PS111 – Weekly report #5 | 17 - 23 February 2018

Ice and water in the Filchner Depression

[26. February 2018] Favourable ice conditions and stable weather with southwesterly winds enabled Polarstern to go far south into the Filchner Depression and to cross directly in front of the Filchner Ice Shelf close to 44° W this week.

On February 20th, the oceanographic program was adapted at short notice owing to the prevailing weather and ice conditions: the polynya in front of Berkner Island reopened so that a CTD section could be completed at this position.

Fig. 1: The CTD brings 288 litres of water samples from various depths in the ocean onto the deck. Their physical, chemical and biological properties are studied by the different research groups on board. The ADCP (Acoustic Doppler Current Profiler) measu (Photo: Alfred-Wegener-Institut)

Fig. 2: Profile of the water column showing temperature (red), salinity (green), oxygen (brown) and turbidity (blue). The bar indicates the Ice Shelf Water with temperatures down to -2.29°C. When such cold water is brought up to the surface, ice crystals (Graphic: Alfred-Wegener-Institut)

Fig. 3: Marcus Huntemann and Nicolas Stoll from the sea-ice group preparing a snow buoy for deployment. (Photo: Alfred-Wegener-Institut)

Fig. 4: Sea-ice researcher Stefanie Arndt analysing the grain size distribution in the snow cover of an ice floe. (Photo: Alfred-Wegener-Institut)

Three further sections across and along the Filchner Depression complete the oceanographic data set that comprises measurements of a number of parameters: salinity, calculated from conductivity, temperature and pressure. The conductivity probe is calibrated daily, allowing salinity to be determined with an accuracy of two thousandths; temperature is measured with an accuracy of one thousandth of a degree. To ensure the highest possible precision, temperature and salinity are measured in duplicate, as a high degree of accuracy is required to identify the different water masses in the ocean. Additional sensors determine the concentration of dissolved oxygen, the turbidity of the water via light transmission and the fluorescence of chlorophyll. The latter is contained in phytoplankton and therefore its concentration is a measure of primary productivity. Two ADCPs (Acoustic Doppler Current Profilers) determine the currents above and below the CTD, making use of the Doppler effect on sound waves reflected from particles in the water.

During a CTD cast, water samples, 12 liters each in volume, are retrieved at various depths selected by the research groups on board. Overall, the CTD brings 288 litres of water samples from up to 24 different depths back onto the ship. On February 19th, a record temperature of -2.290°C was measured with the CTD in the Ice Shelf Water (ISW) flowing out from beneath Filchner Ice Shelf. The complete CTD profile along the Ronne-Filchner ice shelf edge provides a rare dataset that could only be obtained because of the favorable wind and sea ice conditions.

While for many other working groups the growing sea-ice cover along Polarstern’s track can pose a challenge to their work, the sea-ice group is happy about the plethora of sea-ice floes now surrounding the ship in late summer. The work on the sea-ice is as versatile as the ice itself. In addition to snow and ice thickness measurements, the small-scale internal structures of the seven ice floes sampled so far have been analysed. The drilling of ice cores allows vertical profiles of temperature, density and salinity to be measured. Moreover, the snow cover is closely inspected using a magnifying glass: the examination of the grain size distribution in the snow cover provides important information about internal melting and freezing processes. Seasonal changes in snow and ice thickness are detected by autonomous platforms (buoys) deployed on the floes. They will drift with the floes through the Weddell Sea and send the recorded data via satellite directly back to the institute. On this expedition, the buoys are decorated with pictures painted by children from all over Germany. At home, the kids are following the exciting journey of “their" buoy online.

With a contribution from stafanie Arndt / AWI on the sea ice research program.

Best wishes from all members of PS111.

Dr Michael Schröder

Chief Scientist