Seismic profiling in the Bellingshausen Sea

When the RV Belgica got stuck in sea ice over a century ago, her crew could not possibly know that a morphological structure would be named after the vessel in the region in which the ship was trapped. Even more surprising, that the area would be almost completely free of sea ice 125 years later. The ‘Belgica Trough’, as mentioned in a previous blog post (“Belgica Trough and the West Antarctic Ice Sheet”, 21/02/2023), is the largest submarine trough carved into the seafloor of the Bellingshausen Sea. This morphological feature suggests the presence of extensive glaciers in the past, which advanced and retreated over hundred-thousands or even some millions of years and was deeply affected by the dynamics of the West Antarctic Ice Sheet (WAIS). An advance in the WAIS has been related to a cooling of the ocean water in the region, whilst a retreat of the ice sheet has been associated with warmer conditions.

Despite being able to relate the advance of an ice sheet with colder periods and its retreat to warmer ones, the Antarctic system is complex, and its oceanographic, glacial and geological dynamics are difficult to describe or forecast.

As explained in a previous blog post (“Belgica Trough and the West Antarctic Ice Sheet”, 21/02/2023), bathymetric data can give us an idea of how the WAIS behaved in the last few thousands of years, which can help us forecast future conditions. However, we also need data from the last few millions of years to better understand how the dynamic ice sheet cover has affected the structure of the Antarctic ocean floors. So, how can one look into an even further past? How can we look into the deeper past for clues of what might happen in the future?

The answer the scientists of expedition PS134 can provide lies in using seismic profiling. The seismic team, along with the crew of Polarstern, withstood the cold Antarctic temperatures to deploy their equipment and obtain data that might help better understand how the WAIS and the Belgica Trough have been influenced by past glaciation and deglaciation periods.

The 3000 meter long seismic hydrophone cable (“streamer”) is spooled from the winch to the water. The team installs depth-control devices, also called “birds” due to their wings, at particular intervals along the streamer (photo and description by Karsten Gohl).

This type of geophysical surveying uses a seismic pulse source and a long cable of seismic hydrophone receivers able to capture this signal. The seismic source uses compressed air to emit energy which then penetrates the subsurface. Part of this energy is reflected by changes in the sediments below the seafloor. To receive this reflected energy, a 3000 m long cable was used. This equipment is called “streamer” and it is composed of hydrophones, which are capable of capturing the reflected signal. The received signal is then recorded in the seismic lab on board. Via this type of profiles, the seismic team has been able to visualize the deep sedimentary structures formed during the last few million years to analyse how oceanographic, geological and glacial changes have modified the submarine landscape.

This partial image of a yet unprocessed seismic profile shows a nice sequence of sedimentary rock layers on the continental shelf of the Bellingshausen Sea. These layered structures provide indications how and where early ice sheets pushed and deposited sediments. As soon as sophisticated data processing will be performed in the institute to remove the sea-floor multiples, sediment layers can be imaged down to several kilometers deep into the sub-bottom. (Description by Karsten Gohl.)

The region in which the Belgica was frozen in ice is of particular interest to the seismic team because this area is considered to have been affected by oceanographic conditions in the past, potentially by the intrusion of warm deep water. Previous seismic research has suggested that the intrusion of a warm deep ocean mass might have prevented the advance of the West Antarctic ice sheet about 34 million years ago during the so-called Eocene-Oligocene Transition.

The area in which the Belgica was trapped 125 years ago is presently affected by the intrusion of warm Circumpolar Deepwater (CDW). The development of this water mass and how it affects the WAIS is not yet fully understood, however, current research suggests that a retreat of the WAIS might be linked to modifications in strength, intensity, and pathways of the CDW.

In the seismic lab, Ingra, Pascal, Gabi and Thorsten check the control and survey data of the seismic profile on the monitors (photo and description by Karsten Gohl).

With their research the scientists of PS134 hope that evidences of how the WAIS has developed in the past during periods of cooling and warming might help us understand how the region reacts to climate changes. This will give us a hint of why the area where the Belgica was stranded 125 years ago is ice-free in the Antarctic summer of 2023.

This blog post was written by Ingra Barbosa, geophysicist at the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (Bremerhaven, Germany), and student-investigator in the PS134 geophysics team.

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