PS115/2 Weekly Report No 3 | 17. - 23.09.2018
With geophysics and geology towards 83°05’N – the northernmost position of our expedition
Monday/Tuesday (17/18 Sep 18). A new week begins - fog, calm sea, ice free. Nothing reminds us that we are in the Arctic Ocean (but this may change soon when are steaming northwards end of this week!).
Something, however, has changed in comparison to the last weeks. As mentioned at the end of the last weekly report, the geophysicists have started their geophysical profiling, it’s running since Sunday morning, realized by a „wum“ every 15 seconds. What’s behind this “noise”, i.e., what are the scientific goals, what’s the technical approach? This can be best explained by Estella Weigelt, AWI geophysicist and leader of the geophysics team onboard “Polarstern”.
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.
First results of seismic lines along the Lomonosov Ridge look very well (Fig. 2b). At the cross-points to former surveys up to 1600 m thick undisturbed and parallel sedimentary layers can be identified, suggesting ideal drilling locations. Furthermore, the profile through the Amundsen Basin indicates an interesting 400 m high elevation. A part of this seamount has already been recorded in 2014 by sub-bottom profiling and bathymetry surveys. But up to now it’s not clear if this rise results from a mud volcano close to the seafloor, or if it presents a deeper structure. As imaged in the new seismic lines this seamount rises from the crust and penetrates all sedimentary layers, thus indicating a magmatic origin.
After this short excursion into the field of geophysics (Thanks Estella!) back to the daily routine program. Since today (Wednesday 19 Sep) the geology is again the driving force in our research activities. At the moment working at the aft deck is quite convenient, air temperatures around +1 °C, a totally smooth sea surface. Besides the giant box corer and gravity corer, the first use of the multicorer during this expedition is scheduled. By means of the multicorer, totally undisturbed near-surface sediments can be obtained. Eight 50 cm long plastic tubes are cautiously pushed into the sediment by one run. When the plastic tubes are on deck and brought into the sediment lab, an intensive sampling routine starts. The sediment piles are cut into 1 cm thick slices, stored in glass vials or plastic bags, some are stored at -20°C, some at +5°C, etc. What procedure is used depends on the type of post-cruise study to be done later in the home labs.
On Thursday (Sep 20) a highlight is on the program – at least for the geologists. During the Polastern Expedition PS51 in 1998 (from our PS115/2 people, crew members Uwe Grundmann, Lutz Peine, Ecki Burzan und Michael Martens, and scientists Norbert Lensch, Estella Weigelt and the Fahrtleiter have already been onboard at that time – welcome in the club of the “Glorious Seven” and congratulations to the 20. Anniversary of this event!), a “strange” sediment core has been recovered. The lower half of that core looks totally different in comparison to the upper part. Different colors, different grain size and much higher density values, faulted sediment layers – simply strange. Does this abrupt change represent a hiatus? Are several thousands to million years missing? During Expedition PS87 in 2014 we have recovered a sediment core close-by to our present study area (Uwe Grundmann, Michael Martens, Norbert Lensch, Frank Niessen, Michael Schreck and the Fahrtleiter have also been onboard and can certainly confirm this statement) where later studies could prove the presence of a hiatus and an age of 6-9 Ma for the basal sediments of that core. These old sediments have been recovered from an area of Lomonosov Ridge that is characterized by slide scars and steep slopes that allow old sediments cropping out at the surface. This gives us the unique chance to core these old sediments by our simple coring devices we have onboard (i.e., the gravity corer) – an exciting story might be the outcome!!
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).
Back to our approach. How do we proceed? We know now size, extension and steepness of the slide scar area, have selected coring stations considering the Hydrosweep and Parasound data. These coring stations are, however, very close to each other. Thus, a precise navigation is needed to bring the ship, or more important, the coring gear to the right spot. This is only possible if you have the right person on the bridge and an additional person (many thanks to our ELO Winfried Markert) who is following the “pinger” signal (Remark: a “pinger” is attached to the coring gear that allows to record the position of the coring gear very exactly). Fortunately, we have both, the detailed Hydrosweep & Parasound data and the excellent crew members to carry-out this precise coring successfully. Five runs of the gravity corer have been done within a few hours. Many thanks to all the people from crew and science who have been involved in this “coring event“. Now, we have to wait a few days (weeks? months?) because our specialists (maybe Micha Schreck?) have to find the right fossils to prove that these sediments are really a couple of million years old. Let’s cross our fingers for the success!
The weekend is coming – if you look into the labs (Fig. 4) or follow the work on the aft deck, however, no difference is obvious.
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.
- with contributions by Estella Weigelt (geophysics program) and the Bathy Dream Team Melanie Steffen, Sophie Andree & Lamis Oberwinster (Hydrosweep program)