PS101 – Weekly Report No. 5 | 16. October - 23. October 2016
Homebound through the ice.
During the sixth week of expedition PS101 we are making our way home through the ice. A FRAM “superbuoy” station needs to be rescued before we leave the ice.
There was a good 2,000 nautical miles distance between our investigation area, the seamounts of Gakkel Ridge at 86°N and 60°E, and Bremerhaven, the port of RV Polarstern. For about 350 miles of this distance we had to make our way through the thickening ice. Early on the afternoon of the 11th October we left behind the hot vents and sponge gardens of Gakkel Ridge to head towards Svalbard. Not only for a punctual arrival in Bremerhaven, but also because we had a few scientific tasks to fulfill. The first of these was the recovery of equipment making up a large ice buoy station of our FRAM program "Frontiers in Arctic Marine Monitoring", the second being to collect and transport fresh marine sediments from Yermak Plateau for our foraminifera researchers at AWI.
Snow, fog and the increasing darkness reduced visibility greatly during transit, so that we could not use helicopters for the reconnaissance of leads between the ice floes. Using satellite data for navigation, the microwave data showed a near complete ice coverage along our intended course, but the high-resolution radar images of ESA's satellite Sentinel helped us find passages through freshly overfrozen water leads around the growing ice floes (Fig. 2). In addition to the excellent experience of our nautical officers with navigation through the sea ice, we could use our new sea ice geo-information system IceGIS, and were able to make around 4-5 knots through the ice even at this late time of the year, and we did not get stuck once during the transit.
After arriving in the vicinity of the „superbuoy“ station we had to search for its exact location. The buoy was deployed in summer 2015 during the expedition PS94. Several autonomous ice buoys were placed on a thick flow to move with the transpolar drift and to send data for weather, sea ice and ocean research (Fig. 3). These instruments are increasingly used to obtain data when ships cannot be operated in an area. It remains very difficult to find back such equipped floes and recover the buoys from within them, hence the instruments are usually left to sink or float when the ice floes reach Fram Strait and ultimately melt. As these systems can send their data via satellite for periods of 1-2 years, they complement shipboard research programs and send unique data during the fierce autumn and winter seasons.
It was especially relevant to reach this particular superbuoy station, as several instruments sent only their positions via satellite, but not the valuable sensor data which remained locked within their internal memory systems. The need for year-round synchronous data collection is great; hence we did not want to give up on the important information held by the buoy station if retrieving it was a possibility. As the superbuoy station was not too far from our transit route anyway, we decided to attempt the recovery. The hourly transmitted position data and the characteristic look of the big ice floe made it easy to locate and find the station, but as it was one of the last multiyear ice floes, its thickness of around 2 m and because of the strong compression of other floes against this one, we had to stop the ship at a distance of approximately 2 km away from where we located the instruments.
When we arrived weather had improved with little wind and snow. Hence the team of sea ice physicists started the search right away, using skidoos to reach the target location. It was not easy to detect the instruments which were buried in snow and ice after over a year of sea ice drift. After about 3 hrs of search, new positions were available via satellite, and using a drift prediction, the lookouts on board Polarstern was able to guide the team on the ice via radio to the exact location. Three of five devices were brought back from the ice, two were too deeply frozen into the floe – but 100% of all data were recovered from the station, which was a real scientific gain for the FRAM program (Fig. 4 +5).
Two final tasks were completed within the next 24 hours. First an OFOS dive at the ice margin north of Svalbard was carried out. We searched out a relatively steep part of the Nansen basin slope in a few hundred meters water depth, to compare the benthic communities there with those of the Karasik seamount, lying in more oligotrophic waters. The OFOS images showed a lively colorful assemblage of filter feeders, including sponges, starfish, clams and different types of cnidaria including corals (Fig. 6+7).
At first sight, the community looked more diverse compared to the sponge reefs of Karasik seamount. But it had substantially less biomass per area. It remains mysterious what nourishes the giant sponges and their associates at Karasik. We look forward to analysing our samples from the area, to solve this enigma. But at least good news for the Norwegian deep water corals, sponges and fish at 80°N: the ice-covered area we surveyed showed no traces of benthic trawling yet. The very last station of the expedition PS101 took place west of Svalbard at the deep margin of the Yermak Plateaus. Here we sampled the seafloor with a Multiple corer (Fig. 8) to retrieve surface seafloor samples for the AWI biogeochemists, who study foraminifera. In contrast to the ice-covered regions, we noted many traces of trawling down to 850 m water depth.
On 17th October all station works were completed, and only the underway measurements were continued to the morning of the 22nd October, one day before port. In the framework of the FRAM program we tested and calibrated various autonomous sensors and samplers, including analyses for the ground truthing of remote sensing data by satellite, for example with regard to ocean color used to assess primary productivity.
The transit to Bremerhaven ended in the morning hours of the 23rd October, Polarstern arrived at the pier at 08:00 am. The expedition was a great success for all scientific participants, who are happy to be home with large quantities of images and samples from the central Arctic Ocean. In the name of the science crew (Fig. 9) we thank once more Captain Schwarze and his crew for their excellent support with work at sea during Expedition KARASIK (PS101).
Good-bye to all of our loyal readers and many greetings, from the science crew of PS101 and Antje Boetius.