PS111 – Weekly Report No. 4 | 10 - 16 February 2018

From the Ronne Ice Shelf Polynya

[19. February 2018] During this week we completed a transect along the Ronne Ice Shelf front, extending from 50°W to 60°W. Every instrument on board has been used.

The biologists, geologists, and oceanographers acquired numerous samples from a total of 23 stations. The CTD and onboard pump provided sea water, the bongo and multi-net delivered biological samples, and the multi-corer and gravity corer transferred sediments from the sea floor to the ship’s deck. Some samples were distributed among the different disciplines, to be analyzed in separate labs and lab-containers, while others were sealed in plastic bags or bottles for safe transport back home.

Fig. 1: Manta at 400-m depth at ROV station PS111_36-02. The red dots serve as a reference scale 10 centimetres apart. (Photo: Alfred-Wegener-Institut)

Fig. 2: Geological outcrop in the mountains of the Antarctic Peninsula. (Photo: Alfred-Wegener-Institut)

Fig. 3: A halo above the Ronne Ice Shelf polynya. (Photo: Alfred-Wegener-Institut)

It is amazing that the harsh environmental conditions near an ice shelf edge at around 75°S can still sustain life. Here, air temperatures around -18°C, winds of 8 bft, and a wind-chill of -35°C are quite normal for an austral summer season. A gap in the ice shelf front forms an inlet that serves as a sunny beach for some seals, the water column contains lots of plankton, and ice floes are very brownish at the base due to algae growth. The weather conditions are favorable for a geologically/biologically motivated helicopter flight - we have two helicopters on board. However, during the flight, weather conditions change such that the biologist cannot tag any seals and the geologists are left without any rock samples from the nearby mountains of the Antarctic Peninsula. The success of everyone’s work strongly depends on sea ice and weather conditions.

The ROV (Remotely Operated Vehicle) completed three missions at the edges of the Ronne Ice Shelf and iceberg A23-A to observe life in the water column and at the sea floor. Countless ice platelets grow from the ice wall or form within the rising water, which is at sub-freezing temperatures. The shelf water gets colder and fresher as it melts meteoric ice at great depth. As it rises along the sloping ice shelf base, cooled to temperatures as low as -2.5°C, the pressure release and the consequent increase in the freezing point cause the water to become super-cooled. Ice crystals then form and either settle onto the ice shelf base or float out from beneath the ice shelf and thicken the sea ice in front.

Heavy sea ice conditions and strong winds allow us to stay in the coastal polynya for only five days. On 14 February Polarstern arrived at the eastern edge of Ronne Ice Shelf where we turned north heading for iceberg A23-A. Thirty years ago, three big chunks calved from the Filchner Ice Shelf. All touched ground on the shallow Berkner Bank, but two left and now only A23-A remains. From its position, the iceberg still controls the sea ice conditions in front of Filchner Ice Shelf, allowing only a narrow ice-free channel along its edge which, due to northwesterly winds, unfortunately closed as we arrived, making a shortcut into the Filchner Trough impossible. However, a reconnaissance flight with the helicopter revealed a possible passage to the east on the northern side of the iceberg. This route allows us to extend existing CTD sections to the west, onto the western slope of the Filchner Trough. That was impossible during previous Polarstern cruises to this area due to the solid sea ice cover.

Best wishes from all members of PS111

Dr Michael Schröder

Chief Scientist