BLOG of 17-18 August 2008, Author Chief Scientist Antje Boetius (Introduced herself on 25 July 2008)

QUEST 226 - The final dive

After dive 225 we were facing the same situation again: the ROV came up with a twisted cable (picture 1). But being so close to the end of the cruise, our ROV team collected again all their forces and used the 17 August for another repair phase, to permit a final dive.

On the 17th we completed some Parasound transects to the mud volcano area (see BLOG 02.08.2008), and were intrigued by indications of a gas flare above the pie-shaped mud volcano in an area, which we had not covered with our video-transect. Unfortunately, the remaining time was too short to plan further work at these mud volcano structures as well as at the other interesting geostructures around the so called Diapir area close to the submarine Congo canyon, as still a few geological samples were needed in the REGAB area, not to mention the hope for a final dive.

All scientists got together in the afternoon of the 17 August to plan QUEST dive 226. We came up with a list of tasks easily filling 24 hours and requiring a lift operation. Of course this was a risk, because a failure during the final dive would have left our expensive equipment at the deep sea floor. But everybody agreed that we should use the chance of a long final dive before steaming home, and that chance should be on our side. The tasks for QUEST dive 226 included deployment of several profiling instruments, recovery of some experimental moorings, biological samples, water chemistry and the time-demanding “videomosaicking” in the areas where we had worked most. This is a special task for the ROV, where it has to fly at a constant altitude and velocity and known track over the seafloor to visually map organisms and habitats. Producing visual maps of benthic habitats is the only way of arriving at quantitative estimates for the areal coverage of visible structures – for example of the different bivalve species, the tubeworms, the carbonates. For this task, QUEST is equipped with a high quality camera looking downwards, to produce video footage and images, which can subsequently be put together like a “Mosaic”, using the positioning information. This sounds simple, but is really a complicated endeavor due to the difficulties of underwater positioning (also see BLOG 07 Aug).

Summarizing dive 226, it was really a success. Despite the persisting oil leak, we arrived at a 24 hours dive before the reserve (also see BLOG of 3 August) and already accepted as new home to many shrimps (picture 3), which normally prefer tubeworms and mussels as associates.

We even observed a shrimp eating feces of Calyptogena clams, so these critters seem quite promiscuous in their associations. The microprofiler was deployed close to the wood (picture 4), as well as really close to a patch of vesicomyid clams, another risk taking, because the fine microsensors break easily on the shell of a clam (picture 5). Our Eddy system (also see BLOG of 26 July for explanation of in situ payloads) was placed close to and away from vesicomyid clams for community respiration measurements (picture 6). Also, we stopped in the center to watch the beautiful assemblage of organisms in the dense tubeworm forest (picture 7). A peculiar feature of this habitat is the attachment of mytilid bivalves to the tubes, as well as the dense colonization by hydroid polyps.

These are just examples of the fascinating deep sea life, and dive 226 truly gave us a feeling of how things could have been hadn’t there be so many technical issues. Again, we were truly grateful to our ROV team who did not give up and realized this final dive of the GUINECO mission.