Manganese, methane and helium anomalies in sea water above the Mid-Atlantic Ridge : evidence of hydrothermal activity / Anomalies de manganèse, méthane et hélium dans la colonne d'eau de mer au droit de la ride médio-atlantique : des évidences d'activité hydrothermale

Manganese, helium and methane are highly enriched in hydrothermal effluents and their vertical distributions in sea water are commonly used as chemical tracers of hydrothermal activity. The cruise performed during may 1985 with the RV Akademik Boris Petrov (Vernatsky Institut of Geochemistry, Moscow) allowed us to present vertical total dissolved manganese and CH4 concentration profiles at seven stations : four along the south Middle Atlantic Ridge (13°20'S -14°64'W ; 14°12'S -13°58'W ; 21°28'S -11°37'W ; 19°56'S -11°52'W) and three along the north MAR (12°24'N -44°05'W ; 13°47'N -44°59'W ; 14°05'N -45°01'W). Some samples in each profile have been analysed for 3He/4He ratios and 4He amounts. The hydrographie stations have been set up over the axial ridge valley combined with bathymétrie and geochemical rock studies. The results show many hydrothermal tracer anomalies in the water column both in the northern and southern studied areas. Although the amplitude and shape of the anomalies are clearly smaller than those observed at the well documented hydrothermal sites (Galapagos Spreading Center, East Pacific Rise, Juan de Fuca Ridge, TAG), plumes originated by hydrothermal discharge are present in the two southern studied areas as well as in the northern one.