New device able to measure multiple frequencies
For the first time, sea ice physicists at the Alfred Wegener Institute (AWI) have developed a new method of efficiently measuring the distribution and thickness of a sub-ice platelet layer in the Antarctic.
This layer, called platelet ice, comprises delicate ice crystals beneath the coastal sea ice. Although it was discovered more than a century ago, very little is known about it. Nevertheless, the platelet ice is of central importance as it influences sea ice properties and the associated ecosystem in various ways. Thus, it serves as an indicator of the state of the melting ice shelves.
Every year, the ocean surrounding the Antarctic continent freezes to form the sea ice. However, this is not the only ice being formed. Beneath the surface, at the same time, a layer of loose ice crystals is formed. Its appearance is similar to the crushed ice found in cocktail glasses – the difference being that the ice crystals in the platelet layer are disc-shaped and as thin as one millimetre.
In past decades, scientists discovered that platelet ice plays a significant role in the sea ice mass balance in some regions around the Antarctic. The algae that thrive on the platelets serve as food for the many small crustaceans and fish that seek shelter between the platelets from predators like seals and penguins.
Because it is concealed beneath the sea ice, many scientists usually come across platelet ice by accident – for instance, when they drill through the sea ice to measure its thickness. Now, scientists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, the Jacobs University Bremen and Uppsala University have developed an efficient method of determining the distribution, thickness and volume of platelet ice over a large area. Their results have been published in the latest issue of Geophysical Research Letters.
Previously, electromagnetic (EM) induction sounding devices were used to measure the electrical conductivity of the subsurface layers. As the electrical conductivity of the solid sea ice differs from that of the salty seawater below it, the transition between the two can be pinpointed.
However, the transition between the sea ice and platelet ice is less distinct, and conventional EM methods which used just one frequency were inadequate. Scientists then invented a ground-based, multi-frequency EM induction sounding device that used several different frequencies, allowing them to accurately pinpoint the transitions without having to drill holes through the ice and using measuring tapes.
Using the new device, the scientists conducted surveys in the frozen Atka Bay, which is located in the Weddell Sea, near the German research station Neumayer III. The device was placed on a kayak which was attached to a snowmobile. They then repeatedly drove across the sea ice of the bay for weeks, several hours at a time.
“One of the things we noticed was that the evolution of the platelet layer has an annual rhythm,” said AWI sea ice physicist and co-author Dr Mario Hoppmann. The platelets start to accumulate in June, as the Antarctic winter starts. The platelet layer grows over the course of the winter, until it is several metres thick in December. After this time, it shrinks during the summer.
The researchers believe that platelet ice plays an important role in the ice regime of the Antarctic. This is because while the sea ice in Atka Bay has an average thickness of two metres, the thickness of the platelet layer beneath averages five metres over the course of a year. In some places, it is even ten metres.
This means that a considerable amount of ice exists in the form of platelets. According to Hoppmann, “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.”
Link to study: onlinelibrary.wiley.com/../full