Subglacial stream

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Subglacial streams are conduits of glacial meltwater that flow at the base of glaciers and ice caps. [1] Meltwater from the glacial surface travels downward throughout the glacier, forming an englacial drainage system consisting of a network of passages that eventually reach the bedrock below, where they form subglacial streams. [1] Subglacial streams form a system of tunnels and interlinked cavities and conduits, with water flowing under extreme pressures from the ice above; as a result, flow direction is determined by the pressure gradient from the ice and the topography of the bed rather than gravity. [1] Subglacial streams form a dynamic system that is responsive to changing conditions, and the system can change significantly in response to seasonal variation in meltwater and temperature. [2] Water from subglacial streams is routed towards the glacial terminus, where it exits the glacier. [2] Discharge from subglacial streams can have a significant impact on local, and in some cases global, environmental and geological conditions. [3] Sediments, nutrients, and organic matter contained in the meltwater can all influence downstream and marine conditions. [4] Climate change may have a significant impact on subglacial stream systems, increasing the volume of meltwater entering subglacial drainage systems and influencing their hydrology. [2]

Contents

Formation

Subglacial streams derive their water from two sources: meltwater transported from the top of the glacier and meltwater from the glacial bed. [2] When temperatures are high enough to induce melting on the surface of the glacier, typically during summer, water flows down into the glacier. [2] Surface meltwater flows downward through millimeter-sized channels that join together in a network of tributaries, growing in size until reaching the bedrock. [1] Additionally, some water is transported to the surface by moulins (large, vertical shafts up to ten meters wide that range from the surface to a lower elevation, sometimes all the way to the glacial bed). [5] [1] Fractures, crevasses, and cavities between glaciers and valley walls can also provide pathways for water to reach the bed. [5] While surface meltwater can be seasonally dependent, the beds of temperate glaciers are maintained at the pressure melting point (the combination of temperature and pressure at which ice melts). [2] This liquid water at the bed—present in temperate but not polar glaciers—provides a constant input of water to subglacial stream systems. [2] Water from these two sources meets and is concentrated at the bedrock base of the glacier, where pressure from the ice above forces it to move towards the glacial terminus, creating a network of passageways as it works its way out of the glacier. [2]

Hydrology

Direction of Streams

Water in subglacial streams is subject to large amounts of pressure from the mass of ice above; as a result, the direction of water flow cannot be explained in the same way as typical surface streams. [2] Subglacial water flow is, to a large extent, determined by pressure gradients created by the weight and movement of the glacier. [1] As a result, instead of following the slope of the bed, streams can flow up and across slopes. [2] This behavior can be described by viewing the pressure inside glaciers as equipotential surfaces; as the water is pushed from areas of high pressure to areas of low pressure, it travels in a direction normal to these surfaces. [1]

Stream Systems

Subglacial stream systems can be placed in two categories based upon the arrangement and type of passages that make up the system: channelized and distributed. [5]

Channelized

Channelized drainage systems are characterized by water flowing predominantly through tunnels along the bed of the glacier that take meltwater rapidly and directly to the glacial terminus. [5] These tunnels are arranged in a network of tributaries, joining together and growing in size as they near the terminus. [2] Water is fast-moving in these systems, and pressure inside the channels is relatively low compared to pressure in the ice around them. [5] Turbulence in the rapid flow produces heat, which is able to melt the ice walls of the tunnels. [5] While the total water added to the system by this process is insignificant compared to water from the surface and from basal melting, the melting of the channel walls allows the channel to remain open even when the ice pressures surrounding it are much greater than the pressure of the water inside. [5] The constant erosion of the tunnel walls is able to offset the narrowing of the tunnel caused by deformation of the ice. [5] Depending on the water supply and the characteristics of the bed, the tunnels can take different forms, including semicircular tunnels cutting into the ice, broad and low tunnels, and tunnels that cut into the bed rather than the ice. [2] Broad and low tunnels form in channels with variable amounts of meltwater, as melting is concentrated on the tunnel walls rather than the ceiling when the tunnel is not completely full of water. [2] Channels that maintain long-term stability in water flow and location can erode the bedrock over time, resulting in tunnels that cut into the bed rather than the ice above. [2]

Distributed

Distributed drainage systems can consist of a network of linked cavities, porous flow and canals in the sediment, and a thin film between the ice and bed. [5] Films of water between the ice and bedrock form are rarely thicker than tens of micrometers, and form in areas that are isolated from channels and cavities, maintained at the pressure melting point, and are above impermeable bed. [2] Flow in films does not account for a large amount of the total meltwater flux out of the glacier, but may be important in the sliding movement of glaciers. [2] In cases in which glaciers are above porous, unconsolidated sediment, some water can flow through the sediment; like film flow, porous flow does not account for much of the water flux in the system. [2] When the bed is deformable, wide, shallow canals up to 10 cm in width can form in the surface of the sediment, topped by the glacial ice. [2] [6] In glaciers with steep slopes, canal systems are unstable, as they can be easily absorbed by channels above the sediment. [6] As glaciers move over bumps in the bedrock, differences in pressure can separate the ice from the bed behind the bump if the glacier is moving fast enough. [2] [1] This creates cavities between the glacier and the bed, which fill with water. [2] If the water pressure is high enough, the cavity expands, and the water can cause more separation between the ice and bed surrounding the cavity. [1] With sustained water supply, small passages form between cavities, creating a large network of linked cavities which water flows between. [1] Water in linked cavity systems flows, on average, in a direction normal to the equipotential surfaces of pressure in the glacier. [1] However, the path taken is long and indirect, and at times water can be flowing nearly parallel to the equipotential surfaces. [1]

Seasonal Variability

The structure of subglacial stream systems changes significantly over time as a result of seasonal changes in the volume and source of meltwater input. [5] During the winter, subglacial stream systems are dominated by distributed streams. [5] As there is very little surface melting during this season, nearly all meltwater is derived from basal melting and the release of stored meltwater. [5] Both of these sources involve small amounts of water released relatively uniformly throughout the bed of the glacier, making them unlikely to form large drainage channels. [2] Some major tunnels remain in the system year-round, and are the main points of discharge during the winter, but the system at large is characterized by distributed drainage. [5] As temperatures rise and surface melting increases water flux to the bed in late spring, the winter stream system is disrupted. [2] Distributed flow channels, lacking the capacity for increased volumes of meltwater, experience a rise in water pressure and are destabilized. [2] High water pressures lead to the formation of larger tunnels—a process known as channelization—that have a greater capacity for meltwater and allow for pressures to fall. [7] This change can happen gradually or can be triggered by events that rapidly increase meltwater flow, such as consecutive days of high melting or a large rainstorm. [2] The now-channelized system grows in extent throughout the summer as meltwater input continues to increase, with new passages forming and growing in size. [2] In autumn, surface melting decreases, and the volume of meltwater is no longer sufficient to maintain the newly formed channels; deformation of the surrounding ice slowly closes channels that do not generate enough frictional melting along their walls to offset the closure. [2] Eventually, a distributed stream system again becomes dominant. [2] Some perennial channels remain throughout the winter season, but the channels formed in spring disappear—when new tunnels form again the next year, they do not form in the same locations as the ones that closed. [5]

Impact on Glacial Systems

Submarine Glacial Melt

Modeled velocity and temperature of discharge and submarine melt rate with a varying number and size of plumes. Subglacial submarine discharge.jpg
Modeled velocity and temperature of discharge and submarine melt rate with a varying number and size of plumes.

The discharge of subglacial stream systems of marine-terminating glaciers into the ocean has a significant impact on the volume and distribution of glacial melt at the terminus. [8] The discharge of glacial streams into the ocean emerges as plumes that travel up to the ocean surface along the face of the glacier, which can serve as heat sources for glacial melt. [9] Ice melt due to discharge plumes has a significant impact in areas in which discharge rates exceed 100 m3/s−1; with lesser discharge rates, plume-associated heat is insignificant compared to the effects of ocean mixing. [9] Seasonal variability plays an important role in the way that subglacial streams influence glacial melting. [10] During the summer, subglacial stream output is much greater, resulting in plumes that are larger, faster, and more buoyant than during the winter. [10] In addition to the greater volume of discharge increasing glacial melt, the increased buoyancy of the plume results in more turbulence and, consequently, more heat transfer to the glacier, further increasing melt. [10] The effect subglacial stream discharge has on glacial melt is also influenced by the type of subglacial drainage system; distributed subglacial streams result in an output of meltwater uniformly across the grounding line (where the glacier transitions from grounded to floating ice), whereas channelized drainage results in individual, large outlets. [8] Distributed discharge results in glacial melt volumes up to five times greater than that of channelized drainage, as individual strong plumes of meltwater are not as capable of inducing widespread melting as a much greater number of smaller outputs. [8]

Glacial Motion

In temperate glaciers, which are characterized by the presence of liquid water at their base and are able to slide, subglacial streams have a significant impact on glacial movement. The water pressure and friction experienced at the base of a glacier depends in part on whether the subglacial hydrological system is channelized or distributed. [5] Channelized systems are an efficient form of drainage as they are able to rapidly move water out of the glacier, reducing water pressure in the system. [5] By decreasing water pressure underneath the glacier, friction between the glacier ice and the bedrock below increases, slowing the movement of the glacier. [5] Distributed flow systems, contrastingly, are characterized by slow-moving water in small cavities and passages; when water flux into the system increases, such as during periods of high melt, the system is unable to compensate, resulting in large increases in basal water pressure. [5] As a result, friction between the glacier and the bed is reduced, and glacial sliding speed increases. [5] Glacial motion can also cause changes in subglacial stream systems, and there are feedbacks present between the two. [7] As subglacial water pressure increases, the speed of glacial sliding increases. The glacier encounters bumps in the bedrock as it slides: as a result, cavities are created between the ice and the bed. [7] The glacier encounters more bumps due to its higher speed and, since ice moving at a higher speed is less able to maintain connection with the bedrock, faster moving glaciers are more likely to form cavities when passing over bumps. [2] [1] This increases the subglacial space which can be filled with water, decreasing basal water pressure. [7] The interaction between glacial motion and subglacial hydrology creates a negative feedback loop, in which increased water pressure below the glacier increases glacial sliding speed, which in turn decreases pressure and, consequently, sliding speed. Through this mechanism, the effects of speedup events can decay over time. [7] Another control on glacial sliding speed is the process of channelization. [7] Sustained high levels of meltwater input result in a shift from a distributed network of subglacial streams to a more channelized system as larger passages through the ice develop. [7] As larger channels are able to more efficiently remove water from the subglacial system, water pressure decreases, increasing friction between the glacier and the bedrock and decreasing sliding speed. [7] Channelization is the most significant process in terminating speedup events, and is responsible for the slowdown in glacial speed at the end of summer following the speedup commonly observed as meltwater flow increases in spring. [7]

Material Transport

Nutrients and Organic Matter

Subglacial streams carry a significant amount of organic matter and nutrients, originating both from supraglacial meltwater and subglacial processes. [4] Meltwater from supraglacial environments containing microbially-produced dissolved organic carbon, or DOC, flows into glaciers, eventually reaching subglacial stream systems, which carry the organic matter out of the glacier. [4] This source of DOC is supplemented by organic matter produced within subglacial ecosystems, where there are diverse microbial communities. [4] Though the concentration of dissolved organic matter in glacial meltwater is low, the sheer amount of freshwater discharge from glaciers makes glacially-sourced DOC an important source of bioavailable carbon to marine ecosystems. [11] In the Gulf of Alaska alone, glacial runoff provides 0.13 Tg of organic carbon per year, much of which travels through subglacial streams. [11] Subglacial streams also transport various other important nutrients. Geological processes, including the grinding of glaciers on the bedrock below and water-rock interaction, ensure that minerals are continuously fed into the subglacial system. [4] [3] Iron transported by subglacial streams, for example, is mostly sourced from subglacial weathering, and may be responsible for an Fe flux large enough to significantly influence global ocean chemistry over geological timescales. [3] Biological processes also provide nutrients to subglacial streams, with nitrification and denitrification by microbes affecting downstream communities during periods of melt. [4]

Sediment

Subglacial streams can transport, deposit, and remove sediment from the glacier bed; this process is influenced by water supply and the amount and characteristics of the available sediment. [12] The size of sediment particles, the slope of the subglacial stream’s channel, and the roughness of the bed all contribute to whether sediment is mobilized or deposited. [12] Subglacial flooding events can result in significant erosion and sediment transport, and studies modeling subglacial channels suggest that seasonal meltwater flow alone can erode bedrock and transport sediment as large as boulders. [12] Contrastingly, when water pressure is low, such as at the end of a melt season, sediment is deposited. [12] When sediment supply is high enough, the sediment deposition can form an esker: an elongated ridge of sediment that fills the channel of the subglacial stream in which it forms. [13] These eskers can be temporary, lasting only until increasing water pressure during the next melt season flushes out the sediment, or they could be permanent. [12] The permanent formation of eskers is more common in retreating glaciers and ice sheets, as their termini are thinning, which favors the deposition of sediment. [12] Advancing glaciers and ice sheets exhibit steepening termini, which increases shear stresses and, consequently, water pressure, which favors the flushing of deposited sediment out of stream channels. [12]

Climate Change

Anthropogenic climate change is likely to cause significant changes in subglacial stream systems. As glacial melting increases as a result of rising global temperatures, water flux into and discharge from subglacial streams increases as well. [11] Greater water input from surface melting may affect the hydrology of subglacial systems, changing the timing of seasonal variations. [14] As a result of climate change-induced increases in meltwater, greater volumes of water are likely to reach the bed earlier in the year. [14] This would cause the transition from winter distributed subglacial drainage to summer channelized streams to occur earlier in the year. [14] Glacial motion could also be affected: since glaciers dominated by channelized systems have lesser sliding speeds, the earlier transition to this system could result in slower moving glaciers. [14] However, short-term fluctuations in meltwater volume and pressure, which may become more intense as runoff increases, could offset this decrease in sliding by causing localized speedups. [14] Increases in the volume of discharge from subglacial streams are likely to increase the melting of marine-terminating glaciers, as submarine melt rates are highly sensitive to the amount of subglacial discharge. [10]

Related Research Articles

Glacier Persistent body of ice that is moving under its own weight

A glacier is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as crevasses and seracs, as it slowly flows and deforms under stresses induced by its weight. As it moves, it abrades rock and debris from its substrate to create landforms such as cirques, moraines, or fjords. Although a glacier may flow into a body of water, it forms only on land and is distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.

Till Unsorted glacial sediment

Till or glacial till is unsorted glacial sediment.

Esker Long, winding ridge of stratified sand and gravel associated with former glaciers

An esker, eskar, eschar, or os, sometimes called an asar, osar, or serpent kame, is a long, winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North America. Eskers are frequently several kilometres long and, because of their uniform shape, look like railway embankments.

Glaciology Scientific study of ice and natural phenomena involving ice

Glaciology is the scientific study of glaciers, or more generally ice and natural phenomena that involve ice.

Jökulhlaup Type of glacial outburst flood

A jökulhlaup is a type of glacial outburst flood. It is an Icelandic term that has been adopted in glaciological terminology in many languages. It originally referred to the well-known subglacial outburst floods from Vatnajökull, Iceland, which are triggered by geothermal heating and occasionally by a volcanic subglacial eruption, but it is now used to describe any large and abrupt release of water from a subglacial or proglacial lake/reservoir.

Glacial motion Geological phenomenon

Glacial motion is the motion of glaciers, which can be likened to rivers of ice. It has played an important role in sculpting many landscapes. Most lakes in the world occupy basins scoured out by glaciers. Glacial motion can be fast or slow, but is typically around 25 centimetres per day (9.8 in/d).

The Oak Ridges Moraine is a geological landform that runs east-west across south central Ontario, Canada. It developed about 12,000 years ago, during the Wisconsin glaciation in North America. A complex ridge of sedimentary material, the moraine is known to have partially developed under water. The Niagara Escarpment played a key role in forming the moraine in that it acted as a dam for glacial meltwater trapped between it and two ice lobes.

Subglacial lake Lake under a glacier

A subglacial lake is a lake that is found under a glacier, typically beneath an ice cap or ice sheet. Subglacial lakes form at the boundary between ice and the underlying bedrock, where gravitational pressure decreases the pressure melting point of ice. Over time, the overlying ice gradually melts at a rate of a few millimeters per year. Meltwater flows from regions of high to low hydraulic pressure under the ice and pools, creating a body of liquid water that can be isolated from the external environment for millions of years.

Terminal moraine Type of moraine that forms at the terminal of a glacier

A terminal moraine, also called end moraine, is a type of moraine that forms at the terminal (edge) of a glacier, marking its maximum advance. At this point, debris that has accumulated by plucking and abrasion, has been pushed by the front edge of the ice, is driven no further and instead is deposited in an unsorted pile of sediment. Because the glacier acts very much like a conveyor belt, the longer it stays in one place, the greater the amount of material that will be deposited. The moraine is left as the marking point of the terminal extent of the ice.

Fluvio refers to things related to rivers and glacial refers to something that is of ice. Fluvio-glacial refers to the meltwater created when a glacier melts. Fluvio-glacial processes can occur on the surface and within the glacier. The deposits that happen within the glacier are revealed after the entire glacier melts or partially retreats. Fluvio-glacial landforms and erosional surfaces include: outwash plains, kames, kame terraces, kettle holes, eskers, varves, and proglacial lakes.

Ice stream A region of fast-moving ice within an ice sheet

An ice stream is a region of fast-moving ice within an ice sheet. It is a type of glacier, a body of ice that moves under its own weight. They can move upwards of 1,000 metres (3,300 ft) a year, and can be up to 50 kilometres (31 mi) in width, and hundreds of kilometers in length. They tend to be about 2 km (1.2 mi) deep at the thickest, and constitute the majority of the ice that leaves the sheet. In Antarctica, the ice streams account for approximately 90% of the sheet's mass loss per year, and approximately 50% of the mass loss in Greenland.

Tunnel valley Glacial-formed geographic feature

A tunnel valley is a U-shaped valley originally cut under the glacial ice near the margin of continental ice sheets such as that now covering Antarctica and formerly covering portions of all continents during past glacial ages. They can be as long as 100 km (62 mi), 4 km (2.5 mi) wide, and 400 m (1,300 ft) deep.

Meltwater Water released by the melting of snow or ice

Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater can be produced during volcanic eruptions, in a similar way in which the more dangerous lahars form.

Subglacial eruption Eruption of an ice-covered volcano

Subglacial eruptions, those of ice-covered volcanoes, result in the interaction of magma with ice and snow, leading to meltwater formation, jökulhlaups, and lahars. Flooding associated with meltwater is a significant hazard in some volcanic areas, including Iceland, Alaska, and parts of the Andes. Jökulhlaups have been identified as the most frequently occurring volcanic hazard in Iceland, with major events where peak discharges of meltwater can reach 10,000 – 100,000 m3/s occurring when there are large eruptions beneath glaciers.

Fluvioglacial landforms are those that result from the associated erosion and deposition of sediments caused by glacial meltwater. These landforms may also be referred to as glaciofluvial in nature. Glaciers contain suspended sediment loads, much of which is initially picked up from the underlying landmass. Landforms are shaped by glacial erosion through processes such as glacial quarrying, abrasion, and meltwater. Glacial meltwater contributes to the erosion of bedrock through both mechanical and chemical processes.

Ice-sheet dynamics Technical explanation of ice motion within large bodies of ice

Ice sheet dynamics describe the motion within large bodies of ice, such those currently on Greenland and Antarctica. Ice motion is dominated by the movement of glaciers, whose gravity-driven activity is controlled by two main variable factors: the temperature and the strength of their bases. A number of processes alter these two factors, resulting in cyclic surges of activity interspersed with longer periods of inactivity, on both hourly and centennial time scales. Ice-sheet dynamics are of interest in modelling future sea level rise.

Overdeepening Characteristic of basins and valleys eroded by glaciers

Overdeepening is a characteristic of basins and valleys eroded by glaciers. An overdeepened valley profile is often eroded to depths which are hundreds of metres below the deepest continuous line along a valley or watercourse. This phenomenon is observed under modern day glaciers, in salt-water fjords and fresh-water lakes remaining after glaciers melt, as well as in tunnel valleys which are partially or totally filled with sediment. When the channel produced by a glacier is filled with debris, the subsurface geomorphic structure is found to be erosionally cut into bedrock and subsequently filled by sediments. These overdeepened cuts into bedrock structures can reach a depth of several hundred metres below the valley floor.

Glacial stream Body of liquid water that flows down a channel formed by a glacier

A glacier stream is a channelized area that is formed by a glacier in which liquid water accumulates and flows. Glacial streams are also commonly referred to as "glacier stream" or/and "glacial meltwater stream". The movement of the water is influenced and directed by gravity and the melting of ice. The melting of ice forms different types of glacial streams such as supraglacial, englacial, subglacial and proglacial streams. Water enters supraglacial streams that sit at the top of the glacier via filtering through snow in the accumulation zone and forming slush pools at the FIRN zone. The water accumulates on top of the glacier in supraglacial lakes and into supraglacial stream channels. The meltwater then flows through various different streams either entering inside the glacier into englacial channels or under the glacier into subglacial channels. Finally, the water leaves the glacier through proglacial streams or lakes. Proglacial streams do not only act as the terminus point but can also receive meltwater. Glacial streams can play a significant role in energy exchange and in the transport of meltwater and sediment.

A meltwater channel is a channel cut into ice, bedrock or unconsolidated deposits by the flow of water derived from the melting of a glacier or ice-sheet. The channel may form on the surface of, within, beneath, along the margins of or downstream from the ice mass. Accordingly it would be referred to as supraglacial, englacial, subglacial, lateral or proglacial.

Glaciofluvial deposits Sediments/deposits formed from ice sheets or glaciers

Glaciofluvial deposits or Glacio-fluvial sediments consist of boulders, gravel, sand, silt and clay from ice sheets or glaciers. They are transported, sorted and deposited by streams of water. The deposits are formed beside, below or downstream from the ice. They include kames, kame terraces and eskers formed in ice contact and outwash fans and outwash plains below the ice margin. Typically the outwash sediment is carried by fast and turbulent fluvio-glacial meltwater streams, but occasionally it is carried by catastrophic outburst floods. Larger elements such as boulders and gravel are deposited nearer to the ice margin, while finer elements are carried farther, sometimes into lakes or the ocean. The sediments are sorted by fluvial processes. They differ from glacial till, which is moved and deposited by the ice of the glacier, and is unsorted.

References

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