With geophysics and geology towards 83°05’N – the northernmost position of our expedition

A major aim of our expedition is to perform seismic profiling across the central and the southern part of the Lomonosov Ridge for imaging of sedimentary structures and supplement the profiles obtained during “Polarstern” Expedition PS87 in 2014 in the surroundings of proposed locations for future scientific drilling operations within the International Ocean Discovery Program (IODP). To improve the pre-site survey, cross-lines to existing profiles shall enhance the imaging of sedimentary strata and the depth calculation of target reflectors to be reached at the proposed IODP sites.

So, what are the geophysicists doing here onboard “Polarstern”? After a great breakfast with delicious pancakes they have started in the morning of Sep 16 to deploy the streamer (Fig. 1a) and the air-pulsers (Fig. 1b). Four air-pulsers, filled with pressure air of 150 bar, are released every 15 s. The produced sound signal penetrates the water and ground below the sea floor up to several km depths. The sound waves are reflected at different layers and these reflections recorded by a streamer - a 600 m long chain of hydrophones towed behind the ship (Fig. 2a). The recorded reflection signals image the layering and structure of sedimentary packages (Fig. 2). Best records are achieved by a constant speed of 5 Kn, which is no problem for “Polarstern” in the 1-2 m thick sea ice. Additionally, the streamer depth can be controlled and lowered to 30 m depth below thicker ice. But fortunately, we soon reach open water after our start at the Gakkel Deep, and a long survey across the Amundsen Basin, along the Lomonosov Ridge, and finally far into the Makarov Basin has been absolved.

Three days long the monotonous "wum" of the air-pulsers enjoys the geophysicists. But marine animals could be annoyed by such a noise. Thus, a "MMO-watch" (Marine Mammal Observer) has been organized by the scientists, especially the ArcTrain students (Many, many thanks to all of them!). Day and night, they spy for any marine mammals such as whales, polar bears or walruses, always ready to inform the seismic watches. In case these animals would appear in a range of 500 m around the ship, we would immediately interrupt our running seismic profiling. This fortunately has not been necessary so far as no animals have been observed during the first phase of our seismic work.

However, first of all the prerequisite for this is a precise information about the seafloor topography and the structure of the subsurface sediments. The first set of data can be obtained by a Hydrosweep bathymetry survey, the latter from Parasound profiling. Some background about how the Parasound systems works will be explained by our Parasound specialist Frank Niessen in one of the next weekly reports. Some background about the Hydrosweep system will be given by our Hydrosweep Bathy Team Melanie Steffen, Sophie Andree and Lamis Oberwinster in the following:

Our Hydrosweep shipboard swath bathymetry system is used to explore the surface under the deep sea. The ocean has a certain landscape or topography like the land, but it is hidden under vast masses of water. This topography is called bathymetry. To find its characteristics it is necessary to look through the water. Light would make it only for a few meters, but sound is very well propagated in the water column. So we choose sound and in a depth of several thousand meters it makes sense to use a deep frequency such as 15 kHz. Therefore a transducer (very fine device, which is able to send and even receive acoustic signals) is mounted under the research vessel (Fig. 3a). From the timespan it takes the signal to reach the seabed and come back, the depth can be calculated. That is what a multibeam echosounder does. The result is a point cloud of all reflected soundings, which in our cruise, depending on the depth and characteristics of the water, covers a width of 5 – 10 km. From this point cloud a high-resolution map is made. The further we sail, the more complete it gets. Especially undersea features such as seamounts and slide scars make it exciting. It is always fascinating to map a „complete“ area, while unmapped parts leave questions open (“What might the rest look like?”). Before the data are ready to be used, however, it has to be post-processed in multiple steps to clean the data for outliers that may appear due to systematic errors. The final result is definitely worth to do such work as demonstrated in the produced 3D bathymetry map of the slide scar area (Fig. 3b).

On Friday and Saturday (Sep 21/22) routine work is going on as every day in the week. For Sunday (Sep 23), however, a major event is planned, a “Multi-Station” during which – besides the normal geo-station work with giant box corer, multicorer and kastenlot/gravity corer - a major 6-hours sea-ice station on a larger ice floe is planned. In addition, as a highlight and social event, all crew and science members should have the possibility for a Sunday morning walk on the ice floe. This is our exciting plan! Unfortunately, however, it remains a plan! We get up on Sunday morning, stay on the bridge looking for our large ice floe since 05:30 – no success at all. Only small ice floes around, nothing else. The captain and the Fahrtleiter decide to continue for another two hours towards the north – again nothing! Finally, we have to give up our plan of the “Multi-Station” selected for scientific but also social activities. It’s really a pity, especially for the tyros, the young people, students etc. who are in this Arctic Ocean environment for the first time. Instead of having this “Multi-Station”, a two-hours ice station with meltwater pond studies and sampling, ice coring and deployment of drift buoys is carried out at our northernmost location – 83°05’N, 142°30’E (Fig. 5) (about the results of these as well as earlier ice station activities you will get more information during the next weekly report), followed by a normal geology station with giant box corer, multicorer and kastenlot corer. At about 19:00, the geophysicists take over – similar to the last Sunday – and start their seismic profiling towards the south.

That’s it for today. We send greetings to all our families and friends at home.

Ruediger Stein

23.09.2018

- with contributions by Estella Weigelt (geophysics program) and the Bathy Dream Team Melanie Steffen, Sophie Andree & Lamis Oberwinster (Hydrosweep program)