CHEMICAL PLUMES FROM LOW-TEMPERATURE HYDROTHERMAL VENTING ON THE EASTERN FLANK OF THE JUAN-DE-FUCA RIDGE

We report evidence for chemical anomalies in the water column from low -temperature ridge-flank hydrothermal venting. During cruises in 1992 and 1994, samples were taken from the water column for trace metals, n utrients, dissolved gases, and particles near each of three basaltic o utcrops overlying 3.5 m. y. old crust on the eastern flank of the Juan de Fuca Ridge in Cascadia Basin. The water column above one of these outcrops, Baby Bare, which rises about 70 m above a flat turbidite pla in, was the most thoroughly sampled. Thermal, chemical (Mn, Fe, delta( He-3)%, CH4, and O-2), and particulate anomalies in the water column c onfirm the existence of (1) early diagenesis of organic matter in seaf loor sediment which produces a flux of dissolved metals and nutrients to bottom seawater, (2) hydrothermal emissions which are both focused (spring-like) and diffuse, and (3) resuspension of sediment by turbule nt flow of tidal currents about a topographical high. On the basis of data from the water column and thermal and chemical pore water data fr om 46 piston and gravity sediment cores near and on Baby Bare (FlankFl ux 90 and 92), we constrain the composition of seawater in basement an d thus the composition of spring-like water. Given this composition, n o measurable dissolved silica or phosphate hydrothermal anomalies are expected in the water column.