Particle flux in the Rainbow hydrothermal vent field (Mid-Atlantic Ridge):Dynamics, mineral and biological composition

In order to provide information about the export and the distribution of hy drothermal particulate material to the surrounding deep ocean, four mooring s were deployed in the vicinity of the hydrothermal Rainbow vent field (Mid -Atlantic Ridge, 36 degrees 14'N, 2250 m depth). The first mooring was a se diment trap with a current meter deployed at 2 m from a chimney of the Rain bow vent field and 1.5 m above the bottom (a.b.) for 16 days. It represente d the reference for the initial composition of particles produced by the ve nt. The total mean mass particle flux (6.9 g m(-2) d(-1)) was distinctly hi gher than the flux measured at the shallower hydrothermal vents on the MAR segment. This particulate flux showed a high temporal variation at the scal e of a few days and was characterized by a high concentration of sulphur (1 7.2%) and copper (3.5%) and a very low concentration of organic carbon (0.1 4%). Several hundred bivalve larvae belonging to the hydrothermal mytilid B athymodiohis azoricus were collected in this trap at the beginning of the e xperiment. The density of larvae decreased strongly at the end, indicating a patchiness distribution or a discontinuous reproduction of this species. The other three moorings, including sediment traps, current-meters and ther mistor chains, were deployed for 304 days at different distances and altitu des from the Rainbow vent field. The mean speed of the current in the rift valley was low (6 cm s(-1)) and was oriented toward the north. The total me an particle mass flux measured with the five sediment traps varied little, from 10.6 to 25.0 mg m m(-2) d(-1), and displayed temporal variations which are typical of deep-sea environments with seasonal changes in the overlyin g production. However, in the trap at 500 m from the vents 150 m a.b., the presence of the hydrothermal plume can be observed: the sulphur, iron and c opper concentrations of particles were significantly higher compared to the particles sampled in the pelagic reference trap. The plume composition was about 50% hydrothermal particles and 50% pelagic particles and its upper l imit reached 300 m a.b. at this distance. In the traps at 1000 m from the v ents, the elemental composition of particles was similar to the pelagic par ticles and we assume that these traps were not in the plume during the expe riment. The zooplankton obtained in the long-term trap samples revealed hig h density variations in relation to the distance from the vent site. The nu trient enrichment around the hydrothermal area and the abundance of free li ving bacteria explain these variations in zooplankton density.