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SO173-3, 2. Weekly report (11.09. - 17.09.2003)

At the beginning of the week starting 11 September work in the area of "New Mounds" centered at two active structures: "Mound Carablanca" and "Mound Morpho". The designation of the mounds previously was by geographic and other cruise-related terms which caused some confusion. Therefore we named all structures in the "New Mounds" area which were actually investigated, after indigenous species of Central America. For example, the side-scan-sonar back-scatter pattern of Mound Carablanca (=white faced monkey) faintly recalls the visage of monkey with eyes, nose (bright spots) and surrounding mane (changing bright and dark circular bands). The back-scatter pattern of Mound Morpho (=butterfly) suggests the high-contrast patchy pattern of a butterfly wing.

Several gravity-cores, TV-guided multicores, bottom water samples, CTD- and TV-grab-deployments were conducted at the outer margin of Mound Carablanca which is characterized by circular structures as well as at the centre of the mound. All deployments were successful. Studies of sediment fabric revealed clay clasts, slick-in-sides, and other elements suggesting mud extrusions. At Mound Morpho TV-multicorer-deployments on clam fields showed specimens of remarkable size; TV-grab-deployment brought on deck carbonate blocks with chemoherm structures and authigenic carbonate precipitates without sediment embedded.

During 12 -14 September work concentrated on Mounds # 10 and Culebra, located farther SE. These structures had been investigated during an earlier cruise (M54). New was work along an NW-SE striking fault which cuts through the centre of Mound Culebra. After arriving in the area we deployed 2 moorings to collect current data to better understand the dispersal and transport of methane released into the water column. This objective was supplemented by several DTD-casts taken during the time of the current meter deployment.

Cores were taken at the southern flank of Mound # 10 where previous evidence had suggested inflow of fresh seawater. Whereas at the southern flank of Mound Culebra large amounts of clay clasts and altered crystalline clasts were extracted from sediments of a gravity corer. Further we obtained evidence for substantial freshening of pore waters from a core at the NW-SE-fault. This suggests ascent of deep fluids at a segment of the fault at a point where it is slightly off-set. Data from this 9-m gravity core showed a continuous decrease in chloride to as low as 290 mMol. Also other dissolved components, namely calcium, point to deep sourced fluids which ascend relatively slowly but apparently over a considerable area. The fluid chemistry will approximate the pure end-member composition.

During the last part of the week (15-16 September) FS SONNE steamed to the area where several slides have originated from the upper continental slope at shallow depth (300-600m). Based on previous work the following areas were investigated: BGR-slide, GEOMAR-slide, and Quepos-slide. During transit to the area at night a multibeam survey was completed as well as several hydrocasts. We aim at reaching a common understanding of what triggers these slides which so far have been treated as isolated occurences. At the BGR- and the GEOMAR-slides we cored into the slide plane immediately below the head wall where only thin hemipelagic sediments had accumulated since the slide event. The sediments showed considerable over-consolidation as well as high shear-strength; density estimates by logging confirmed this interpretation. We also cored into an undisturbed slide block at the base of the head wall. Laminations, cross-bedding and slightly rotated clasts support this preliminary interpretation. The chaotic slide mass farther downslope was not investigated although the high methane-content in the bottom water clearly suggests that the slide mass itself or the undisturbed sediment beneath may be the source of degassing.

At the Quepos-slide TV-guided multicorer deployments and bottom-water-sampling showed a considerable fluid venting activity which was previously not known. Extented bacterial mats were known from the base of the slide and the samples retrieved suggest a seep of ground water instead of deep sourced fluids. Such a phenomenon had previously been assumed because of "light" oxygen and relatively "heavy" carbon (non-biogenic) isotope values obtained from a carbonate-crust. Now pore water analyses further support typical fresh water criteria for the venting fluid: low Cl- and Ca-valus.

After completing work in the morning of 16 September we took course towards Caldera. Before arriving we deployed two current meter moorings at the axis of Jaco Scarp. At 16:00 FS SONNE docked at the pier and between 18:00 and 20:00 the exchange of scientists and crew members took place. The following day several more crew members were exchanged and we departed Caldera at 20:00 on 17 September.

Erwin Suess
Chief scientist SO 173 / 3&4

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