Sea ice physicists of the Alfred Wegener Institute (AWI) have developed a new method that allows them for the first time to efficiently determine the distribution and thickness of what researchers call a sub-ice platelet layer. This several metre thick layer of delicate ice crystals is predominantly found beneath coastal Antarctic sea ice, and at present knowledge about its spatial distribution is very limited. This phenomenon, which is also known as platelet ice, is of central importance in the coastal regions of the Antarctic, influencing sea ice properties and the associated ecosystem in various ways, and serving as an indicator for the state of melting ice shelves. The researchers published their results with open access in the current issue of the journal Geophysical Research Letters.

Infographic: Formation of the platelet layer in the Southern Ocean. Graphic: AWI, Mario Hoppmann
Infographic: Formation of the platelet layer in the Southern Ocean. Graphic: AWI, Mario Hoppmann

Every winter in the Southern Hemisphere, the ocean around the Antarctic continent freezes. The “normal” sea ice formed on the surface of the Southern Ocean, however, is not the only ice that forms in the sea. During the same period, a remarkable habitat develops hidden beneath the solid sea ice cover: a several metre thick layer of loose ice crystals. Some areas underneath sea ice in coastal Antarctica then resemble a cocktail glass filled with crushed ice – the difference being that the crystals in this layer grow to disc-shaped, millimetre-thin platelets.

Close-up of a ice platelet, taken from the Atka bay, Weddell Sea, Antarctic. The Platelet layer consists of individual crystals up to 20 cm in diameter. Pho­to: AWI, Mario Hoppmann
Close-up of a ice platelet, taken from the Atka bay, Weddell Sea, Antarctic. The Platelet layer consists of individual crystals up to 20 cm in diameter. Pho­to: AWI, Mario Hoppmann

This phenomenon was discovered by researchers more than a century ago, but for a long time, little was known about this peculiar ice type. In the past decades, researchers have discovered that platelet ice plays a significant role in the sea ice mass balance in some regions around the Antarctic, and that it represents a unique and productive habitat. Countless algae thrive on the platelets, which are food for a myriad of small crustaceans and fish that seek shelter between the platelets.

Knowledge about the thickness and extent of platelet ice is still very limited, because of its “hidden” nature. Most of the time, researchers only came across it by chance – for example, while drilling through the sea ice to measure its thickness. A team of researchers of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), the Jacobs University Bremen and Uppsala University have now succeeded to develop an efficient method to determine the distribution, thickness and volume of platelet ice over large distances.

Screenshot of an underwater image of the platelet layer in the Atka bay, Weddell Sea, Antarctic. The platelet layer consists of individual crystals up to 20 cm in diameter. Photo: AEWI, Tauchzentrum
Screenshot of an underwater image of the platelet layer in the Atka bay, Weddell Sea, Antarctic. The platelet layer consists of individual crystals up to 20 cm in diameter. Photo: AEWI, Tauchzentrum

As the scientists report in a recent issue of the journal Geophysical Research Letters, they used a so-called multifrequency EM device to determine the platelet ice volume. EM is short for "electromagnetic induction sounding", a geophysical method sensitive to the electrical conductivities within the subsurface. The electrical conductivity of solid sea ice, for instance, differs significantly from that of the salty seawater underneath. This means that, using the EM device, scientists are able to identify the transition from sea ice to seawater, thus allowing them to calculate the ice thickness.

However, the transition between the solid sea ice and the loose platelet layer is much less pronounced. Conventional EM methods that only use a single frequency are unable to distinguish the platelet layer from sea ice or seawater. Using several different frequencies, scientists are now able to reliably determine these transitions without having to drill holes into the ice and measuring the thicknesses using measuring tapes.

For measuring the thickness of the platelet layer, scientists mounted the multi-frequency EM instrument in a kayak and pulled it over sea ice with a skidoo. Photo: AWI, Mario Hoppmann
For measuring the thickness of the platelet layer, scientists mounted the multi-frequency EM instrument in a kayak and pulled it over sea ice with a skidoo. Photo: AWI, Mario Hoppmann

"We were really surprised by our own results," says AWI sea ice physicist and co-author Dr Mario Hoppmann. "With our new approach, we weren’t only just able to determine the thickness of the platelet layer. We were even able to calculate the fraction of ice within this layer by subtracting the volume of sea water in between the platelets."

In order to collect as much data as possible, the researchers conducted several surveys across large parts of the frozen Atka Bay. The bay is located in the Weddell Sea, near the German Antarctic research station Neumayer III. The researchers placed the multifrequency EM instrument in a kayak, which in turn was attached to a snowmobile. They moved the tandem across the sea ice of Atka Bay for many days and several hours at a time. "One of the things we noticed was that the evolution of the platelet layer has an annual rhythm," says Mario Hoppmann. In June, at the beginning of the Antarctic winter, platelets begin to accumulate under the sea ice. Over the course of the winter, the layer grows; by the end of the winter, in December, it is several meters thick, after which it shrinks again during the summer.

The researchers are convinced that platelet ice plays an important role in the ice regime of the Antarctic. After all, the seasonal sea ice in Atka Bay freezes to an average thickness of two metres in the winter. The platelet layer underneath, however, reaches an average thickness of five metres over the course of a year. In some places it was up to ten metres thick. This means that a significant amount of the ice exists in the form of platelets. "To understand the situation of the Antarctic sea ice and to assess a possible influence of climate change, it is likely that more account must be taken of platelet ice," says Mario Hoppmann.

Source: AWI, Bremerhaven