WATER COLUMN ANOMALIES ASSOCIATED WITH HYDROTHERMAL ACTIVITY BETWEEN 11-DEGREES-40' AND 13-DEGREES-N ON THE EAST PACIFIC RISE - DISCREPANCIES BETWEEN TRACERS

During the HYDROFAST cruise (R.V. Jean Charcot, December 1986) on the East Pacific Rise (EPR), entrainment and vertical transport of deep ocean water by buoyant hydrothermal plumes were clearly seen between the two (12-degrees-53' and 12-degrees-37'N) Overlapping Spreading Centers (OSC). Geochemical anomalies (CH4, Mn, He-3), coupled with deviations from ambient T/S linearity, confirmed the existence of a hydrothermal field around 12-degrees-50'. New intense hydrothermal venting around 12-degrees-43' was also discovered. These two submarine hydrothermal fields produce effluent plumes which coalesce and form a 250 m thick layer. T/S diagrams show the complex structure of temperature anomalies in the water masses affected by hydrothermal inputs. The hydrothermal clouds are depleted in dissolved oxygen relative to deep-sea water. Simple relationships do not exist between temperature and chemical tracers. The discrepancies result from a combination of various reasons: variable vent fluid characteristics, residence time of tracers in seawater, scavenging and oxidation kinetics of elements and biological activity. However, CH3/He-3 and Mn/CH4 ratios are comparable to those observed in other hydrothermal fields at spreading axes. These elements and compounds can be used at zero age to describe the variation in hydrothermal activity.