PS115/2 Weekly Report No. 4 | 24.09. – 30.09.18
A mosaic of silence, storm, sea ice and kastenlot
Monday (24 Sep). A new week begins – quite similar to the last one. For the next three to four days we have to listen to the “music of the geophysicists”, every 15 seconds the monotonous “wum” will enjoy us.
Tuesday (25 Sep) – hump day is reached. 20 days are behind us, 20 days are ahead of us. There are a couple of other exciting aspects that make this day unique: a beautiful sunrise, calm sea, no ice at all, many birds are surrounding the ship. Are we really in the Arctic Ocean? I am sure we are as I trust our nautical officers on the bridge.
The Arctic sea ice simply has reached another prominent minimum as has been observed in several different Arctic regions this year and described in the public literature and press releases a few weeks ago. The Arctic marginal seas and their adjacent continental margins are more or less ice-free whereas the central Arctic Ocean is still covered by heavy pack ice (Fig. 1).
But let’s come back to this unique beautiful sunset and the very calm sea – is this extreme silence (of course, here we have to exclude for some minutes the “music of the geophysicists”) real or is it misleading? Is it perhaps the calm before the storm?? Listening to Weather Max’s report in the morning this is not an unrealistic option!! Weather and model forecasts predict stormy weather with high waves for Wednesday/Thursday (26/27 Sep). Who really knows what’s ahead of us?
Two days later (Thursday 27 Sep, 08:15) we know the truth! Max’ weather forecast from Tuesday was not so bad – later today or latest during the coming night we will get very strong winds (8-9 Bft) with 3 to 4 m high waves. Thus, our geophysicists decide to bring the 3000m long streamer as well as the airguns on deck. The danger is too high to damage the streamer. In less than four hours, streamer, blubs and airguns are on deck, and we are ready steaming towards the north, ready for another geological station. This has been at least our plan!
Friday (29 Sep), 06:00 in the morning – ready to run the geological gears. Meanwhile, however, waves have become higher and higher, a strong swell has been established. The ship is moving up and down, and the work with the heavy geological gear becomes difficult and – even more important – dangerous. After a short inspection of this situation on the aft deck, captain, chief mate and chief scientist immediately decide to stop all running activities, the geological station is cancelled. Instead, we are steaming towards the east into the ice (Fig. 2), looking for a station where ice and swell conditions allow hazardless operation of our geological gear.
Four hours later, around 10:00, we stop for running a geological station with giant box corer, multicorer and kastenlot corer. As the weather and visibility conditions are good enough as well, contemporaneously to the geo-station work two helicopter flights are scheduled, making our ice people happy (to have happy people around is always good and important for the whole life onboard). They get the chance for ice observations and the deployment of a drift buoy.
Ice observations from the helicopter and the deployment of drift buoys, these are two key aspects of interest of our ice people onboard. Thus, they should get the chance to introduce themselves and tell us what they are doing here and why. Go ahead, ice people!
Our sea ice team focuses on three different topics. Niels Fuchs, Gerit Birnbaum and Marcel König investigate melt ponds, which presently refreeze. They are currently only present on second-year ice floes, which did not melt completely during the summer season. The new ice on the melt ponds is considerably darker than the surrounding floe surface. Hence, melt ponds decrease sea ice albedo also in the period of refreezing. The project aims at increasing the quality of areal fraction and melt pond albedo derived from airborne and satellite remote sensing measurements. Heike Zimmermann is mainly interested in the bottom part of the second-year floes and the ice algae that live there. She develops methods to search for ice algae DNA in sediments that are thousands of years old, and with a little luck, to find clues about the sea ice extension of the past. Gunnar Spreen deploys buoys in our working areas to contribute measurements of air temperature and air pressure to the Year of Polar Prediction (YOPP). Additionally, a dense net of buoys is deployed, which allows to analyze ice drift and ice deformation on a local scale.
In their project, Niels, Gerit and Marcel link airborne measurements by using the “Polarstern” helicopter to ground-based measurements on the ice. In order to obtain so-called ground-truth data and to validate the airborne measurements, ideally an area with melt ponds should be first observed with the airborne cameras followed by ground-based measurements. This ideal combination of measurements could not be performed so far due to unfavourable ice and weather conditions. Up to now, Niels and Gerit have carried out four measurement flights. Images taken by an RGB-camera and a hyperspectral camera will allow to derive pond characteristics (areal fraction, albedo) in a radius of 90 km around Polarstern. Almost all ponds on smaller and bigger ice floes in our working areas were melted entirely through during summer. A 5-10 cm thick layer of grey ice with a bulk salinity of about 10 g/kg is now covering them. Only their shape makes them distinguishable from new ice in intermediate floe areas.
During the two-hour ice station on 23 Sep 2018, Marcel could perform ground-based measurements on a refrozen melt pond (Fig. 3). He measured spectral reflectance of the ice surface and ice thickness, ranging from 5 to 7.5 cm, at different sites. The quality of the reflectance measurements could be increased by means of a double-spectrometer setup, which allows a correction of changes in illumination during a measurement cycle. Videos taken with an underwater camera below the ice cover proof that the pond has melted through the ice floe before refreezing.
Sea ice contains a high diversity of microorganisms like unicellular algae. These can be found in small channels filled with brine, especially in the bottom part of the ice that is connected to the sea water, which provides nutrients for the algae. In summer, the sunlight can penetrate the ice, allowing an algae bloom at the ice bottom, which is visible as a yellow-brownish coloration of the ice. The drilling of ice cores on an ice floe by Heike on the 23.09.2018 permits the collection of algae samples. By applying DNA metabarcoding (a specific part of the genetic code can be used similarly to a barcode in a supermarket) the samples can be used to make inferences about the species composition in the ice.
In the Arctic, and especially in the remote location of the Arctic we are in, there is only a small amount of weather observations like air pressure and temperature available. Numerical models therefore have problems to reliably predict the weather and climate in the Arctic, which also influences the quality of weather forecasts for more southerly latitudes like Europe. It is the goal of the Year of Polar Prediction (YOPP) to change that. From 2017 to 2019 a huge amount of extra observations are taken in Polar regions, models are getting improved, and also we are a small part of this international project. By now Gunnar deployed seven of the nine YOPP buoys, which measure air temperature, pressure, and sea ice drift. These buoys will drift with the ice for more than a year and every hour transmit their observations by satellite to observational centers. The drift of the buoys can be followed under http://meereisportal.de. Some of the buoys already traveled more than 100 km since their deployment. The YOPP buoys were deployed equally along the route from the ship or by helicopter to sample an as large area as possible (Fig. 4, red diamonds).
In a second experiment, an array of buoys measures the drift and deformation of the sea ice on a more local scale (Fig. 4, orange diamonds). Next year in October, “Polarstern” will be frozen in and drift with the sea ice for one complete year, starting from a location a bit north of the one we are currently. In the framework of this large international MOSAiC project also an array of autonomous measuring stations will be deployed in a radius of 40 km around “Polarstern”. The eight buoys deployed by us will provide data about sea ice drift and deformation from this year, to have a comparison to and as an outlook on the expected situation during MOSAiC next year.
Many thanks to the ice people for this detailed excursion into their ice business! But now let’s come back to our running daily business. It’s still Friday (28 Sep), the kastenlot (my subfinal!?) is just under the way to the seafloor. Exactly at lunch time, the kastenlot core is on deck – 9 m in length!! The full splendor, however, becomes first obvious on Sunday afternoon after the kasten (the box) has been opened. A sedimentary section of 8.8 m in length (the longest core we got so far during this expedition!) characterized by beautiful color cycles, changes in structure and texture, etc. – just beautiful!! What secret is behind all these cycles and changes, what kind of story in terms of climate change these sediments can tell us? For answering these questions we need the results of our future studies of these cores. That means, we have to wait of a couple of months, maybe years!
At the end of this 4th weekly report – today is Sunday (30 Sep) – I would like to share with you a little nice, interesting and touching contribution, a school essay by Emil Schmengler, dealing with the self-selected theme “Polarstern” and sent to his mom here onboard (Emil has certainly recognized and described the duty and importance of the captain of “Polarstern” quite correctly).
That’s it for today. Kind regards to all our friends and families at home from the “Polarstern”, also in the name of all cruise participants,
- with a contribution of our sea ice people Gerit Birnbaum, Niels Fuchs, Marcel König, Gunnar Spreen und Heike Zimmermann)