Geological and hydrothermal controls on the distribution of megafauna in Ashes Vent Field, Juan de Fuca Ridge
Authors: Arquit Anne M. 
Published: February 2008

A computerized data base was constructed to aid in the interpretation of biological and geological observations recorded from 7662 photographs taken of Ashes vent field (located along the SW wall of the summit caldera of Axial Volcano, Juan de Fuca Ridge) during 1985–1986 using the Pisces IV submersible and a towed camera system. The transition region between the locus of high‐temperature vents in Ashes vent field (i.e., Inferno, 326°C; Hell, 301°C; and Virgin Mound, 298°C) and more typical environmental conditions for the summit caldera of Axial Volcano as a whole is zoned spatially with respect to sediment type and organism assemblage. Three general ecological zones are identified within the vent field: (1) the central vent zone (within 100 m of a high‐temperature vent), dominated by vent‐associated organisms (vestimentiferan tube worms, clams, bacterial mats) and sedimentation (high‐temperature, plume‐derived and low‐temperature, in situ deposits); (2) the distal vent zone (100–725 m from any high‐temperature vent), characterized by extensive fields of iron oxide, iron silicate and silica chimneys and sediment (nontronite assemblage material), as well as maximum densities of most nonvent fauna; and (3) the nonvent impact zone (725–1300 m), indicated by elevated densities of nonvent organisms relative to regional (i.e., caldera‐wide) values and maximum Bathydorus sp. sponge densities. The distribution of vestimentiferan tube worms is limited to within 90 m of known high‐temperature venting (central vent zone); and anemones were observed only between 30 and 40 m from hot vents. Clams and microbial mats are concentrated in the central vent zone, as well, but occur sporadically up to 1250 m from the hot vents in association with hydrothermal nontronite that is probably precipitating in situ from <60°C vent fluid; thus megafaunal distributions are a useful indicator of poorly defined, often diffuse low‐temperature hydrothermal activity on the seafloor. Maximum densities of nonvent echinoderms (i.e., holothurians, sea stars, urchins) are associated with the distal venting zone. Distributions of megafauna not endemic to the vent environment are sensitive to both local geology (particularly the nature of the substrate) and effects of hydrothermal activity such as enhanced food availability or hydrothermal sedimentation. Although evidence for active venting is not apparent in the nonvent impact zone, photographic data suggest that the type and number of megafauna living in this region are significantly influenced by proximity to low‐ and high‐temperature hydrothermal sources that support enhanced microbial production and produce sediment of hydrothermal origin, which may be advected to the peripheral environment. Statistical interpretation of photographic data suggests an outer limit to the extent of vent influence on benthic megafauna (i.e., Bathydorus sp. sponges) of about 1000–1300 m for the hydrothermal system active at Ashes vent field in 1985–1986. On the scale of a single vent field, hydrothermal activity apparently results in a less patchy distribution of benthic, nonvent organisms within the central and distal vent zones than is common in the nonvent environment.