Zonation of peripheral indicators of midocean ridge hydrothermal vent activity

In relation to the regional flux of organic C and pelagic sedimentation through the water column to the deep sea, inputs from individual hydrothermal vent fields may be regarded as point sources. A detailed tabulation of observations recorded from 13,000 photographs taken within Ashes Vent Field (Juan de Fuca Ridge) indicates that distributions of all of the megafauna observed can be described in terms of maxima which coincide with three distinct zones defined in the study. These zones are the following: (1) the central vent zone (0-100 m from {dollar}>{dollar}298{dollar}spcirc{dollar}C vents), dominated by vent biota and sediment containing high-temperature mineral phases; (2) the distal vent zone (100-725 m), marked by low-temperature hydrothermal precipitates and elevated densities of some nonvent megafauna; and (3) the nonvent impact zone (725-1,300 m), in which elevated abundances of organisms indicate vent influence, although direct evidence of active venting is absent. Density maxima for vent-associated organisms (i.e., tube worms, clams, bacterial mats, crabs) occur in the central vent zone; for echinoderms, in the distal vent zone; and for sessile sponges, in the nonvent impact zone.; The mineralogy and chemistry of sediments are also affected by hydrothermal processes. At 85{dollar}spcirc{dollar}50.5{dollar}spprime{dollar}W lon. on the Galapagos Rift, the presence of hydrothermal mineral phases is limited to within 500 m of presumed vent exit sites. The hydrothermal sediment suite consists of three endmember mineralogies: (1) Fe oxide with Zn, Fe, and minor Cu sulfide; (2) nontronite; and (3) Mn oxide (birnessite and todorokite). The hydrothermal precipitates are quite similar to deposits described at the Galapagos Mounds hydrothermal field, in an off-axis setting 20 km to the south. Although elevated metal contents of ridge flank sediments reflect axial hydrothermal inputs on a regional scale, disequilibrium conditions on the seafloor and patterns of plume transport result in a constrained distribution of hydrothermal phases. Only very near hydrothermal sources do sedimentation rates (up to 141 cm/1000 yrs in this study) balance loss due to dissolution and the steady accumulation of pelagic sediment in the region (about 5 cm/1000 yrs).