Mid-Atlantic Expedition 2005, MARSÜD 2 - Cruise No. M64 - April 2 - June 6, 2005 - Mindelo (Cape Verde) - Dakar (Senegal) (English)

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The three main goals of the M64/1 cruise were to study the (1) volcanic, hydrothermal, and biological processes at the newly discovered vents at 4°48'S on the Mid-Atlantic Ridge (MAR), (2) sample an extensive young volcanic eruption site at 8°48'S, and (3) explore and find more hydrothermally active regions on the MAR further south. Thus, the first part of the cruise was dedicated to the observation and sampling of the three hydrothermal sites near 4°48'S, which had been discovered in the previous month during a British-American cruise. Seven dives with the MARUM (Univ. of Bremen) remotely operated vehicle (ROV) were performed at the sites near 4°48'S which yielded numerous fluid, biological and rock samples as well as photographic and film material and measurements. The most important results were the observation of a boiling fluid at 3000 m water depth with a temperature of 400°C, a change of the mineralogy of the precipitated minerals at these vents, and evidence for very young volcanism. It appears that a recent volcanic eruption led to a significant change in the composition and temperature of the fluids. The second main study area of M64/1 was a large volcanic field on segment A2 at 8°48'S which was identified on the side-scan maps of the M62/5. This feature was sampled during two ROV dives and with the wax corer. First results indicate that the volcanism in this field may be older than originally suggested by the reflectivity and preliminary geochemical data imply that the large lava formations consist of two chemically distinct lava types. Thus, although the lava field probably formed within a brief period of time, the composition of the lavas in the northern part is different from the southern part implying different magma reservoirs. The third studied area was the northern part of segment A3 which is relatively shallow (about 1500 m depth) and lies on thickened oceanic crust (about 11 km thick). Here, we found a new hydrothermal field, the so-called Lilliput diffuse field, which is the southernmost known hydrothermal field along the MAR. The occurrence of predominantly juvenile mussels suggests recent reactivation of the hydrothermal activity, possibly following an eruptive event. The investigations of cruise M64/2 are a continuation of the program started in 2004 (cruise M60/3) between 14°45'N and 15°05'N on the Mid-Atlantic Ridge. While basic geochemical and biological studies were carried out during the 2004 cruise in the Logatchev hydrothermal field, the emphasis of the 2005 cruise was on the temporal variability of fluid emanations, fluid temperature and chemistry, microbial activities and associated fauna at selected hydrothermal vent sites. Biological, microbiological, petrological and hydrological samples were taken using the ROV, TV-grab, and CTD where several fluid chemical parameters as well as, for example, methane oxidation rates were determined immediately on board. Hydrothermal fluids and sulfides were sampled at several of the hydrothermal chimneys in order to determine the differences in composition between the different structures of this ultramafic-hosted site. Plume exploration and mapping of the seafloor with the ROV revealed more hydrothermal activity in the area of the Logatchev field. Several long-term observation systems were successfully installed on the seafloor in order to measure the variation of temperature at a hydrothermal vent mussel bed, the temperature variation in the bottom water, as well as the seafloor tilt and pressure variations. Furthermore, several experiments were conducted to determine variations of VI METEOR-Berichte, Cruise 64, 2 April - 6 June 2005 temperature and fluid compositions in the range of several hours to days which were combined with the sampling of fauna at the respective vents. The results of these measurements will allow the study of the links between the geochemical energy supply from hydrothermal fluids and the vent organisms. Geochemical gradients in vent fluids along mussel beds were determined and mussels were collected along these gradients for analysis of the biomass and activity. Furthermore, a relocation experiment of hydrothermal mussels was carried out where the mussels were removed from hydrothermal activity in order to determine the temporal evolution of the methane oxidation rates and the relationship between symbionts and host mussels.

  • Title:
    Mid-Atlantic Expedition 2005, MARSÜD 2 - Cruise No. M64 - April 2 - June 6, 2005 - Mindelo (Cape Verde) - Dakar (Senegal)
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    METEOR-Berichte ; M64 ; 1-103
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    DFG-Senatskommission für Ozeanographie
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    103 pages
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    Electronic Resource
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    DDC:    550 Earth sciences and geology
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