Mid-Atlantic Ridge site surveys of sediments and basement in North Pond (western flank of the Mid-Atlantic Ridge, 23°N) - Cruise No. 11, Leg 1 (English)

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North Pond is an isolated small sediment pond (6 km by 15 km) located on the western flank of the Mid-Atlantic Ridge (23°N) which offers the opportunity to study microbial communities and their activities in deeply-buried sediments and the underlying basement. North Pond has been previously studied by a series of DSDP and ODP drill holes and seafloor observatories. However the existing site survey data are not sufficient for the approved North Pond IODP drilling expedition, scheduled to take place in FY 2011. Therefore a site survey cruise on the German RV Maria S. Merian (MSM 11/1) took place from February 17 - March 12, 2009 (Fort-de- France, Martinique to Dakar, Senegal) with the goal to map in detail the sediment-basement interface and increase the heat flow measurement coverage. Due to an accident on board the scheduled working days on site were reduced from 14 to 6 days. Therefore a number of detailed investigations especially in the southern part of the basin had to be abandoned. A set of geophysical survey data was obtained comprising 14 lines (about 240 km) of multibeam data in combination with single channel seismics and sediment echosounding, 61 successful heat flow measurements, and recovery of 68 m of core. The survey confirms the existing results based on two seismic profiles from 1989 but the new data show the sedimentbasement interface in much more more detail. New heat flow measurements complement the existing heat flow coverage of Langseth et al. (1992) and confirm the hydrothermal circulation pattern with inflow of cold seawater at the southern rim of the basin and upflow of warm water at the north/north-western boundary. The new detailed geophysical data set will allow to position the proposed drillholes and constrain hydrogeological modelling of the circulation in the upper crust.