Volatile und Fluide in Subduktionszonen

The fifth week of the cruise SO163-1 was nearly entirely devoted to surveying with the TOBI system of our colleagues from Southampton. As launch and recovery operations of such deep-towed systems require some time, such deployments are scheduled, whenever possible, over a time span of several days without interuption. The TOBI survey, started the week before, continued until wednesday. This survey was carried out on three parallel profiles along the middle slope and frontal tectonic prism crossing in the south the Cocos Ridge. The structures of the middle slope and the frontal prism were mapped in great detail. In the area between Quepos Plateau and Cocos Ridge, we observed several small indentations of the deformation front, scars of seamounts currently impacting the margin. Close to the western crest Cocos Ridge a circular cone showing extremely high backscatter, surrounded by several concentric rings, was imaged.

The TOBI survey stopped near Cocos Ridge in order to run several OFOS missions in between wednesday morning and thursday afternoon, aimed at ground truthing the structures with high backscatter in the TOBI images.

By thursday afternoon, TOBI was relaunched again for a mission to map the area landward of Quepos Plateau up to the upper slope. This area, characterised by the large Quepos Slide, was surveyed during cruise SO-144 with an OFOS (OFOS-4). The observations then showed considerable indications for fluid venting. The 40 hours TOBI survey mapped this part completely. TOBI data showed several mounds with high backscatter as well as small bands of slightly increased backscatter, downwards of these mounds. OFOS observations and TV-grabs across these structures are already scheduled for the weekend to reveal the reasons for this high backscatter.

During all of the above mentioned experiments, the Simrad EM120 multibeam system was operated continuously. The results demonstrate the great progress achieved with this system. During the DTS-1 survey, for instance, the EM 120 was recording with a focused fan of reduced range, but with a dense data coverage. In combination with the slow speed of 3.5 kn of the ship a dense data coverage and oversampling was achieved. A high resolution digital terrain model was calculated with a grid cell spacing of only 25 m, yielding a detailed map compared to the earlier achieved 200 m grid cell spacing.

The different surveys, processing and interpretation of the data recorded so far, and contributions to the cruise report keeps everybody busy. But, everybody enjoys the dolphins and swordfishes frequently playing very close around the ship and performing a fantastic show.

With best regards of all aboard
Wilhelm Weinrebe