Near-bottom biological and mineral particle flux in the Lucky Strike hydrothermal vent area (Mid-Atlantic Ridge)

To provide information about the export and the distribution of hydrotherma l material to the deep ocean, two sediment traps with current-meters were m oored on the Lucky Strike segment of the Azores Triple Junction (Mid-Atlant ic Ridge). The results of a 25-day experiment with a single trap deployed 1 .5 m from a chimney (2 m above the bottom) are used as a reference for the composition of particles produced by the vent. A 392-day experiment with a time-series sediment trap positioned 500 m from the Lucky Strike vents (17 m a.b.) showed seasonal variations in the particulate flux and in its compo sition. Particles sampled by the trap close to the chimney (264.3 mg m(-2) day(-1)) were characterised by a high concentration of sulphur (10.8%) and barium (3.19%), and a low C/N ratio (5.5). From the current data recorded a t the same time, we estimate that this small trap was under vent influence for about 10% of the experiment duration. The calculated mean particle flux obtained with the multisample sediment trap 500 m away (7.7 mg m(-2) day(- 1)) was among the lowest recorded in the Atlantic Ocean. This situation may be explained by a very low primary production in the Azores region. No sig nificant influence of the Lucky Strike vent particles was recorded in this trap although near bottom currents were regularly favourable to transport p articles towards it. No major effect was observed on the trap efficiency by variations of the hydrodynamic conditions during this experiment. Particle s in the trap included bivalve larval shells. A comparison between their hi nge structure and that of postlarvae of the mytilid species found on these vents suggests a non-hydrothermal origin for the larvae in the trap. (C) 20 00 Elsevier Science B.V. All rights reserved.