Iceberg Alley — what lies beneath?

Davis research station is renowned for its view of ‘Iceberg Alley’, where hundreds of icebergs sit, seemingly unchanged, year after year. Now, seabed mapping by a team of scientists, hydrographers, engineers and coxswains, has found that a barrier of shallow underwater banks prevent these frozen sentinels from setting sail.

During the 2016–17 Antarctic summer, a team from Geoscience Australia, the Royal Australian Navy and the Australian Antarctic Division undertook a comprehensive hydrographic and seabed mapping survey in the waters adjacent to Davis research station and along the coast of the Vestfold Hills. The work was conducted as part of a collaborative project between the three organisations, which aims to map and characterise the seabed of the nearshore environment adjacent to Australia’s stations. The project builds on previous work conducted at Davis in the 2009–10 season and at Casey in 2013–14 and 2014–15 (Australian Antarctic Magazine 27: 6–8, 2014).

Using multibeam echosounders and sub-bottom profilers, which use sound to visualise the shape and depth of underwater features, along with sediment grabs and underwater cameras, the team now have a detailed picture of what the seabed off Davis looks like, and its geology and biology. This information will be used to understand the geological history of the area, including modern sediment processes, as well as the influence it has on marine life. The new data also help explain the presence of the spectacular icebergs sitting in Iceberg Alley.

The main aim of the project is to obtain bathymetry (depth) data to compile nautical charts for navigation and safety of life at sea. However, the baseline geoscience data can be used for many other applications. For example, the data is used to assist decision-makers responsible for developing environmental management strategies, including Marine Protected Areas, scientific studies on oceanography, paleoclimate, biodiversity, and ice sheet dynamics, meeting international obligations such as International Hydrographic Office and Antarctic Treaty System protocols, and aiding effective logistical operations.

The nearshore waters around Australia’s Antarctic research stations are of interest because they are high-use areas. Davis is visited twice per year by Australia’s icebreaker, Aurora Australis, and hosts frequent international visitors, such as the Chinese ship Xue Long. There are also numerous small boat operations in the area. Yet there are large areas of uncharted waters and these are often very shallow — as protruding rocks reveal. It is also an important biological site and key habitat for birds and mammals, with numerous penguin colonies, seal haul-out sites, and seabird nesting sites.

While these types of surveys have been done by Geoscience Australia and the Navy in the past, this was the first time such as survey was conducted using two workboats operating in tandem. This allowed the survey team to achieve maximum survey coverage during the short field season, and also meant that the boats could act as Search and Rescue watch for each other, allowing the survey team to go further from station than before.

The main focus area for the survey was the shipping channels coming into Davis, as well as Ellis and Long Fjords (see map). Previous work has identified a number of interesting features in these areas, including unique habitats such as polychaete (marine worm) reefs in the mouth of Long Fjord and high suspended sediment loads in front of Ellis Fjord. It was also thought that these areas offered the best possible location to identify glacial features that would provide insight into ice dynamics in the area.

Once underway, the teams covered a lot of ground (actually, a lot of seabed!). In 23 survey days the team acquired approximately 130km2 of multibeam bathymetry and backscatter data, 18 sediment samples, 25 camera deployments (co-located with sediment samples) and 33 km of sub-bottom profiles.

The bathymetry data reveal a picture of the seabed in incredible detail. Overall, the seabed environment looks remarkably similar to the terrestrial environment onshore in the Vestfold Hills. It is characterised by flat plains interspersed with hard shallow banks and sediment-filled basins, similar to the hills and lakes found onshore. The sub-bottom profiles reveal the basins are infilled with only a few metres of soft marine muds, and the plains are draped with a thin veneer of sediment, indicating there is little modern sediment input into these waters.

Throughout the nearshore area, and particularly to the west of Gardner Island, are numerous hard shallow banks. These banks vary in size and depth but they all sit proud of the surrounding seabed and often rise to within metres of the surface. These banks essentially form a barrier to icebergs passing through the area with the currents from the north. Larger bergs can get wedged against these banks for years, possibly decades, until they melt, roll and continue to move south. Some of the icebergs mapped in the 2009–10 season are still in the same location seven years later.

As the icebergs move across the seabed they leave scour marks in the soft sediment. These range from enclosed, rounded wallow marks 10–40m diameter and one metre deep, through to meandering tracks up to one kilometre long, often in the direction of the prevailing local currents.

While the waters around Davis preserve few glacial features, the team mapped one large moraine (an accumulation of glacial debris) in approximately 50m of water. The crescent-shaped moraine is 1.2km long and about 50m wide and five metres high, and sits on top of a shallow bank. It is similar in size and form to moraines found in the Vestfold Hills, including one at the mouth of Heidemann Bay (see photo).

Initial observations of the underwater video reveal a diversity of benthic marine communities across the survey region, including mixed macroalgae communities on the shallow banks, complex sessile invertebrates on the rocky slopes, and sparse motile invertebrates in muddy basins. More detailed data analysis will be undertaken to improve our understanding of the marine life in the area and its relationship with the seabed, including mapping the distribution of macroalgae communities that are commonly found on the shallow banks.

The survey was an exciting opportunity for the team to explore the uncharted waters around Davis and to find out what lies beneath Iceberg Alley.

Jodie Smith
Geoscience Australia