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SO173-4, 3rd and last weekly report (18.09. - 29.09.2003)

After exchanging several crew members, scientists and guests, welcoming the lander-team on board, and loading air freight, FS SONNE departed Caldera in the evening of 18 September and took course to the nearby working area Quepos Slide. Observatiuons with TV-guided systems, measurements of methane in the near-bottom water column, and pore water analyses showed that at the foot of the slump massive fluid emissions are responsible for the wide spread colonisation by bacteria and the freshening of pore waters. As start-up of the lander systems for the determination of fluid flow rates would have still required extra time, we relocated to the area of the mounds north of Caldera. On the way, methane measurements at selected stations which had been visited previously (SO144 Cruise in 1999, SO163 Cruise in April 2003 and M54 Cruise in July and September 2002) showed that surprisingly all known anomalies now were significantly reduced. This strongly suggests that not only currents and circulation influence the methane distribution but that the source strength of the fluid emissions themselves seem to be subject to large scale inter-annual fluctuations. During the course of the expedition this observation was confirmed whereby Mounds #11 and #12 showed additional anomalous behaviour. Here the maximum concentration of methane in the water column was also reduced compared to previous measurements but within a large area surrounding Mound #12 the near bottom methane emissions were greatly enhanced which had not been observed before.

Work at mounds Culebra and #10 turned out to be difficult, as during the TV-multicorer deployment at 2300m the underwater power supply system was completely destroyed by implosion of the pressure housing. To ensure subsequent deployment of this important instrument parts of the lander system were installed which however, required constant change over of parts between the instruments and hence cost precious time. Furthermore, substandard material used to attach the weights at the benthic chamber lander (BC-L) caused premature launches. This problem could fortunately be solved during the latter part of the cruise. In addition to these instrumental and material problems the targets for the lander deployment exhibited a complicated topography, contained carbonate structures, and were exceedingly small such that no successful deployment was possible during the scheduled time at these sites. However, several TV-multi cores and gravity-cores compensated for this failure.

Before FS SONNE departed the northern working area on 23 September the Long Ranger was picked up. This lander contains an upward looking current meter (ADCP = Acoustic Doppler Current Profiler) which for several days had registered data at the summit of Mound Culebra. During the deployment time of the Long Ranger an array of CTD-casts was sampled around Mound Culebra to monitor the methane distribution. Also before departing, another gravity core was taken at the NW-SE-trending fault which cuts through Mound Culebra. The pore waters again showed apparent discharge of deep fluids based on decreasing Cl-contents. Based on 2 cores at differing distances to the fault zone we could define an area affected by ascending fluids.

On steaming towards Mounds #11 and #12, two current moorings were retrieved at Jaco Scarp, before intensive lander deployment were carried out. The Benthic Chamber Lander (BC-L) and the Fluid Flow Lander (FLUFO) were first deployed at the discharge area at Quepos Slide and later at Mounds #11 and #12. At all locations we succeeded in optimally positioning the equipment onto the most active fluid vents for a 24-hour-deployment. At Quepos Slide all systems worked well and preliminary results document high rates of fluid- and methane-flow, intense material turnover, complete oxygen depletion, exceedingly high methane contents near gas saturation levels, and a significant build-up of peculiarly shaped authigenic carbonate crusts. Also at Mound #12 both lander types recorded optimal data. This is extremely interesting in light of the increased colonization by bacterial mats in the area. Unfortunately, a technical failure of the BC-Lander on Mound #11 precluded registration of equally good data at this location. But here as well as at Mound #12 TV-multi-corer deployments yielded undisturbed sediment cores with clear negative Cl-anomalies which suggests the presence of deep fluids as well as a high flow rates.

Between the lander deployments OFOS- and multibeam-surveys were conducted. OFOS was used to ground-truth back-scatter anomalies as well as to investigate new hitherto unknown features of fluid-flow. The multibeam survey mapped a possible slide in detail (Quepos East) at the upper slope, east of the active Quepos Slide. This structure is well-defined bathymetrically; it is about 5 n.m. long, 3 n.m. wide at the shallow end and 5 n.m. wide at its deeper end. The head wall lies at 230 m of water depth, has its steepest gradient between 350 and 280 m and ends at a water depth of 500 m. Quepos East shows no slump mass at the foot, instead 3 canyons traverse along the longitudinal axis with numerous feeder channels. It is probably an old slump feature, similar to the Quepos Slide but significantly bigger. Currently it is denuded by erosion and its canyons have taken over sediment transport from shelf to the slope. No detailed OFOS-obseravtion nor methane measurements were carried out because of time limitation; therefore it is still unclear whether or not Quepos East is still actively venting fluids.

Similar to Mound Culebra also at Mound #12 a detailed program of the methane and current measurements was carried out. The Long Ranger was deployed as well as a conventional current meter mooring and seven hydrographic stations complemented the array to obtain a simultaneous picture of methane and current distribution. Approaching the end of the cruise on 27 September, a TV-grap deployments was planned in the area of the increased bacterial colonisation at Mound #12, however, this plan was abandoned because of a damaged fibre optic cable. As its repair would have taken more time than was available, two gravity-core deployments were carried out. As is almost tradition, the last one of these contained several layers of gas hydrate at between 200 - 300 cmbsf (= cm below sea floor) Obviously, at this bacterial site the increased flow of methane is reflected in the gas hydrate formation as well as the increased methane contents within the bottom water layer.

With the scientific program coming to an end in the evening of 27 September, the following 36-hour transit from the working area to Balboa/Panama brought the vessel into port by midday Monday, 29 September.

In summary several high-lights and considerable station work were accomplished. Of the results directly and significantly contributing to the SFB-objectives were the discovery of the Quepos Slide vent, which might be a ground water flow from as far as 80 km off-shore and which may have triggered the slide. Furthermore, equally significant are the documentation of deep fluid flow along the Culebra Fault as well as the wide spread inter-annual fluctuation of methane-emissions. Gas hydrates of a biogenetic (?) and a thermogenetic (?) methane source side by side, at Mounds #12 and #11, respectively documents the high special variability and differing source-depths of dewatering. The identification of different types of clasts related to extrusion features in the mound sediments considerably enhances our understanding of fluid- and mass-transport from depth. Once again, the importance of our Lander systems cannot be emphasized enough in order to obtain direct fluid and dissolved component flow rates. Although this approach is technically complex and demanding, it is unique world wide and provides essential data directly contributing to the overall SFB-objectives. They determine the quantity of element-recycling in an observational time frame which may be extrapolated to the subduction framework and they validate modelling.

Such results can only be achieved through the excellent and continued cooperation between the vessel's crew and the scientific party, the cooperation among the SFB subprojects, as well as the dedication of all participants. Thanks to all of the persons involved.

Erwin Suess
Chief scientist SO 173 / 3&4

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