion in response to the glacial overprint of the landscape. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

10.
The paraglacial reworking of glacial sediments by rivers and mass wasting is an important conditioning factor for modern sediment yields in mountainous catchments in formerly glaciated regions. Catchment scale and patterns of sediment storage are important influences in the rate of postglacial adjustment. We develop a quantitative framework to estimate the volume, sediment type, and fractional size distribution of legacy glacial materials in a large (1230 km2) watershed in the North Cascade Mountains in south‐western British Columbia, Canada. Chilliwack Valley is exceptional because of the well‐dated bounds of deglaciation. Interpolation of paleo‐surfaces from partially eroded deposits in the valley allows us to estimate the total evacuated sediment volume. We present a chronology of sediment evacuation from the valley and deposition in the outlet fan, based on infrared stimulated luminescence (IRSL) and 14 C dating of river terraces and fan strata, respectively. The effects of paraglacial sedimentation in Chilliwack Valley were intensified through a major fall in valley base‐level following ice retreat. The steepened mainstem valley gradient led to deep incision of valley fills and fan deposits in the lower valley network. The results of this integrated study provide a postglacial chronology and detailed sediment budget, accounting for long‐term sorting of the original sediments, lag deposit formation in the mainstem, deposition in the outlet fan, and approximate downstream losses of suspended sediment and wash load. The mass balance indicates that a bulk volume of approximately 3.2 km3 of glacial material has been evacuated from the valley. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

11.
Research over the last decade has shown that the suspended sediment loads of many rivers are dominated by composite particles. These particles are also known as aggregates or flocs, and are commonly made up of constituent mineral particles, which evidence a wide range of grain sizes, and organic matter. The resulting in situ or effective particle size characteristics of fluvial suspended sediment exert a major control on all processes of entrainment, transport and deposition. The significance of composite suspended sediment particles in glacial meltwater streams has, however, not been established. Existing data on the particle size characteristics of suspended sediment in glacial meltwaters relate to the dispersed mineral fraction (absolute particle size), which, for certain size fractions, may bear little relationship to the effective or in situ distribution. Existing understanding of composite particle formation within freshwater environments would suggest that in‐stream flocculation processes do not take place in glacial meltwater systems because of the absence of organic binding agents. However, we report preliminary scanning electron microscopy data for one Alpine and two Himalayan glaciers that show composite particles are present in the suspended sediment load of the meltwater system. The genesis and structure of these composite particles and their constituent grain size characteristics are discussed. We present evidence for the existence of both aggregates, or composite particles whose features are largely inherited from source materials, and flocs, which represent composite particles produced by in‐stream flocculation processes. In the absence of organic materials, the latter may result solely from electrochemical flocculation in the meltwater sediment system. This type of floc formation has not been reported previously in the freshwater fluvial environment. Further work is needed to test the wider significance of these data and to investigate the effective particle size characteristics of suspended sediment associated with high concentration outburst events. Such events make a major contribution to suspended sediment fluxes in meltwater streams and may provide conditions that are conducive to composite particle formation by flocculation. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

12.
One of the key issues associated with the hypothesis of catastrophic subglacial drainage of the Livingstone Lake event is whether flows of such large magnitudes are physically feasible. To explore this issue, a one‐dimensional hydraulic network flow model was developed to investigate the range of peak discharges and associated flow parameters that may have been carried by a tunnel channel network in south‐east Alberta, Canada. This tunnel channel network has been interpreted elsewhere to carry large discharges associated with subglacial meltwater flows because of the convex longitudinal profiles of individual channels. This computational modelling effort draws upon established and verified engineering principles and methods in its application to the hydraulics of this problem. Consequently, it represents a unique and independent approach to investigating the subglacial meltwater hypothesis. Based on the modelling results, it was determined that energy losses resulting from friction limit the maximum peak discharge that can be transported through the tunnel channel network to 107 m3 s−1, which is in reasonable agreement with previous estimates of flood discharges for proposed megafloods. Results show that flow through channels with convex longitudinal profiles occurs when hydraulic head exceeds 910 m (Lost River) and 950 m (Sage Creek) , respectively. These are considerably below the maximum head capable of driving flow through the system of 1360 m, beyond which ice is decoupled from the bed across the pre‐glacial drainage divide. Therefore, it is concluded that these model results support the hypothesis of catastrophic subglacial drainage during the Livingstone Lake event. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

13.
Temporal variability in suspended sediment delivery processes was studied during three seasons in a 7·9 km2 catchment at Cape Bounty, Melville Island, Nunavut in the Canadian High Arctic. Discharge was controlled primarily by the magnitude of snowmelt, with limited inputs from ground ice melt and precipitation. Years with greater snowpack non‐linearly increased sediment yield and resulted in seasonal counter‐clockwise hysteresis, while a year with low snowpack resulted in reduced sediment yield and clockwise hysteresis, and indicates that sediment was increasingly available after the onset of streamflow. In addition to the event‐scale hysteresis observed during the nival discharge peak, diurnal clockwise hysteresis was observed during all three seasons and suggests daily exhaustion of sediment supplies. These results indicate that the channel snowpack plays a primary role over sediment accessibility during the nival discharge peak. Similarly, grain size analysis of suspended material in the river showed that the coarsest mean grain size was transported during the early phase of peak nival discharge and indicates that isolated sources of coarse material were being accessed by high velocity flow. Snowpack is present through the peak nival period and conditions sediment availability by isolating channel sediments from high‐energy flow. These results indicate sediment delivery characteristics in small High Arctic catchments reflect complex interactions with channel snowpack and disproportionate responses to flow conditions that differ from glacial and temperate settings. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

14.
Cold‐based polar glacier watersheds contain well‐defined supraglacial, ice‐marginal, and proglacial elements that differ in their degree of hydrologic connectivity, sources of water (e.g., snow, ice, and/or sediment pore water), meltwater residence times, allochthonous and autochthonous nutrient, and sediment loads. We investigated 11 distinct hydrological units along the supraglacial, ice marginal, and proglacial flow paths that drain Joyce Glacier in the McMurdo Dry Valleys of Antarctica. We found that these units play unique and important roles as sources and/or sinks for dissolved inorganic nitrogen and dissolved inorganic phosphorus and for specific fractions of dissolved organic matter (DOM) as waters are routed from the glacier into nutrient‐poor downstream ecosystems. Changes in nutrient export from the glacial system as a whole were observed as the routing and residence times of meltwater changed throughout the melt season. The concentrations of major ions in the proglacial stream were inversely proportional to discharge, such that there was a relatively constant “trickle” of these solutes into downstream ecosystems. In contrast, NO3? concentrations generally increased with discharge, resulting in delivery of episodic pulses of dissolved inorganic nitrogen‐rich water (“treats”) into those same ecosystems during high discharge events. DOM concentrations or fluorescence did not correlate with discharge rate, but high variability in DOM concentrations or fluorescence suggests that DOM may be exported downstream as episodic treats, but with spatial and/or temporal patterns that remain poorly understood. The strong, nutrient‐specific responses to changes in hydrology suggest that polar glacier drainage systems may export meltwater with nutrient compositions that vary within and between melt seasons and watersheds. Because nutrient dynamics identified in this study differ between glacier watersheds with broadly similar hydrology, climate, and geology, we emphasize the need to develop conceptual models of nutrient export that thoroughly integrate the biogeochemical and hydrological processes that control the sources, fate, and export of nutrients from each system.  相似文献   

15.
Drumlins are landforms essential to understanding of ice sheet movement over soft beds, sediment transport along the ice/bed interface, and the formation of a wide range of glacial deposits. Although investigated more than any other glacial landform, the origin of drumlins remains contentious. Using high-resolution LiDAR imagery and field data, we investigate the geomorphology and internal composition of one of the biggest drumlin fields in the North European Lowland. The Stargard drumlin field consists of over 1300 drumlins and related streamlined subglacial bedforms in a terminal part of a major Weichselian palaeo-ice stream of the southern Scandinavian Ice Sheet. The drumlins are typically 600-800 m long, 200-250 m wide, 3-6 m high and have axial elongation ratios ~2 but in some cases exceeding 15. Several subzones inferred from drumlin morphometry exist reflecting different ice flow dynamics. The most elongated drumlins occur in areas where ice moved down-slope and where thick fine-grained deposits of low hydraulic conductivity occur in the substratum. The largest portion of land occupied by drumlins and the greatest frequency density of drumlins occur where the ice moved up-slope. Stargard drumlins are composed of a wide variety of glacial deposits including various types of tills and meltwater sediments, which range from undisturbed to heavily deformed. There is no correlation between the deposits in the drumlins and the drumlin forms indicating that the deposits pre-date the drumlinizing process. It is suggested that the drumlin field was generated by a combination of direct glacial erosion and subglacial meltwater erosion by removing antecedent material from the inter-drumlin areas and streamlining the resultant bumps. Our data support the search for a unifying theory of drumlin formation and suggest erosion as the most plausible single mechanism generating drumlin landscapes. © 2019 John Wiley & Sons, Ltd.  相似文献   

16.
Stable oxygen isotope analysis and measurement of several dissolved cations and anions of bulk meltwater samples have provided information about the hydrochemical environment of the glacial hydrological system at Imersuaq Glacier, an outlet tongue from the Greenland ice‐sheet, West Greenland. The samples were collected at frequent intervals during the period 20–28 July 2000 in a small (<20 L s?1) englacial meltwater outlet at the glacier margin. The results document the following findings: (i) a marked diurnal variation of δ18O is related to the composition of oxygen isotope provenances, mainly near‐marginal local superimposed ice and basal up‐sheared ice further up‐glacier; (ii) a relationship is seen between all base cations (Na+, K+, Ca2+, Mg2+), SO42? and δ18O, indicating that solute acquisition is provided by solid–solution contact with the up‐sheared ice—as the relationship with Cl? is weak the influence of seasalt‐derived solutes is small in the area; (iii) when the melt rate is high, two diurnal maxima of δ18O values and solute concentrations are measured, and it is suggested that a snow meltwater component is responsible for the second maximum of δ18O—a short residence time leads to a delayed decrease in ion concentrations. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

17.
The summer discharge pattern of the Skeldal River, which drains a 560 km2 partly glacierized catchment in north‐east Greenland, is dominated by diurnal oscillations reflecting variations in the melt rate of snow and ice in the basin. Superimposed on this diurnal pattern are numerous short‐lived discharge fluctuations of irregular periodicity and magnitude. The larger fluctuations are described and attributed to both rainfall events and periodic collapse of the glacier margin damming flow from beneath the Skelbrae glacier. Other minor fluctuations are less readily explained but are associated with changes in the channelized and distributed reservoirs and possibly temporary blockage of subglacial conduits caused by ice melt with subsequent damming. Fluctuations in suspended sediment concentration (SSC) are normally associated with discharge fluctuations, although examples of ‘transient flushes’ were observed where marked increases in SSC occurred in the absence of corresponding discharge variations. A strong relationship between the event discharge increase and event SSC increase for rainfall‐induced events was established, but no such relationship existed for non‐rainfall‐induced events. There is some evidence for an exhaustion effect in the SSC patterns both at the event time‐scale and as the month proceeds. A mean suspended sediment load of 1765 ± 0·26 t day?1 was estimated for the study period, which would be equivalent to a suspended sediment yield of 732 ± 4 t km?2 year?1. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

18.
In this study, we used an archive of borehole logs from the British Geological Survey to collect information on the spatial structure of weathering that extends from the surface to competent bedrock across the Triassic Sherwood Sandstone Group outcrop (750 km2), in the East Midlands, UK. The borehole logs were used to estimate the thickness of the soil (n = 280) and soil and saprolite (S&S) to competent rock (n = 500). The weathering profile of the sandstone consisted of soil (median thickness ～ 1·5 m) overlying a transition zone of compacted and weakly cemented weathered sandstone saprolite over bedrock. Topographic analysis using a NEXTMAP 5 m × 5 m digital elevation model (DEM) revealed no significant relationships between slope properties (relief, flow length, flow accumulation or slope angle) and soil or S&S thickness. A weak, but statistically significant correlation was found between the thickness of the soil and S&S (rs = 0·25, p < 0·001, n = 192). The variation in soil thickness may be related to changes in current and historic and land‐use, variation in sandstone properties and the influence of glacial/peri‐glacial processes. The thickness of the saprolite was more variable towards the southern part of the study area, where it increased to a maximum 40 m. We hypothesize and provide evidence that the greater weathering thickness is related to the occurrence of increased faulting in this part of the study region, allowing increased access to meteoric waters. A possible source of increased water supply is meltwater from Quaternary ice sheets; the overburden of ice may have increased sub‐glacial pore water pressure, with the fractures and faults acting as a drainage system for the removal of dissolved weathering products. British Geological Survey © NERC 2010  相似文献   

19.
Accelerated runoff and erosion commonly occur following forest fires due to combustion of protective forest floor material, which results in bare soil being exposed to overland flow and raindrop impact, as well as water repellent soil conditions. After the 2000 Valley Complex Fires in the Bitterroot National Forest of west‐central Montana, four sets of six hillslope plots were established to measure first‐year post‐wildfire erosion rates on steep slopes (greater than 50%) that had burned with high severity. Silt fences were installed at the base of each plot to trap eroded sediment from a contributing area of 100 m2. Rain gauges were installed to correlate rain event characteristics to the event sediment yield. After each sediment‐producing rain event, the collected sediment was removed from the silt fence and weighed on site, and a sub‐sample taken to determine dry weight, particle size distribution, organic matter content, and nutrient content of the eroded material. Rainfall intensity was the only significant factor in determining post‐fire erosion rates from individual storm events. Short duration, high intensity thunderstorms with a maximum 10‐min rainfall intensity of 75 mm h?1 caused the highest erosion rates (greater than 20 t ha?1). Long duration, low intensity rains produced little erosion (less than 0·01 t ha?1). Total C and N in the collected sediment varied directly with the organic matter; because the collected sediment was mostly mineral soil, the C and N content was small. Minimal amounts of Mg, Ca, and K were detected in the eroded sediments. The mean annual erosion rate predicted by Disturbed WEPP (Water Erosion Prediction Project) was 15% less than the mean annual erosion rate measured, which is within the accuracy range of the model. Published in 2007 by John Wiley & Sons, Ltd.  相似文献   

20.
Kuannersuit Glacier, a valley glacier on Disko Island in west Greenland, experienced a major surge from 1995 to 1998 where the glacier advanced 10·5 km and produced a ～65 m thick stacked sequence of debris‐rich basal ice and meteoric glacier ice. The aim of this study is to describe the tectonic evolution of large englacial thrusts and the processes of basal ice formation using a multiproxy approach including structural glaciology, stable isotope composition (δ18O and δD), sedimentology and ground‐penetrating radar. We argue that the major debris layers that can be traced in the terminal zone represent englacial thrusts that were formed early during the surge. Thrust overthrow was at least 200–300 m and this lead to a 30 m thick repetition of basal ice at the ice margin. It is assumed that the englacial thrusting was initiated at the transition between warm ice from the interior and the cold snout. The basal debris‐rich ice was mainly formed after the thrusting phase. Two sub‐facies of stratified basal ice have been identified; a lower massive ice facies (SM) composed of frozen diamict enriched with heavy stable isotopes overlain by laminated ice facies (SL) consisting of millimetre thick lamina of alternating debris‐poor and debris‐rich ice. We interpret the stratified basal ice as a continuum formed mainly by freeze‐on processes and localized regelation. First laminated basal ice is formed and as meltwater is depleted more sediment is entrained and finally the glacier freezes to the base and massive diamict is frozen‐on. The increased ability to entrain sediments may partly be associated with higher basal freezing rates enhanced by loss of frictional heat from cessation of fast flow and conductive cooling through a thin heavily crevassed ice during the final phase of the glacier surge. The dispersed basal ice facies (D) was mainly formed by secondary processes where fine‐grained sediment is mobilized in the vein system of ice. Our results have important implications for understanding the significance of basal ice formation and englacial thrusting beneath fast‐flowing glaciers and it provides new information about the development of landforms during a glacier surge. Copyright © 2010 John Wiley & Sons, Ltd.  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