Uncharted island soon to appear on nautical charts

A 93-strong international expedition team has been exploring the northwestern Weddell Sea in the Antarctic on board the Alfred Wegener Institute's icebreaker Polarstern since 8 February 2026. In this key region for global ocean currents, the focus has been on the outflow of ice and water from the Larsen Ice Shelf and the astonishing sea ice retreat of recent years. When the research work had to be interrupted due to rough weather conditions in order to seek shelter in the lee of Joinville Island, the scientists and ship's crew were surprised by the sudden appearance of an island that had previously only been marked as a danger zone on the available nautical charts.

“On our route, the nautical chart showed an area with unexplored dangers to navigation, but it wasn’t clear what it was or where the information came from,” reports Simon Dreutter from the Bathymetry section at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). As a specialist in underwater mapping, this aroused his curiosity. “I scoured all the coastlines we had here in the bathymetry lab and went back to the bridge. Looking out of the window, we saw an ‘iceberg’ that looked kind of dirty. On closer inspection, we realised that it was probably rock. We then changed course and headed in that direction and it became increasingly clear that we had an island in front of us!”

The navigators on the bridge drove the Polarstern carefully towards the island, always with at least 50 metres of water under the keel. This allowed the icebreaker to approach it to within 150 metres, circumnavigate it and survey the seabed with the onboard multibeam echo sounder. A drone was also used and the image data was analysed photogrammetrically to obtain an elevation model and a georeferenced aerial image to measure the coastline. This was the first time the island had been systematically surveyed and recorded. The result: the island is about 130 metres long, 50 metres wide (slightly longer than the Polarstern with its 118 metres and about twice as wide) and protrudes about 16 metres out of the water.

It is unclear to the experts why the island is marked as a danger zone on the nautical chart but not as a coastline in other data sets and why the position shown on the nautical chart is about one nautical mile off the actual position. On the satellite images analysed, the island could hardly be distinguished from the numerous icebergs drifting around in the immediate vicinity due to its ice cover.

As there is no official international registration of the island by name, the task now is to go through the naming process for such a discovery. Dr Boris Dorschel-Herr, head of AWI bathymetry and also on board the Polarstern, already has experience with this: In 2014, he and his team had ensured that two underwater mountains were plotted on the nautical charts of the South Atlantic and the Weddell Sea. The team will publish the exact position of the island once the naming process is complete and will also ensure that the information is added to international nautical charts and other important data sets. Such information is essential for bathymetric sea floor maps such as IBCSO (International Bathymetric Chart of the Southern Ocean) in particular, as the thin coverage of measurement data and interpolation means that such unmapped objects are simply erased.

The bathymetry team works closely with other research groups on board, for example with physical oceanography. This enabled the scientists to track various water masses along several sections from the deep sea to the continental shelf and investigate the colonisation of the sea floor. In doing so, they gained important insights into the decline of Antarctic deep water in comparison to the long-term data collection that the AWI has been conducting in the region via oceanographic measurements as part of the Hybrid Antarctic Float Observing System (HAFOS) since 2002. In addition, the outflow paths of cold water from the Larsen Ice Shelf have been narrowed down. These water masses have a significant influence on global ocean currents and the melting of sea ice, particularly on the continental shelf.

Unlike the sea ice in the Arctic, the Antarctic sea ice was considered relatively stable for a long time. However, the summer sea ice extent in the north-western Weddell Sea has declined sharply since 2017, presumably as a result of warmer surface water. Prof Dr Christian Haas, head of the Polarstern expedition and AWI Sea Ice Physics, comments on the initial results of SWOS (Summer Weddell Sea Outflow Study): “The ice thickness showed great regional variability. On the western, shallow continental shelf in particular, the ice was up to four metres thick, which we can attribute to strong deformation caused by the tides and the proximity to the coast. The ice further east came from the large Ronne and Filchner ice shelves and was less deformed with thicknesses of around one and a half metres.”

Overall, the sea ice showed surprisingly strong surface melting, which mainly affected the snow cover and the uppermost ice layers and led to almost Arctic conditions, where the ice is covered with many melt ponds. Christian Haas reports: “Although we only found very few melt ponds, the ice was often almost free of snow and had a bluish or greyish surface. Thanks to novel measurements of the water directly under the ice using turbulence and biological probes, we found larger quantities of sweet meltwater in and under the ice. This has a strong effect on the biological colonisation of the ice and the interactions with the seawater under the ice, because such freshwater lenses keep the heat from the ocean away from the sea ice.”

Future analyses and modelling will show what contribution the organisms living in and under the sea ice make to the carbon cycle in the Southern Ocean, for example. However, the researchers will only carry these out after the expedition, which is scheduled to end on the Falkland Islands (Malvinas) on 9 April 2026. From there, the Polarstern will start its transit across the Atlantic and is expected to return to its home port of Bremerhaven in mid-May.