RRS Discovery Cruise 228, 21 May-28 Jun 1997. The Fluxes at AMAR Experiment: FLAME

The principle objectives of the cruise were to study the physical, geochemical and biological dispersion of the neutrally-buoyant hydrothermal plume overlying the Rainbow hydrothermal field on the Mid-Atlantic Ridge, near 36°15'N; to investigate the interacting processes active within the dispersing plume; to better constrain the source of active venting on the seabed; and to quantify the physical, geochemical and biological fluxes to the water column on the segment scale. A secondary objective was to better constrain the source of strong dissolved methane concentrations which had been observed previously in the FAMOUS segment further north. An additional objective, which evolved during the course of the cruise programme, was to investigate the dispersion of vent-larvae through hydrothermal plumes along a section of the MAR extending from the Rainbow area to the previously known Lucky Strike hydrothermal field at 37°17'N. Initially, a series of hydrographic CTD stations were occupied, complete with a lowered Acoustic Doppler Current Profiler (L-ADCP) to provide instantaneous measurements of prevailing current directions with depth in the water column. This was coupled with a suite of deep-tow CTD tow-yo sections using the hydrothermal plume instrument BRIDGET. This preliminary data set yielded an understanding of the nature of plume dispersion which was then utilised to target further water column sampling using a combination of further CTD hydrocasts for water column samples, in situ filtration of particles for mineralogical, geochemical and microbiological investigations and RMT 1+8 Net trawls for biology. The strategy was largely successful and the neutrally buoyant plume, which was revealed to be dispersing under topographic control, was traced to a distance of greater than 50km down-stream. As the programme progressed a grid of closely spaced (0.5 nautical miles) orthogonal survey lines were occupied across the suspected site of venting, yielding a resolution of closest approach to the source of better than 200m. Finally at Rainbow, a suite of 8 current-meter moorings were deployed around the vent-site to monitor long-term (>12 month) fluxes of physical parameters including suspended particulate material away from the site of venting. In addition to sampling at Rainbow, 6 CTD stations were occupied in the southern portion of the FAMOUS segment and RMT 1+8 Net Trawls were completed in the Southern AMAR, AMAR, FAMOUS, North FAMOUS and Lucky Strike segments as well as in the non-transform discontinuity (NTD) offset immediately to the south of the Lucky Strike segment