HYDROTHERMAL EFFECTS WEST OF THE JUAN-DE-FUCA RIDGE

The Juan de Fuca Ridge in the northeast Pacific Ocean is a region of e xtensive hydrothermal activity, and plumes emanating from the ridge ca n be dispersed by the oceanic circulation at about 2000 m. Off-axis te mperature-salinity characteristics and water-property maps suggest a h istory of variability in hydrothermal activity on the ridge. Distincti ve bulges in theta-S curves from stations several hundred kilometers w estward have characteristics very similar to those near and over the r idge, possibly representing plume remnants from prior venting which de tached from the source and drifted with the regional circulation. If t hey were advected from the ridge at about 1 cm s-1, they would represe nt venting events which occurred 2-3 years earlier. Although not concl usive, currents observed on the ridge in 1984-1985 showed a 6-month wi nter interval of continuous off-axis flow to the west, but, unfortunat ely, it occurred between annual shipboard observations. Year-to-year v ariations are shown by relatively large increases in the near-ridge po tential temperatures from 1987 to 1988, possibly suggesting another ev ent that may propagate westward. The thermal observations here are con sistent with simultaneous distributions of silica, suspended particula tes and helium described by others. The variations in temperature, as well as flow variability near the ridge, are sufficiently large that t hey need to be considered when interpreting other water column observa tions, not only near hydrothermally active ridges but also at least se veral hundred kilometers away from the venting sites.