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SO163-1, 2. weekly report, March 18 to 25, 2002

During the second week of the cruise we carried out mapping with the TOBI system of our colleagues from Southampton. TOBI, the deep-towed ocean bottom instrument, carries a 30 kHz side-scan sonar, a 6 to 10 kHz CHIRP subbottom profiler, and some additional sensors. The system records the acoustic backscattering strength of the seafloor in a 6 km wide swath, with a resolution on the order of some meters. Mapping concentrated during the first 4 days in the northern part of the area of investigation northwest of Nicoya Peninsula. The bathymetric data of this area shows several mound like, small circular structures, interpreted as mud volcanoes. The TOBI system, on the other hand, recorded high backscattering strength for only very few of those. The most spectacular is a 1.5 km in diameter and more than 100 m high mud volcano with high backscatter, located at position 10° 18' N, 86°18.3' W in a water depth of 1,600 m. We explored this conical structure with the TV-sled OFOS, and took two samples with the TV-guided grab (TVG). Due to prevailing easterly winds and currents the only direction to run OFOS was to the east, and due to the steep slope of the structure only from the centre radially outwards.

The top of the mud volcano is characterized by ridges about 30 m high. Swath bathymetry indicates that these ridges may be concentric in orientation and, thus, resemble crater-like rims. On the top surface of these ridges massive carbonate is exposed, which also outcrops at the flanks suggesting that the whole structure is cemented. We found no evidence of mudflows. At the top of the rim chemoautotrophic species, vesicomyid clams and pogonophoran tubeworms, indicate that venting is active. We deployed two TVG in order to sample the carbonates. Both grabs felt over during the closing of the jaws, however, we retrieved some pieces of rocks, sediment with sulfide odour and shell fragments. The calcareous cemented sediments are probably formed in the course of methane venting.

This small survey has raised questions about the development and the correctness of the term mud volcano. This structure is definitely a result of venting of methane rich fluids from depth and transported material from deeper sediment strata to the surface. However, it is largely unknown to what extent expelled sediments or carbonate precipitation has built up this structure.

Mapping work was continued southwards with an additional survey with TOBI during the weekend. The characteristic mud volcano field seaward of Central Nicoya Peninsula locally studied during cruise SO-144 in 1999, will be the main target of forthcoming investigations.

The strong wind we observed frequently since the begining of the work in the northern area, seems to be a local phenomenon, as it has made way for a gentle breeze in the southern parts closer to the coast. Not only that accounts for everybody aboard to feel well. The first exciting data, as well as the continuous professional support of the whole crew contributes considerably to this feeling.

On behalf of all aboard
Wilhelm Weinrebe

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