Overflow, Circulation & Biodiversity - Cruise No. M85/1 - June 24 - August 02, 2011 - Brest (France) - St. John's (Canada) (English)

RV METEOR cruise M85/1 was conducted in the framework of the BMBF-funded cooperative research program North Atlantic, sub-project 2.1. It involved groups of the University of Bremen (UniHB) and the Federal Maritime and Hydrographic Agency (BSH) in Hamburg. One of the project-relevant tasks was the servicing of two deep-sea mooring arrays installed at the Mid-Atlantic Ridge (MAR) and in the Deep Western Boundary Current (DWBC) off Flemish Cap. Both arrays, consisting of three moorings each, serve to estimate variations in the strength of the deep water export and the near-surface circulation as well as the composition of water masses. Additionally, inverted echo-sounders equipped with pressure sensors (PIES) are installed at the MAR since 2006 that deliver data to estimate the strength of the subpolar gyre at the MAR. Besides fixed-station-tied work another objective pursued during cruise M85/1 was to conduct a large-scale survey of the western North Atlantic delivering distributions of the anthropogenic tracers chlorofluorocarbon (CFC) and sulphurhexafluoride (SF6). These data allow estimating the formation, aging and spreading of deep water components. With some exceptions the major research objectives related to cruise M85/1 were achieved. Out of six deep-sea moorings five were successfully recovered. One mooring from the DWBC array was lost during the recovery at conditions of reduced visibility. The mobile hydrophone which can be used to contact the acoustic releases while cruising and which is attached to the hydraulic extension units was unfortunately not available during the cruise. While following a search pattern course this made time-consuming stops of the vessel and subsequent manual lowering of a hydrophone necessary. By optimizing the vertical arrangement of the recovered DWBC instruments, however, three DWBC and three MAR moorings could be installed again. Two PIES showed erroneous signal transmission, which made an emergency recovery necessary. Both instruments were successfully retrieved, but any redeployment was hampered by technical instrument failures. Due to the installation of two spare instruments, the PIES array at the MAR was completely equipped with four instruments again at the end of the cruise as planned. The number of instruments installed along 47°N, however, had to be reduced. The large-scale dualtracer survey (CFC and SF6) could not be carried out as anticipated, since the necessary analytical system could not be transferred in time from the Southern Ocean towards Europe due to an average of the transporting vessel. This circumstance was compensated by a large-scale CFC-only tracer sampling. Different from water samples determined for a dual analysis of CFC and SF6, CFC-only samples can be measured at the Bremen lab after the end of the cruise. This analysis is, however, very time-consuming and was completed in July 2014.