ARK-XXII/2, Weekly Report No. 5
26 August - 2 September 2007
Towards the end of last week we reached our northernmost position at 88° 40N. Of course, we had expected that even here the ice would be as eroded and loose as in all other regions that we visited during the past weeks which has allowed us to maintain a speed of up to 6 kn. But a whole day of rain within 150 km of the North Pole came somewhat as a surprise! For the past few weeks, one low-pressure system after another has continuously carried warm air from northern Siberia (15°C at the Lena estuary!) towards the central Arctic Ocean. In this way the sea ice disintegrates more and more right before our eyes.
We continued our section that had started in the Voronin Trough across the Amundsen Basin up to the Lomonosov Ridge that separates the two large basin systems of the Arctic Ocean, the Eurasian Basin and the Amerasian Basin. Along the crest of the ridge we sailed northwards. The reason for this detour was a depression on the ridge crest, called the Intra Basin. During an expedition with the Swedish icebreaker Oden two years ago, the bathymetry of the ridge was surveyed in great detail. In the past the deepest sill between the two basins was believed to be located here at 2500 m. However, the survey with Oden had shown that the sill depth here is only 1870 m. Obviously, the exchange of deep water across the Lomonosov Ridge depends on the sill depth and the direction of the exchange depends on differences in the water masses on both sides of the sill. Former investigations provided evidence that deep water is flowing from the Amundsen Basin (Eurasian side) to the Makarov Basin (Amerasian side). Because recently the upper layers of the Arctic Ocean as well as the deeper ones are changing, it seems plausible that the direction of the deep cross-ridge flow varies with time. Hence, in 2005 the deep flow direction was opposite to what was thought before. We took stations at narrow intervals across the ridge and in the Intra Basin. In the small basin, and at the sill, we found Makarov Basin water with higher salinities and temperatures than the deep water of the Amundsen Basin. Obviously the direction is the same now as in 2005.
At each station we sample tons of water for lab analyses. Many dissolved substances like to attach to particles. This behaviour can be used to investigate not only the substances themselves but also the sinking of particles. Sinking marine organic particles (rotten algae, fecal pellets, dead zooplankton) are an important pathway in the global carbon cycle that removes carbon from the atmosphere by carrying it to the deep sea or even depositing it as sediment. Certain radionuclides love to attach to organic particles, causing them to sink with the particles to the sea floor. From the knowledge of the half-life times of the parent and daughter nuclides one can calculate the theoretical ratio between the two in the water. Deviations from that ratio in a particular water layer mean that a percentage of the nuclides have left that layer using the particles as a vehicle: the more of the daughter that is missing, the more particles have sunk. Also trace metals are particle-reactive. Hence when we measure the trace metal content both in water and at particles we can use the nuclide method not only to derive the rate of the export of particles but also that of trace metals. On the other hand, to convert that into carbon export information is required about the nature of the particles: big, small, light, heavy?
To sort out the whole multitude of the required parameters, a never-ending filtration business begins after each water sampling station: some filter to get rid of all particles in order to measure dissolved substances only, some filter to get rid of the water and keep the particles, the next ones size-sort the particles with a centrifuge, others run the water over chemical filters that absorb those substances they want to quantify and so on. Often we run four or five casts of the CTD/water sampler before everybody is happy.
Due to the low sea ice cover we proceed very fast and the stations follow in a narrow sequence. Consequently there are many very tired faces on the ship. On the other hand we really enjoy that the ice conditions enable us to extend our research far into the Makarov Basin. We don't know if we will have a similar chance again soon. Despite the fact that the ice conditions along most of our cruise track fully confirm the long-term trend of Arctic sea ice decrease, it does not tell much about the regional distribution. North of Svalbard, at the beginning of our cruise we had a hard time moving at all! Like ten years ago, it is still difficult to plan exactly where an Arctic Ocean expedition will go.
Best regards in the name of all participants,