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SO163-2, 3. Weekly report 12.05.02

at Sea at 9°50´N/86°20´W

In the previous week we completed investigations in both southern operating areas. We shot a profile net of 4 lines across the mega lens and 3 profiles across the detachment reflection. In total, 125 OBH/S have been deployed and successfully recovered so far. Data is of very good quality so that we are optimistic regarding the goal of the cruise, i.e. the derivation of lithologic parameters from amplitude variations. Two units were launched with an anchor rope 1000 m in length so that we have a detailed knowledge of the sensing area of the various air gun sources.

The two tiltmeters positioned on the Quepos Plateau were recovered after about 8 days of operation. Unfortunately, one of the units failed to supply useful data; we have as yet not detected the cause of fault. Due to the vessel's movements (even with the sea being very calm), laboratory inspections and measurements on site have become impossible, and we will probably not be able to use this unit any longer.

In the seismological net of Jaco Scar, 7 stations were recovered after 17 days of operation; 3 of these were deployed again to remain on site until the beginning of October. We are going to launch further stations here towards the end of our working schedule. Presently, we are positioning a profile near the northern end of Nicoya peninsula where BSR reflections are to be studied.

In the preceding week, work on the water column included above all the imaging of a hydrographical profile (CTDs 15-20) south of the operating region (perpendicular to the continental slope south of the Quepos Plateau) to achieve a higher resolution of the upper water column. Besides the omnipresent CH4-plume in the oxygen minimum zone, the formation of an unusually strong maximum beneath the intermixed layer (oversaturation: up to 6 times) has to be emphasized. With growing distance to the coast, the depth of this maximum, which is obviously the controlling source of methane venting into the atmosphere, increases from 30 m to 80 m. In addition, another hydrocarbon - presumably photochemically generated Ethene (C2H4) - was detected in the gas-chromatographic spectra of the area between the surface of the water and a depth of approx. 100 m. However, laboratory calibrations ashore have yet to be performed for verification. Last night, we took samples at a water depth of 2250 m (CTD21-MUC10) from a further station in the region of Jaco Scarp. This profile aims at delimiting the lateral extension of the methane plume, which was found at the beginning of the second week of the leg, to a range between 1850 and 1700 m. It shows an extremely high CH4-occurrence in the entire range between 2000 und 1600 m, with concentrations of approx. 80nmol/L at 1930 m. Its structure implies that there are several methane sources at varying depths. Results from successful minicorer operation performed at this station have not been obtained yet. Owing to such preliminary studies, Jaco Scarp is certainly one of the most promising regions where to operate video-guided equipment in the course of future legs. Therefore, we will try to extend the Jaco-profile further south next week.

Other events of last week: For the 2000th time, our working group deployed and recovered an OBH! On this occasion, Costa Rican TV was present aboard all day long. And we celebrated half-time of the leg. A big octopus, tenderly embracing our seismometer, emerging from a depth of more than 2000 m, is worth mentioning. Unfortunately, it did not survive its journey to the surface.

In good spirits we are looking forward to the last week of this leg. At fine weather we are sending our regards to those at home.

E. Flueh

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