Essay: Formation and presence of meltwater channels in glacial landscapes

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  • Formation and presence of meltwater channels in glacial landscapes
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Meltwater channels in a glacial environment are considered an erosional feature that’s part of the landscape geomorphology. They are what has been carved out and left behind in the landscape, by the force and strength of the water flow in a drainage pathway (Shaw & Sharpe, 1987). The effect of pressurised water on the proglacial areas and the hydrology of glaciers exerts a strong influence on different characteristics of the glacier, and meltwater channels are evidence of this. Where meltwater channels carve from glacier margins to the sea the land is forcefully shaped, this influences rates of erosions, glacier movement and the deposition of glacial till. (Benn & Evans, 2010). It is this imprint of meltwater channels left behind in the landscape that allows scientists to conduct the challenging yet effective task of glacier reconstruction.

The aim of this paper is to review the formation and presence of meltwater channels in glacial landscapes and determine their eligibility in glacier reconstruction.

Before going any further it’s important to note the difference between drainage pathways and meltwater channels. Drainage pathways are the route through a glacier which the water has taken. These pathways are part of complex drainage systems of the transportation of water within an dentritic network. (Singh, Singh & Haritashya, 2011). Meltwater channels are the landform left behind once the water has drained away. It is this that tells scientists about past glaciation.

An understanding of the formation of these drainage pathways must be exhibited to understand the formation of meltwater channels. The changes in state of ice due to the global energy exchange between surface ice and the atmosphere, can bring about surface melting. This is when there is a surplus of solar energy when the temperature of the ice is 0ᵒC (Benn & Evans, 2010). Subglacial melting occurs due to the development of high pressure at the glacier base as it flows over bedrock. This basal sliding causes frictional heating and can lead to changes to the shape of the glacier (Goudie, 2004). Geothermal heating from beneath the ice also causes melting. An example of this process occurring is in the West Antarctic where the Pine Island Glacier is melting from heat produced from a volcanic rift system below the surface (Watts, 2014). These heat processes are the primary causes of ablation in glacial systems(Goudie, 2004)

Formation of Meltwater Channels

Meltwater channels are classed as mesoscale landforms and are formed as a result of glacial erosion. The two main types of meltwater channels formed from this process are subglacial and lateral. When crevasses develop within the glacier due to extensional flow, water can travel down to a subglacial position via a moulin. This meltwater erodes the bedrock to carve out a subglacial meltwater channel. As the water erodes downwards into the sediment it firstly creates an N-channel, a landform associated with meltwater channels (Bennett & Glasser,2009). Subglacial channels are mainly formed in temperate glaciers and the N-channel that forms can be a single channel or a network of channels (Singh, Singh & Haritashya, 2011). Lateral meltwater channels run along the margin of the glacier at a shallow angle to the slope (Unknown, 2014). In contrast to subglacial meltwater channels, they generally form at cold-based glaciers. These glaciers are frozen to the substrate preventing the infiltration of water, therefore water is forced to travel down the side of the glacier. Here it erodes the margin leaving behind a lateral meltwater channel (Singh, Singh & Haritashya, 2011).

The formation of these meltwater channels is influenced by hydraulic potential. This occurs when water is not flowing freely due to atmospheric pressure under the influence of gravity, but when water flow is driven by at potential gradient (Knight, 2006). In terms of subglacial channels the pressure of the water under the ice mass can cause water to flow uphill giving the channel a humped profile. (Singh, Singh & Haritashya, 2011). Thermal regime influences the temperature of the ice. It is a function corresponding with air and ground temperatures as well as geothermal heating that controls whether the glacier will be warm based such as the Alps in temperate regions, or cold based like many in Antarctica. It is this difference in temperature that occasionally leads to the formation of either lateral or subglacial channels (Davies, 2014)

Their capability in reconstruction

Palaeo ice sheet reconstruction of glacier extent and dynamics is off great importance to scientists because it gives them an idea of the climatic sensitivity of past glaciers and the role of erosional and depositional process caused by meltwater flow. This flow can influence the transfer of ice mass which in turn will enable scientists to determine glacial retreat history (Carr & Coleman, 2007). Other aspects of glacial reconstruction that meltwater channels provide is the thermal regime and patterns of former ice streams. In general the task of reconstruction will allow for a better understanding of the earth system, former climate and future sea level change.

The use of meltwater channels in the past has enabled successful reconstruction of the Pleistocene British Ice Sheet. By mapping channel distribution these channels have showed past glaciological conditions, the process of deglaciation and identified different channel types and flow patterns (Singh, Singh & Haritashya, 2011).

Landforms associated with meltwater channels can indicate glacier retreat history as they are usually formed during the retreat stage. Erosional landforms include the channel itself, glacial lakes and fossil shorelines while depositional landforms include outwashed fans and eskers. Together with ice dynamics such as glacial lineations and moraines, these give an idea of the deglaciation stage particularly the direction of retreat (Marigold, 2012). It has also been discovered that channels flow parallel to ice flow (Unknown, 2014). As a glacier looses mass it can erode into the landscape to create a notch. As the glacier retreats this process continues to create a sequence of notches to reveal lateral channels within the landscape as well as provide an indication of the nature of retreat. This process is not associated with modern fluvial drainage and it has generally been found that glacial meltwater channels don’t follow the same flow pattern of modern channels today. This is shown in figure 5. A reason for this as mentioned is the humped profile of meltwater channels caused by the flow of pressurised water (Benn & Evans, 2010)

The thermal regime of glaciers can also be identified by meltwater channels. As identified cold based glaciers are generally associated with lateral meltwater channels. These channels can also tell scientists the retreat of margins and former polythermal glacier margins. In Canadian and Greenland High Arctic these have been extensively marked by lateral meltwater channels and have shown glacier recession (Benn & Evans, 2010). On the other hand warm based glaciers are characterised by sub glacial meltwater channels. Glacial lakes can also be associated with warm based glaciers and are an important feature in identifying glacial retreat history (Marigold, 2012).

Conclusion

Despite the reconstruction potential of meltwater channels, there are some limitations of their use in the models and methodology of reconstruction. A study using remotely sensed data and digital elevation models was conducted in northern Finland, that used meltwater channels to indentify fossil glacial lake shorelines. The remote sensing data did not recognised any shorelines even though this area is known to consist of them (Marigold, 2012) Data on reconstruction is scarce with reliance on the evidence collected and interpreted by other scientists. Glacial reconstruction of the iconic Laurentide Ice Sheet has been conducted in the past by Boulton and Clark in 1990 with the use of Landsat MSS imagery and aerial photographs. They were able to reconstruct the evolution of the ice sheet but made no effort to include marginal channels and meltwater landforms in its reconstruction (Knight, 2006). With the complexity of glacier reconstruction it is evident that plausible data collection must be carried out in a rigorous manner in order to be reliable. The vast potential of the use meltwater channels in reconstruction must be considered in future studies.

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