Seismic Profiling

In 2008, The British Geological Survey (BGS) took seismic equipment to Disko Bugt with Cape Farewell. The aim was to use sound waves to image the seabed, and the sediment layers beneath the seabed to a depth of ~ 300 meters, in order to understand the past history of glaciation in this area.

As glaciers and ice streams advance and retreat, they erode, re-work and re-deposit sediments. Sediments pushed out at the snout of a glacier form terminal moraines that generally run perpendicular to the direction of ice flow. These can form recessional suites of moraines that mark the retreating phase of a glacier or ice stream. Other features that can be observed are lateral or medial moraines that build up generally parallel to the direction of ice flow. Sediments will also settle out into depressions left by retreating glaciers, filling the carved out channels with layers of glacial till through to fine grained muds. This constant interplay between glaciers advancing and eroding sediments and then retreating and depositing sediments forms a sequence of layers that reveal how glacial and interglacial periods in the past have shaped the submarine landscape.

Imagine cutting down through a multi-layered cake and looking at it side on. You would see icing, sponge and filling in different layers – all very distinct from each other. In a similar way we can use soundwaves to image a slice down through the seabed, and see all the layers left behind by the interplay between ice, sea and land. Sound is transmitted into the water column by a source that is towed behind the boat (Figure 1). Some sound bounces off the seabed, whilst the remainder penetrates into the sediment layers beneath, reflecting off boundaries between sediments of a different type, size or consolidation (compaction). The returning sound waves are picked up by a series of hydrophones (underwater microphones) that receive the reflected sound signals and note the time delay between when the sound was transmitted and when it was received, giving a depth.

Figures 2, 3 and 4 show three of the profiles collected by BGS and Cape Farewell in 2008, with preliminary interpretations. They suggest that in areas previously occupied by ice streams there has now been significant deposition of laminated sediments, in areas up to 250m thick. They also reveal the possible extent in the past of ice streams and glaciers, suggesting that there has been significant ice retreat in historical times. However, these profiles cannot tell us about dates, and how quickly the ice responded to past changes in climate.

Further collaborative work is therefore required to fully understand the complexity of historical glacial behaviour in this region of the Arctic. By understanding the archive records held in the submarine environment, we can better understand the interplay between present day climate change and the potential impact on the sensitive Arctic region.

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