HYDROTHERMAL EFFECTS WEST OF THE JUAN-DE-FUCA RIDGE

The Juan de Fuca Ridge in the northeast Pacific Ocean is a region of extensive hydrothermal activity, and plumes emanating from the ridge can be dispersed by the oceanic circulation at about 2000 m. Off-axis temperature-salinity characteristics and water-property maps suggest a history of variability in hydrothermal activity on the ridge. Distinctive bulges in theta-S curves from stations several hundred kilometers westward have characteristics very similar to those near and over the ridge, possibly representing plume remnants from prior venting which detached from the source and drifted with the regional circulation. If they were advected from the ridge at about 1 cm s-1, they would represent venting events which occurred 2-3 years earlier. Although not conclusive, currents observed on the ridge in 1984-1985 showed a 6-month winter interval of continuous off-axis flow to the west, but, unfortunately, it occurred between annual shipboard observations. Year-to-year variations are shown by relatively large increases in the near-ridge potential temperatures from 1987 to 1988, possibly suggesting another event that may propagate westward. The thermal observations here are consistent with simultaneous distributions of silica, suspended particulates and helium described by others. The variations in temperature, as well as flow variability near the ridge, are sufficiently large that they need to be considered when interpreting other water column observations, not only near hydrothermally active ridges but also at least several hundred kilometers away from the venting sites.