5 December to 13 December 2007

Tuesday 5 Dec noon, the first instrument fell overboard, intentionally of course. Polarstern had reached the position of its first scientific sampling site, the oceanographic station 13 for its 24th Antarctic expedition. With the free fall Lander, biological processes in the top 10 cm of the 3000 m deep-sea sediments are measured for 24 hrs. Most geochemical reactions on such sediments take place in the first few millimetres or centimetres of the sediment surface. Now after the long and dark winter period, we do not expect to observe much activity but measuring the background level will become important later during the cruise when sinking of organic material to the seafloor enhances biological activities. As soon as rich organic matter reaches the otherwise nutrient poor deep-sea floor, the additional load of organic carbon, nitrogen and phosphorus will readily be consumed by the deep-sea benthos, mostly comprising bacteria, protozoa and a few tube dwelling benthic foraminifera. The more the metabolic activity will increase, the more decrease will occur in oxygen concentration, while nutrient concentrations and pH will also change accordingly in the sediments. In reverse, the depth profile of these parameters will indicate the intensity of the respective turnover rates.

To sample the deep-sea we have special technology on board: the 3D microprofile Lander, the free fall amphipod traps, the Multicorer MUC, the box corer, the deep-sea camera, the benthic sledge and the Agassiz trawl, and the Bottom Water Sampler BWS. The Lander has the shape of a space robot landing on moon. Three legs with heavy weights attached reach out to about 2m. In the middle of the instrument, a metal cylinder is attached near the bottom with an array of 11 tiny glass needles, the micro-probes, sticking out. The tips of these sensors have 0.050-0.100 mm diameter and are used to determine oxygen in the sediments with high vertical resolution. The Lander is deployed as free falling instrument. Once released from the ship's crane, it sinks to the sea floor with a speed of 1m/sec. There it will rest for 30 min to let the cloud of fine sediment produced upon landing, drift away. Thereafter a stepping motor drives a spindle that lowers the microprobes slowly into the underground, recording the parameters at 0.5 mm intervals. This process is repeated about 9 times so that thereafter the sediment looks like a Swiss cheese but the data will allow reconstruction of three dimensional oxygen concentration distribution and two dimensional organic carbon fluxes in an area of 18x34 cm. After the sampling time has passed, the ship will transmit an acoustic signal that triggers the releaser to unhook the bottom weight. Due to the uplift of 16 pressure-resistant glass spheres, the instrument surfaces again and transmits its position back to the ship via radio beacon, Argos GPS position and during night-time also by flashlight.

The 24-hour deployment time of the Lander is used to deploy the other instruments. Another free-fall instrument, the amphipod traps are designed to attract scavenging organisms by exposing fish bait. In the deep-sea, food falls are scarce and the mobile organisms can smell rotten flesh in minute concentrations.

As we investigate the connection of ocean currents, upper ocean production, sinking of organic material – a subject of another weekly report - and the benthic response, on station all gear used during this expedition was deployed. Besides sampling the water column, more instruments were lowered to the muddy deep-sea floor. The Multicorer MUC looks like a big spider with its long but thin legs sticking out. In the centre of the MUC heavy lead plates are attached over 12 tubes made of Plexiglas that in turn face downwards. Lowered by the deep-sea winch, the ends of the legs will eventually touch the sea floor well outside the designed sampling spot. After standing on the sediment, a hydraulic driven by the heavy weights will very gently, slowly but steadily lower the tubes up to 30 cm into the sediments. While pulling up the cable and lifting the MUC, flaps close the tubes on both sides, protecting the sediment from sliding out and the overlying water from mixing with water on the instrument's way to sea surface. The Box Corer works similarly, but forces a 50x50 cm stainless steel box into the white-brownish sediment by its heavy lead weights of about 1 tonne. Oxygen and nutrients are determined in the pore water of the sediment and slices are cut from the MUC cores to obtain parameters to determine geochemical processes and pathways in the sediments. In addition, MUC and box core samples are gradually sieved through a series of different mesh sizes to retrieve all organisms in the light sediments. Under the microscope the secret of the sediments were revealed, the remains of some planktonic diatoms - the minute algae of the upper surface that produce most of the organic carbon in the ocean – contributed most to these sediments. Diatoms carry a transparent frustules made of silica, that after the cells´ death sinks to the sea floor, at least for some species. These glass frustules allow a reconstruction of the ocean productivity from previous times and we found species like Fragilariopsis kerguelensis and Thalassiothrix spp. that are typical for plankton blooms in this area, whether natural blooms or stimulated ones. Pelagic foraminifera contribute also a great share to these sediments.

The benthic life in the sediments was rather poor. People spent long hours sitting in the 1°C cold container looking through a microscope and picking minute forms out of the mud. It became obvious that these sediments do not see food falls very often but such results are also quite important to contrast with areas of strong coupling of upper ocean productivity with rich benthic life. Other sampling gear was towed along the sea floor but this again is subject for a later report.

As we were busy also on St Nicolas day 6 Dec we simply shifted the party for the two happy birthday persons just one day ahead. Once station work was done, crew and scientists gathered in the decorated working areas inside the ship and had relaxed hours with music, dance, and drinks, and talking.

A few days ago, we reached Antarctic waters, as we have crossed 60°South. The sea-ice is not so strong and Polarstern easily breaks its way by pushing the 80 cm floes aside or breaking them. Ocean swell has disappeared but the sounds of the crashing ice and bumps every now and then remind us of the hostile environment outside that one tends to forget in the comfortable warmth of the ship. Even with closed cloud cover, the light is bright and reflects from the white surface around us and people have to wear sunglasses and should use skin protection.

We wish such white impression for the time before Christmas also at home. At the end of the week we expect to reach Neumayer and all of us are keen to see the white continent.

Uli Bathmann
13 December 2007