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

Abstract

In order to provide information about the export and the distribution of hydrothermal particulate material to the surrounding deep ocean, four moorings were deployed in the vicinity of the hydrothermal Rainbow vent e eld (Mid-Atlantic Ridge, 36° 14 9N, 2250 m depth). The e rst mooring was a sediment trap with a current meter deployed at 2 m from a chimney of the Rainbow vent e eld and 1.5 m above the bottom (a.b.) for 16 days. It represented the reference for the initial composition of particles produced by the vent. The total mean mass particle e ux (6.9 g m 2 2 d 2 1 ) was distinctly higher than the e ux measured at the shallower hydrothermal vents on the MAR segment. This particulate e ux showed a high temporal variation at the scale of a few days and was characterized by a high concentration of sulphur (17.2%) and copper (3.5%) and a very low concentration of organic carbon (0.14%). Several hundred bivalve larvae belonging to the hydrothermal mytilid Bathymodio- lus azoricus were collected in this trap at the beginning of the experiment. 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 thermistor chains, were deployed for 304 days at different distances and altitudes from the Rainbow vent e eld. The mean speed of the current in the rift valley was low (6 cm s 2 1 ) and was oriented toward the north. The total mean particle mass e ux measured with the e ve sediment traps varied little, from 10.6 to 25.0 mg m m 2 2 d 2 1 , and displayed temporal variations which are typical of deep-sea environ- ments with seasonal changes in the overlying 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 copper concentrations of particles were signie cantly 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 limit reached 300 m a.b. at this distance. In the traps at 1000 m from the vents, the elemental composition of particles was similar to the pelagic particles and we assume that these traps were not in the plume during the experiment. The zooplankton obtained in the long-term trap samples revealed high density variations in relation to the distance from the vent site. The nutrient enrichment around the hydrothermal area and the abundance of free living bacteria explain these variations in zooplankton density.