Preface The expedition ARK XIX/3 with the German icebreaking RV ...

winch operated gears were used again. On Sunday morning we started our fourth dive
operation with "Victor 6000" aiming at sampling sites close to the centre of the mud
volcano. During a previous dive close to this position we observed gas bubbles coming
out of the sediment. This location, easily to be revisited through the ultra short baseline
navigation system installed onboard "Polarstern", was in the focus of scientific interest
during this dive.
On monday evening at 9:30 the dive of "Victor 6000" did end. Shortly after recovery the
shuttle was released and needed exactly 17 minutes for his way from 1250 m water depth
to the sea surface. A lander, subsequently released, needed about 24 minutes for the
same ascending. After all gears were safe on deck the huge sample material was
distributed to the scientists involved in the respective missions. Another lander was
deployed before midnight in the northern area of the mud volcano, followed by a CTD
station and the horizontal bottom water sampler. At 3 o‘clock in the morning an intense
sonar survey programme followed aiming at elucidating the influence of tidal cycles on
methane release. On Tuesday, based on the sonar information obtained during the
survey which started in the early morning, we knew much more about the spatio-temporal
distribution of methane in the water column down to the seabed. Scientists onboard, while
processing the data, were already discussing the preliminary results which afterwards
had an influence on our further station planning.
In the early evening of Tuesday the shuttle, equipped with two autonomous instruments
to be positioned later with the ROV, was deployed. About one hour later "Victor 6000"
followed, this time for the last mission during the expedition ARK XIX/3 with one of the
two multi beam sonar systems which were hired for this cruise from commercial
companies. Final gaps in our data set on the microbathymetry of the HMMV had to be
filled.
The very good cooperation between crew and scientists did help us on Saturday to finish
all our station work in time. Therefore, "Victor 6000" was launched even earlier than
scheduled for his pre-last dive at the HMMV. During this 48 hours dive we started in the
south-east for some samples to be taken there, crossed then the centre to work finally in
the more northern part of the crater. Because we used so many instruments we had to
launch and recover the shuttle two times during the dive. However, for the first time
during this cruise leg the weather influenced our operation: fog led to visibility below 100
metres. Because the shuttle is only equipped with a flag and an underwater beacon for
position determination we decided to send the release command later. Since "Victor
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