PROSA Process Studies in the Eastern South Atlantic – Cruise No. M76 – April 12 – August 24, 2008 – Cape Town (South Africa) – Walvis Bay (Namibia) (English)

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In: METEOR-Berichte   ;  M76 ;  1-180  ;  2012

METEOR cruise M76 comprised of four legs to three working areas in the eastern South Atlantic the shelf and continental slope off Namibia, the deep-sea fan of the Congo River with the giant pockmark REGAB, and the DIAPIR area off Angola; Fig. 1). Due to a high security risk, originally planned investigations in the area of the Niger fan could not take place. The main objective of the first leg (M76/1a,b) was to provide the scientific party with new samples for deep biosphere research. Microbial life in deeply buried sediments has only moved more and more into the focus of geoscientific and microbial research in the mid of the nineties. Main questions concentrate on the composition of the comunities and the energy sources for the material cycles. With long sediment cores from the upper continental slope off Namibia sample material for a variety of interdisciplinary investigations should be gained. For this purpose the new wire-line drilling technique of the sea floor drill rig (MeBo) was tested for the in time in unconsolidated marine sediments. Unfortunately, but typical for the season, strong low-pressure areas around the Southern Ocean caused high swell up to 6m in the whole working area during most of the time, which very much restricted the time for deployments of MeBo. Nevertheless, very valuable information could be gained which led to important improvements for the next uses of the equipment, e.g. during METEOR cruise M78/3. Conventional devices were used to take a huge amount of new and very promising samples for subsequent analysis in the home labs. Furthermore, the waiting time for better weather conditions was used two times for intensive hydroacoustic surveys of a relatively small area (about 360 km²) in the southern working area, where very conspicuous pock mark structures could be identified. During the second leg of this cruise (M76/2), reactions of geochemical environmental conditions on natural, temporal variations in the hydrodynamic circulation of the Benguela costal upwelling off Namibia and Angola were investigated. Therefore a CTD station grid was occupied that consisted of transects perpendicular to the coast. Observations in the northern Benguela suggest that the summer season 2008 was characterised by exceptionally high surface temperatures and an increased frequency of coastal hydrogen sulphide outbreaks. Not only from the biogeochemical perspective, an on-line measuring system (FerryBox; 4H-JENA) could be used successfully to document spatial and temporal variations of the oceanographic and biogeochemical situation during the expedition M76/2. A very extensive data set could be generated which allows more detailed investigation on the complexity of the biogeochemical interrelations within an upwelling system. Especially, new information on the nitrogen cycle could be retrieved. With a suite of autonomous instruments, including benthic landers, conditions in the benthic boundary layer (BBL) were investigated. Additional velocity measurements show that surface waves induce high current velocities in the BBL even at water depths of about 100m. Oxygen measurements seem to correlate with wave frequency and indicate a possible vertical transport mechanism for solutes and resuspended matter. The main objectives of the first of the expedition GUINECO (M76/3a) were to investigate the controls of fluid and gas migration, the accumulation of free gas in and beneath the gas hydrate stability zone, and the geochemical and microbiological interactions at the sediment-water interface, with the help of high-resolution seismo-acoustic mapping, video observations and sediment coring on the West African continental margin. Main questions include on one hand the influence of facies, grain size, and permeability variations within the sediment; and on the other, the effect of salt tectonic deformation on gas accumulation and migration. Different lithological settings were selected for comparative multi-scale investigations with swath bathymetric, sediment echo-sounder, multichannel seismic, and backscatter systems as well as geological and bio-geochemical sampling. The immediate products of the cruise were a complete picture of the structural and sedimentary framework; near-3D images of vent sites, migration pathways and gas reservoirs; the identification of areas of recent vent activity, an overview information for subsequent ROV work during leg M76/3b and a quantification and characterization of active vent sites and shallow gas hydrate deposits in the light of different geological conditions. During the second leg of the expedition GUINECO (M76/3b) the different target areas and active venting sites identified during the previous cruise were visited with ROV QUEST. Beside further visual observations, intensive sampling of escaping gas bubbles, sediments and organisms were carried out. Also in situ microsensors and a respiration chamber were used. Furthermore, several profiling instruments and carbonate colonization experiments were deployed, while some experimental moorings from former expeditions could be recovered.


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