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 eld (Mid-Atlantic Ridge, 36°149N, 2250 m depth). The rst mooring was a sediment trap with a current meter deployed at 2 m from a chimney of the Rainbow vent 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 ux (6.9 g m 2 2 d 2 1) was distinctly higher than the ux measured at the shallower hydrothermal vents on the MAR segment. This particulate 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 Bathymodiolus 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 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 ux measured with the 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 environments 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 concentrationsof particles were signi 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.

Evidence of seasonal reproduction in the Atlantic vent mussel Bathymodiolus azoricus, and an apparent link with the timing of photosynthetic primary production

“…Interestingly, larval settlement occurs at a time of the year when phytodetrital £ux has reached its minimum (this study; Khripouno¡ et al, 2001), a behaviour which could be seen as an example of trophic opportunism on the part of the metamorphosing larvae. Interestingly, larval settlement occurs at a time of the year when phytodetrital £ux has reached its minimum (this study; Khripouno¡ et al, 2001), a behaviour which could be seen as an example of trophic opportunism on the part of the metamorphosing larvae.…”

Prefrontal bore mixing

“…This tight linear relation was established from shipborne conductivity‐temperature‐depth (CTD) profile near the mooring site at about 3 km from Rainbow vent. Implicitly, these data thus confirmed historic observations [ German et al ., ; Khripounoff et al ., ] that the vent is not traceable in temperature at distances >1 km away from it horizontally. In the data sets under investigation, salinity intrusions disturbing this relationship did not occur.…”

Rhachotropis species (Crustacea: Amphipoda: Eusiridae) of hydrothermal vents and surroundings on the Mid‐Atlantic Ridge, Azores Triple Junction zone

“…459N, 33u52.819W, 1937 m) during the same cruises and for the same period. The top-collection was at 310 m above the bottom (Khripounoff et al 2001 …”