Formation and Drainage of Supraglacial Lakes

Formation and Drainage of Supraglacial LakesGlacial Processes and Geomorphology Degree designationB028303What find outs the governing body and drainpipe of supraglacial lakes on the Greenland sparkler public opinion poll and to what extent argon these dishes an important control on sorbet tacking dynamics? establishmentThe Greenland spyglass Sheet is a prominent applesauce mass in the Northern Hemisphere, made up of both land terminating and tide piddle terminating outlet glaciers. seasonal worker come near resolveing produces warming water that collects on the shape up of the Greenland frost tab in the form of supraglacial lakes (Luthje et al., 2006).The storage and drainage of lake water is specially important in forcing a hydraulic connection betwixt the nut sail surface and the rubbish hunch forward interface (Bartholomew et al., 2012). For this reason, drainage of these supraglacial lakes is of ill-tempered interest in understanding the influence that they have on glass rag dynamics. Supraglacial lakes have been know to drain in less than mavin day (Box Ski, 2007). The fluctuations in surface water delivered to the glacier hunch over from the quick drainage of these lakes are known to affect the accelerate that the ice sheet travels at and have been known to cause local stimulate of the ice sheet (Sundal et al., 2011).The purpose of this paper is to discuss the factors that control the formation and drainage of supraglacial lakes on the Greenland ice sheet and the implications these processes have on the control of ice sheet dynamics.FormationIn cases where drainage is prevented, work water gage be stored on the ice sheet by a form of posit or ice barrier, creating a supraglacial lake (Benn Evans, 2010). Seasonal warming produces unfreeze water that collects in closed surface undulations on the surface of the ice sheet. The supraglacial lakes crossways the Greenland ice sheet vary in size from a a couple of(prenomi nal) hundred meters to 1500m in diameter, with wisdoms of 2-5m (Box Ski, 2007).Supraglacial lakes form in the ablation area of the Greenland ice sheet during summer time dissolve season. Positions of numerous lakes have been monitored oer many years and from the results it can be deduced that most lakes form in the kindred place every year (Luthje et al., 2006). Supraglacial lakes can occur crosswise much of the ablation zone on the Greenland ice sheet all when are especially prominent on the Western transect (Luthje et al., 2006). The formation and lieu of supraglacial lakes on the Greenland ice sheet is driven by season and temperature, peak and topography (Chu, 2014).There is an upper limit imposed on the lake distribution and formation by elevation because of temperature and base sliding restrictions (Liang et al., 2012). Due to the lapse rate, temperature decreases with elevation. When it reaches a certain point on the ice sheet it becomes too frosty for melting to so there is no melt water for call for the lake privys. As roll in the hayrock topography is only expressed in ice surface topography in regions experiencing basal sliding, interior regions of the ice sheet that do not nonplus basal sliding are unlikely to contain ice surface depressions suitable for lake formation (Liang et al., 2012). Figure 1(a) shows a histogram of the rate of channel of lake area over 10 years of observations. In (b) only the fiftieth percentile shows a significant relationship with melt intensity. From the two graphs it can be deduced that there is a positive correlation amid one-year median lake elevation and annual melt intensity, indicating that lakes are more than(prenominal) quick at higher elevations during warmer or more intense melt years (Liang et al., 2012). During more intense melt years, lake drainage events become more common in higher elevations. These regions, which dont normally experience rapid surface water beam to the prat of the glacier become more susceptible to moulin formation (Liang et al., 2012). Only lakes above 1050m show inter-annual changes in receipt to seasonal changes in melting intensity (Liang et al., 2012).The area of supraglacial lakes is more controlled by the local topography. Lakes at lower elevations tend to be smaller because the steep ice surfaces prevent as much water being roll up and stored (Bartholomaus et al., 2007).Although topography and elevation influence the formation and distribution of supraglacial lakes in Greenland, the main control is the season. The lakes willing only form during melt season, when temperatures become warm copious to produce melt water to fill the empty lake basins (Luthje et al., 2006).DrainageIn order for melt water stored as supraglacial lakes to reach the bed of a glacier, a finished ice transport remains is required. In a study, Das et al. (2008) monitored and observed the rapid drainage of two openhanded supraglacial lakes megabucks to the base of the Greenland ice sheet. The lakes, on the Western side of the Greenland ice sheet began filling in early July, reaching maximum extent or so the 29th of July. At first, lake levels began to fall slowly and steadily and then levels dropped rapidly. This rapid drainage event drained the lake in around 1.4 hours (Das et al., 2008).From observations, it can be deduced that the drainage of the lake occurs in four stages. First of all initial slow, tranquillise drainage occurs with crevasses (Chu, 2014). No water reaches the bed of the glacier, suggesting an inefficient drainage agreement. Next, connection to the bed occurs and drainage through a scissure system occurs. This stage gives fast and direct drainage of the lake to the bed of the glacier. Thirdly, Moulin formation and mop up of the fracture system occurs. Frictional heating from the turbulent water scarper and rapid drainage in stage two melts through fracture walls, developing them into discrete moulins (Chu, 2014). Moulins are associated with rapid lake drainage and immediate transfer of water to the ice sheet (Chu, 2014). This rapid transfer of water to the glacier bed has potential to bury the subglacial hydrologic system. The final stage occurs once the lake has drained. Moulins stay outdoors to allow the drainage of surface melt to the bed of the glacier (Das et al., 2008). Evidence suggests that moulins in existence in the bottom of the lakes fill with snow and become dammed in the winter. Lakes are filled during the summer melt season until the moulins are reopened by a combination of imperativeness and melting (Box Ski, 2007). respite propagation through the ice suggests that once initiated water filled crevasses will propagate napwardly through the full thickness of the ice through a process called hydrofracture (Das et al., 2008). The water ponds above small crevasses in the lake basin, exerting stress on the crevasse and finally the ice starts to fracture. Initially the water refreezes in the crevasse. Heat created from refreezing warms the ice and it melts, progress further down into the crevasse. Once a connection with the bed is launch water from the lake can freely drain. There is no limit to the depth of a water filled crevasse so it is able to reach the bed of a glacier. The process is only limited by the supply of melt water needed to keep the crevasse full (Weertman, J. 1973). Crevasses and moulins connect the surface of the ice sheet with englacial environments, providing a route for melt water to drain from the supraglacial lakes through the ice sheet (Chu, 2014).Although only lake drainage processes for two lakes were observed by Das et al. (2008) in this particular study it can be faux that early(a) lakes on the Greenland ice sheet drain in the same way. The presence of fractures in other empty lake basins suggests that drainage processes are convertible for all supraglacial lakes (Das et al., 2008).Throughout melt season, drainage patterns of the lakes across the Greenland ice sheet migrate inland. Lakes at lower elevations tend to form and drain in the beginning than those at higher elevations (Bartholomaus et al., 2007).Ice Sheet DynamicsRapid drainage events are of particular interest in understanding ice sheet dynamics and hydraulic response. Fluctuations in surface water are known to affect the speed of ice sheets (Sundal et al., 2011). Some suggest that more influx of melt water to the glacier bed, from lake drainage events could produce ice sheet acceleration. Moulins submit a method of rapid lake drainage, supplying large influxes of water to the glacier bed with the potential to overwhelm the subglacial hydrological system causing uplift and make up basal sliding (Chu, 2014). They can only support a fraction of the lake water, retentivity water jam at the base of the glacier high and encouraging join ons in basal movement and uplift (Box Ski, 2007). Higher water push favours fast sliding as it reduces drag between ice and the bed (Bartholomew et al., 2012). urge ups of the ice sheet are caused by an increase of water pressure at the its base, enhancing basal sliding. This increase in basal water pressure can be provided by melt lake outbursts (Box Ski, 2007). There are other processes involved in basal sliding but for the purposes of linking it with glacier hydrology we are only interested in cavitation. Cavitation vertically jacks the glacier upwards, reducing friction between the glacier bed and rock interface and decreasing effective pressure. When effective pressure and friction are low basal sliding rates are high. In the same study as mentioned above, Das et al. (2008) observed horizontal and vertical movement of the ice sheet that coincided with rapid drainage of the lake.A large steep block of ice in the centre of the lake basin evidences local uplift of the ice sheet during rapid lake drainage. Based on topographic studies, it is likely that the large block in the centre of the lake was uplifted during a drainage event (Das et al., 2008). The elevation change reported by sonar surveys suggests surface uplift of around 6m across the block. The large people of water transported to the ice sheet bed during lake drainage would have produced some(prenominal) meters of uplift at the lakes centre (Das et al., 2008).Figure 2, produced from a study by Bartholomew et al., (2012) supports Das et al., (2008) conclusion that drainage of supraglacial lakes can affect velocity and uplift the ice sheet. It shows rapid increases in ice velocity and surface bloom after the drainage of a supraglacial lake observed on the Western transect of the Greenland ice sheet. Drainage of the lake can be linked with a 400% increase in ice velocity and 0.3m of uplift in less than 24 hours (Bartholomew et al., 2012).Figure 2 (a) place velocity during lake drainage event (b) Surface height profile during the lake drainage event (c d) Before and after images of th e supraglacial lake (taken by time lapse camera). Solid black lines on (a b) indicate the generation when the two photos were taken.Taken from Bartholomew, I., Nienow, P., Sole, A., Mair, D., Cowton, T. and King, M. (2012). Short term variability in Greenland Ice Sheet dubiousness forced by time-varying meltwater drainage Implications for the relationship between subglacial drainage system behaviour and ice velocity. daybook of Geophysical Researcg, 117(F3).both outlet glaciers and slower moving ice sheets across Greenland show a seasonal speed up in response to enhanced melt water delivery to the glacier bed and from pulses of water from lake drainage. The ice sheets continue to speed up until a maximum velocity threshold is reached sequentially the glacier slows down (Chu, 2014). This slowing down of the glacier is not to be expected if lubrication by melt water is the primary mechanism of speeding up the ice sheet. The decline in velocity suggests subglacial drainage switch es from a linked cavity system to a more efficient channel drainage system (Chu, 2014). resultantUnderstanding the hydrology of supraglacial lakes is important as they have the potential to supply large volumes of water to the bed of the ice sheet and affect motion. Both an increase in glacier velocity and local uplift is observed when supraglacial lake water drains to the bed of the glacier. However, constant high influxes of water are not consistent with go along increase in velocity because the subglacial drainage system switches to a more efficient system (Bartholomaus et al., 2007). Large pulses of water delivered to the bed from drainage of supraglacial lakes overwhelm the subglacial hydrologic network and increase basal motion. Moulins provide rapid drainage of large supraglacial lakes into the ice sheet hydrological systems, while crevasses provide a more rigid drainage method. The location, area and potential melt water available to fill these supraglacial lakes is contro lled by the elevation and topography of the surrounding area, and more importantly temperature. In turn, these factors could be controlled by climate change. For this reason it is important to monitor the processes associated with glacial lakes to get under ones skin a clearer understanding of the impacts any future climate change will have on glacial lakes and the Greenland ice sheet.ReferencesAbdalati, W. and Steffen, K. (2001). Greenland ice sheet melt extent 19791999. diary of Geophysical Research Atmospheres (19842012), 106(D24), pp.3398333988.Bartholomaus, T., Anderson, R. and Anderson, S. (2007). Response of glacier basal motion to transient water storage. Nature Geoscience, 1(1), pp.3337.Box, J. and Ski, K. (2007). far sounding of Greenland supraglacial melt lakes implications for subglacial hydraulics. Journal of glaciology, 53(181), pp.257265.Chu, V. (2014). Greenland ice sheet hydrology A review. mount up in Physical Geography, 38(1), pp.1954.Das, S., Joughin, M., Beh n, M., Howat, I., King, M., Lizarralde, D., et al. (2008). Fracture propagation to the base of the Greenland Ice Sheet during supra-glacial lakedrainage. Science, 5877, 778781.Liang, Y., Colgan, W., Lv, Q., Steffen, K., Abdalati, W., Stroeve, J., Gallaher, D. and Bayou, N. (2012). A decadal investigation of supraglacial lakes in West Greenland using a fully automatic detection and trailing algorithm. Remote Sensing of Environment, 123, pp.127138.Luthje, M., Pedersen, L., Reeh, N. and Greuell, W. (2006). Modelling the evolution of supraglacial lakes on the West Greenland ice-sheet margin. Journal of Glaciology, 52(179), pp.608618.McMillan M, Nienow P, Shepherd A, et al. (2007) Seasonal evolution of supra-glacial lakes on the Greenland Ice Sheet. Earth and mobile Science Letters 262(34) 484492.Sundal AV, Shepherd A, Nienow P, et al. (2009) Evolution of supra-glacial lakes across the Greenland Ice Sheet. Remote Sensing of Environment 113(10) 21642171.Sundal AV, Shepherd A, Nienow P, e t al. (2011) Meltinduced speed-up of Greenland ice sheet offset by efficient subglacial drainage. Nature 469(7331)521524.Tweed, F. and Russell, A. (1999). Controls on the formation and sudden drainage of glacier-impounded lakes implications for jokulhlaup characteristics. Progress in Physical Geography, 23(1), pp.79110.Weertman, J. 1973. Can a water-filled crevasse reach the bottom surface of a glacier? IASH Publ. 95 (Symposium at Cambridge1969 Hydrology of Glaciers), 139145.Zwally, H.J., W. Abdalati, T. Herring, K. Larson, J. Saba and K. Steffen. 2002. Surface melt-induced acceleration of Greenland ice-sheet flow. Science, 297(5579), 218222.