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SO173-3, 1. Weekly report (03.09. - 10.09.2003)

On 3-4 September the containers and equipment were unloaded and loaded during the port stop at Caldera (Costa Rica) under rainy skies and humid, 30° C temperatures. Thunderstorms and heavy rain showers in the late afternoons brought a welcome cooling off and a break in these activities as did the visit of the impressive 3-mast full sailing ship "Gloria Columbia" of the Columbian navy which docked alongside RF SONNE. At 1700 hrs on 4 September we departed Caldera in a north-easterly direction towards the margin off Nicaragua. Here our investigations were concentrated in the area known as "New Mounds" as well as on an oceanic plate transect running parallel to the El Salvador - Nicaragua offshore line.

On the way north we retrieved a long-term mooring at "Mound Culebra" that should have recorded the current distribution over 2 months but did not. We reached the New Mounds area in the morning of 6 September and started work. Here a large number of fluid venting targets had been documented by Side Scan Sonar surveys during the previous expedition SO 173 Leg 1. All together 126 possible sites for detailed investigation of fluid- and gas-venting had been mapped by several surveys consisting mostly of mounds, canyons, slides and slumps. We eventually selected several mound structures and patches of different back-scatter patterns with and without morphological expressions. All mounds investigated showed manifestations of active fluid flow based on OFOS-, TV-grab and TV-multicorer deployments as well as methane anomalies in the water column.

The mounds were dominated by authigene carbonates of considerable thickness and extent either as crusts, concretions, and odd-shaped chemoherms, followed by colonies of vent clams and pogonophorans. Bacterial mats were conspicuously absent.
The mounds located between 900-1300m of water depth seem to be more completely covered with impenetrable carbonate caps and contained fewer vent organisms, than those situated on the lower slope, e.g. between 1400-1700m. The former appeared larger and more mature than the latter ones.

A list of results included preserved fluid structures in semi-cemented sediments and interlayered carbonate-sediment strata retrieved by TV-grabs (iron-sulfide-coated channels, "fritted carbonates" carbonate-lined burrows, enrichment of coarse particles, formation of pseudomorphose etc.). Gravity-cores and TV-guided multi-cores contained overconsolidated mud-clasts, which indicate extrusions of material from considerable depth. These samples are important in settling the question of whether muds erupt or extrude and if continuous or periodic material transport causes the build up of these mounds. The pore-water investigations provided indications of diagenetic processes which proceed more intensely in sediments affected by ascending fluids than in those surrounding vent sites.

The small scale inverstigations in the New Mounds area alternated with large-scale sampling of volcanic ash distribution patterns. An array of gravity cores was set up which extended up to 100 nautical miles from the margin onto the oceanic plate.
As the research clearance for the EEZ of El Salvador had expired just days before, we were not able to extend the transect NW-ward as originally planned. Never-the-less the distribution pattern of ash layers over an area of more than 1000 sq-km was sampled which is reached by the volcanic chain of Central America. In total 59 ash layers in 6 cores were identified on the basis of their causing density and magnetic susceptibility changes in the core logs. These layers were samples for detailed shore-based analyses and a preliminary correlation of the layers among each other was established. Sampling of ash layers was complemented by the investigation of pore waters of the adjacent strata which provided information on alteration reactions of ashes with sediment and seawater. Some prominent eruptions known from land-based studies of Nicaragua, El Salvador and Guatemala were documented in the sediments. Comparison with previously sampled material gave reason to assume that the ashes covered the last 150.000 years. Along the coring profiles a hydrographic transect of the uppermost 1000m recorded the distribution of methane and other parameters in the oxygen-minimum-zone. This provided criteria for the differentiation of methane from venting and methane generated "in-situ" in the water column.

Erwin Suess
Chief scientist SO 173 / 3&4

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