A 6-YEAR TIME-SERIES OF HYDROTHERMAL PLUMES OVER THE CLEFT SEGMENT OF THE JUAN-DE-FUCA RIDGE

The causal links between magmatic/tectonic processes and hydrothermal venting along the mid-ocean ridge axis will be difficult to discern without broadband observations of the temporal variability of hydrothermal activity. I report here on temporal variability in the intensity and distribution of chronic and event plumes generated by continuous and episodic hydrothermal discharge at two vent fields on the Cleft segment of the Juan de Fuca Ridge. Hydrothermal temperature (DELTAtheta) and light attenuation (DELTAc) anomalies were comprehensively mapped between 1 and 4 times per year during nine cruises from 1986 to 1991. Intensity of DELTAtheta and DELTAc in the chronic plume overlying the north Cleft vent field showed more variability between years than within a given year. Both DELTAtheta and DELTAc were exceptionally high in 1986, fell to moderate levels during 1988 to 1990, and declined to minimum values in 1991. The chronic plume over the south Cleft vent field varied similarly. Superimposed on the chronic plumes were brief and intense releases of hydrothermal fluid that created event plumes distinguishable by their symmetry and exceptional rise height. Event plumes at the north Cleft field ranged from the 1986 megaplume, with a rise height exceeding 1000 m, to smaller events in 1987 and 1989 with rise heights of approximately 400 m. Chronic hydrothermal heat (660 +/- 277 (1sigma) MW) and mass (204 +/- 86 (1sigma) g s-1) fluxes from the north Cleft field were determined by assuming these fluxes were equivalent to the long-term net advection of a mean cross-section of the vertically integrated DELTAtheta and DELTAc in the plume. This 6-year history of hydrothermal plumes indicates that episodic discharge has been an important component of the recent hydrothermal flux and that the aggregate heat and mass fluxes declined significantly, though not consistently, between 1986 and 1991.