Sunday, 23 March 2008

Easter in the Weddell Sea
 
Easter Sunday in the western Weddell Sea: With -13°C and 5 Bft we are back in the ice earlier than expected. In this respect we were not able to go for our Easter promenade as described by Goethe, however the pancake ice fields glittering in the sun while gently oscillating in a weak swell has its own special charm. Real Easter sentiments are felt in the messes where a full population of Easter bunnies was found on the tables and on the walls.

The beginning of the week was still overshadowed by the aftermaths of tragic helicopter accident. On Monday the funeral of Willem Polman took place and on Tuesday the obsequies for Stefan Winter. Simultaneously with the ceremonies on land we stopped the work on board and came together for a commemoration.  In the solemn company of our ceremonies we were with our thoughts near to the deceased and their families. We share their pain.

Still, we can also recognize and follow via email that our injured comrades have returned safely to their families and are making good progress recovering from their injuries. Even if it is hard for us to overcome our pain, the work on board has to go on. The deep gaps which are left by the deceased and injured colleagues in our hearts and at work have to be bridged as adequately as possible. In this sense a helicopter technician became a winch driver for the ultra-clean CTD. With solidarity and even more enhanced efforts we will continue the programme in the sense and as an appreciation of the victims.  

We have now reached the foot area of the continental slope of the Antarctic Peninsula in the Western Weddell Sea. The rhythm of the  programme is given by the sequence of lowering and hoisting of the „normal“ and ultra-clean CTD and the processing of the never ending flow of sampled water. Most of the profiles reach to the sea bottom, but frequently shallow casts (200 to 300 m) are needed to provide large quantities of water for experiments or for extraction of trace substances to be sampled. The moorings which we recover and deploy are always a particular challenge.

In our programme the deployment of sound sources is of particular interest. To obtain measurements in the winter and under the ice, floats were developed which drift at 800 m depths. Once every 10 days they descend first to 2000 m depth and then return to the surface. If there, they are informed of their position and they transfer the measured data by satellites. So far, this is the global Argo system, in the context of which about 3000 such floats operate in the open ocean and to which we are contributing. However, under the ice this procedure does not work because the floats are not able to reach the surface. For this reason our floats are located by means of the sound sources and the travel times of the signals they transmit. They recognize that they are under the ice because the near surface water temperature is close to the freezing point. Then they stop their ascent and return to depth again. When they reach open water again, they transmit the full recorded data set. So far during our cruise we have recovered 4 sound sources and deployed 5 of them. Unfortunately we had to take note that two of the recovered sound sources had failed. We have launched 53 floats so far.

In two moorings we have recovered, we encountered a particular phenomenon. The moorings contain buoyancy elements (floats) which are supposed to keep the mooring wire upright in the water column. They consist of glass spheres in plastic housings. In the two last moorings which we recovered we found only remnants of the deepest floats which consisted of the smashed plastic housings which contained sand like glass flour. Those remnants are an impressive demonstration of the impact of the implosion of a glass sphere in about 4500 m water depth. The discussion on the potential causes is not yet finished.

In the week to come we will continue our work across the continental slope on to the shelf and finally reach King George Island where this second phase of the cruise will come to an end.
 
With our best regards from all on board
Eberhard Fahrbach