A “palaeo” perspective on Patagonian glaciers

By Jacob Bendle, 1st year PhD student

Patagonia boasts breathtaking and beautiful scenery that excites and intrigues geographers and non-geographers alike. However, Patagonia’s vast landscapes also contain important strands of evidence with which we can unravel the history of past glaciers and ice sheets.

This landscape provides the basis for my PhD research into the glacial history of Central Patagonia.

So, why should we study glaciers that no longer exist, in a remote and often inhospitable (it can be incredibly windy!) part of the globe?

Well, Patagonia is currently home to the North and South Patagonian Icefields (see picture below), which together form the largest expanse of land-based ice in the Southern Hemisphere.

Note the emphasis on the word currently – as these icefields are shrinking at an alarming rate, not only putting downstream communities at risk of catastrophic flooding, but also contributing to global sea level rise.

OLYMPUS DIGITAL CAMERA

Picture of Glacier Exploradores, an outlet glacier of the North Patagonian Icefield. Notice the proglacial lake in the foreground, which is developing as the glacier is melting and downwasting.

Unfortunately, we cannot currently predict with certainty the future behaviour of these (and other) icefields because historical records of glacier change are limited in terms of the time (only the last few decades in many cases!) and area they cover.

To overcome this problem, we can turn to the geological, or “palaeo”, record (i.e. deposits laid down in the past that contain information about the history of Earth surface processes) in order to:

1) better understand how glaciers evolve over longer timescales (e.g. over thousands of years)
2) better predict the future behaviour of glaciers to climate change

For this reason the initial focus of my research has been to collect evidence of glacier extent and behaviour for a period of deglaciation that has occurred in the past.

This evidence takes the form of glacial landforms. These are deposits that were formed at the Earth’s surface by the action of glaciers, and are commonly found littering the valley-floor of formerly glaciated regions. For my study, I am working within the Lago General Carrera (Chile) valley (the Argentinean name for the valley is Lago Buenos Aires), which was occupied by a large outlet glacier (i.e. a fast-flowing glacier that drains the inner portion of ice sheets) ~20,000 years ago.

To identify glacial landforms, I study satellite images and digital elevation models (i.e. 3D models of Earth’s surface terrain) to map a variety of surface features, which are often difficult to see on the ground. This desk-based mapping is then carefully checked in the field where landforms are drawn onto basemaps and their location pinpointed using a handheld GPS (in the same way that a SatNav can pinpoint a car’s location on the road).

The types of landforms I have identified (see pictures below) include moraines, meltwater channels, trimlines, and areas of smoothed bedrock. These relate to the former extent, thickness and flow patterns of the glacier. I have also mapped lake shorelines, raised deltas and terraces, and these show that a lake formed around the glacier whilst it melted, as glacial meltwater became trapped between moraines and the retreating glacier snout.

landform plate

Pictures of glacial landforms from the Lago General Carrera valley. Top left, moraines with faint shoreline etched into moraine surface. Top right, glacially-smoothed bedrock. Bottom left, meltwater channel cut into bedrock. Bottom right, raised deltas.

The landform patterns tell us that the Lago General Carrera glacier was very dynamic when it was melting. The evidence for a proglacial lake at different levels in particular suggests that the glacier could retreat rapidly during one phase (causing part of the lake to drain and lower) and become stable in another (causing a shoreline to be created).

It is one thing to understand the size and likely behaviour of past glaciers and ice sheets, but this does not tell us about the rates of change – in this case the rate of glacier melting. For this we need a timescale.

This forms the next part of my research, and will involve the detailed study of finely-layered sediments that were deposited into the proglacial lake.

The layers that make up these sediments relate to specific intervals of time – a layer forms in the summer during the glacier melt season (the lighter layer in the picture below) and is followed by a layer that forms in the winter when the lake surface freezes and fine particles settle (the darker layer in the picture below). When summed together, they equal a year’s worth of time.

Therefore, by counting the number of annual layers that occur within a sediment sequence I will be able to develop a precise timescale for glacier retreat.

This will allow us to understand how quickly glaciers may respond to changing climate, and will help in predicting the likely response of the Patagonian Icefields in the future.

IMG_1356

Picture of finely-layered sediments that were deposited into a proglacial lake. The lighter (silt) layers were formed in the summer as the glacier was melting, whist the darker (clay) layers were formed in the winter when the lake surface was frozen.

 

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