17 - 24 September 2007

The last week of our research took us along the Gakkel Ridge to the south. The Gakkel Ridge is a spreading zone from which the oceanic and continental plates drift apart. It is the continuation of the Mid Atlantic Ridge in the Arctic Ocean. In the Laptev Sea, the Gakkel Ridge is directly linked to the continental plate and consequently its spreading is particularly slow. At our first crossing of the Gakkel Ridge a few weeks ago we found anomalies in the contents of trace metals in the deep water, indicating hydrothermal venting. Hydrothermal vents are a common feature of spreading zones and thus we went through the eastern part of the ridge searching for traces of other vents of which we intended to take a comprehensive and systematic survey. The plumes in the water appear not only as mineral anomalies but also show anomalous temperatures and light transmissions which are measured electronically, allowing us to detect them immediately. Unfortunately, we did not detect a signal as strong as at the first encountered vent; but it was worth trying since in the near future there will be no opportunity to have a cruise in this region with the state-of-the-art Ultra Clean system and the tracer analytics onboard.

Towards the end of the week we reached the south-eastern end of the Gakkel Ridge where the sea floor rises gently towards the Laptev Sea. Within a few days of each other, we sampled both the deepest and shallowest stations during our cruise, at 5300 m and 48 m depth, respectively. This section across the continental slope was the third of our cruise that covered the transition from the shallow shelves to the deep sea where we captured the flow of the Atlantic water that is topographically steered by the slope. In addition, we measured the transition from the shelf to the deep sea to trace the spreading of the huge amount of river water that enters the Siberian shelf seas. On the shelf the river water mixes with ocean water, staying near the surface due to its low salinity. This river water spreads out from the shelf, not like a pancake on the surface, but in an anticlockwise bow towards the shelf edge and in a broad stream across the Arctic Ocean towards the North Atlantic, driven by wind and internal dynamics. The river water carries a lot of land-derived dissolved and particulate material; but when flowing over the shallow shelf many substances are exchanged or modified through contact with the sediment. This is also the case for the low salinity Pacific surface water that enters much farther east through the 50 m deep Bering Strait. This water is modified through interaction with the bottom of the Chukchi Sea. Both fresh water currents converge in the central Arctic. On our transect to the Makarov Basin we had passed the front between the two water masses and could take samples for many components which we are still analysing onboard. The pathways, and maybe the properties, of both fresh water flows are likely to vary with different atmospheric circulation patterns.

The purpose of the Laptev Sea transect was to identify the properties of this fresh water near its source using the wide spectrum of different tracers available and also to determine processes that change the water mass properties. We measure “classical” properties such as nutrients; fluorescence, including “yellow substances” (terrestrial humics); and natural radionuclides such as radium which we can then compare with data from past cruises. In addition, we systematically determined for the first time the distribution of trace metals in this area. Here we find enormous differences between the mixture of Siberian river water and Atlantic Water, and the Pacific Water.

Meanwhile, the station work is finished; the last samples have been taken the night before Monday. The analyses and several experiments are still going on in the labs. The extent of the results cannot yet be fully assessed but there is already strong evidence that our expedition will bring us a step forward in our understanding of the current changes taking place in the Arctic. Arctic research is not only a question of good science but also of extraordinary professionalism of the seafaring parties. We want to express our deep gratitude to Captain Stefan Schwarze and his entire crew for getting us here and for friendly, engaged and reliable assistance in all matters during the past two months!

One encouraging correction has to be made to the last weekly report: In the last moment before giving up the search, our Russian colleagues on the “Academic Fedorov” found a suitable floe for the manned drifting ice station east of the Taimyr Peninsula. We send our best wishes to the team for a good overwintering and a successful drift!

For the last time best regards from all of us from the Arctic,

Ursula Schauer