“Frazil ice”

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28 October 2006

The last couple of days have left us a bit confused as we have tried to interpret strange patterns on our depth sounder. At the beginning of each CTD cast we need to obtain the depth in order to avoid hitting the bottom with the sounding equipment. Up until now we have received a strong regular signal, indicating a hard rocky seabed at 1800m – 2000m depth. However, this week we have been unable to ‘see the bottom’, instead picking up a variable echo at about 30m.

Jean Claude Gascard (Director of the DAMOCLES project) provided us with an explanation. What we were actually seeing on the sounder was the formation of “Frazil Ice”. The images directly under the ice show a chaotic scene with blocks stacked one on top of another as they jostle for position with the ice movement and pressure.

This week we have also started to drift slowly in reverse, making a southerly course back towards Siberia. The ice began to move again last night and we now have some open water along the starboard side of Tara. Evidence that even though the temperature has dropped to around -20?C and ice has formed all the way to the Russian coast, the sea ice remains very dynamic and we cannot relax our vigilant watch.

Grant