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

hydrotroilite and gas hydrate, this sediments were mostly bioturbated. Main difference
between these layers is color and consistence changes are due to diagenesis. All
these layers are represented by the same lithology sandy mud.
B. 4 Water column investigations above the Håkon Mosby Mud Volcano
Sauter, E., Muyakshin, S., Rogenhagen, J., Rohr, H., Wegner, J., Baumann, L.,
Gensheimer, M., Schlüter, M., Boetius, A.
During the previous cruise ARK XVIII/1b of "Polarstern" to the Håkon Mosby Mud
Volcano in the year 2002 a large plume image of about 600 m height and 500 m in
diameter was discovered by the use of the scientific echosounder EK60. The plume
was suggested to be related to mud expulsion and fluid discharge from the volcano’s
centre. So far, no gas bubbles had been observed at HMMV, neither by manned
submersibles, TV guided devices like TV-MUC and OFOS, nor by ROV observations.
Furthermore water column methane concentrations above the mud volcano have
been found to be far below saturation. Accordingly, the plume has been suspected to
consist of fine sediment particles rather than of gas bubbles.
Behind this background water column investigations of various kinds have been
performed during this cruise. Main objectives of this were to obtain clarification of the
nature and origin of the acoustically observed plume above the mud volcano’s centre.
Fortunately, several locations of bubble and fluid discharge have been discovered
and visually observed with "Victor 6000" (Fig. B4-1). The proofed existence of gas
bubbles makes the further interpretation of acoustic plume images much easier.
Acoustic plume survey
The scientific echosounder EK60 is one of the fixed sensor installations onboard RV
"Polarstern". It provides an acoustic image of the water column and is normally used
to detect fish shoals. The EK 60 provides four operating frequencies, ranging from 38
to 200 kHz. The data is displayed online and recorded in digital format. Two
frequencies (30 and 70 kHz) were operated regularly, while the other two channels
were too noisy or off less performance in regard to depth range. The EK60 interferes
very much with other sounding devices of RV "Polarstern". To achieve high quality
data, the EK60 was used as a stand alone sounding device with Hydrosweep and
- 186 -