nontronite chimneys mark regions of <^60°C venting in distal zones 

 of lower velocity flow. Such variability is observed within 

 200 m of the locus of high temperature venting in Ashes Vent 

 Field. 



BIOLOGICAL ZONATION 



In addition to geochemical zonation recorded in hydrothermal 

 vent chimney samples and sediment, biological zonation is also 

 observed along gradients of temperature and water chemistry 

 radiating from hydrothermal sources. Vent habitats encountered 

 within Ashes Vent Field range from diffuse, low temperature seeps 

 to black smokers venting fluid in excess of 270°C. 



Both biological and geological information obtained from 

 photographs of the Ashes Vent Field ( R/V Discoverer , 1983; 

 Alvin, 1984; Angus , 1984) have been included in a computer data 

 base and mapped with respect to reconstructed bottom track lines 

 on a SeaBeam base map of the site. These data suggest that for 

 both vent and nonvent organisms, species distribution and 

 megafaunal abundance are zoned similarly to sediment type (i.e., 

 with distance from a vent source). Dense thickets of tube worms 

 and bacterial mat communities extend to 100 m from the highest 

 temperature venting recorded; clams, to 200 m; and yellow-orange 

 hydrothermal nontronite and iron oxide sediment, to at least 

 500 m from the hot vents (Arquit et al., 1985). 



No bacterial mats or high abundances of megafaunal 

 communities are associated with the zone of hydrothermal sediment 

 surrounding the high temperature vents. About 4.7 percent of 

 Alvin photographs taken in the Ashes Vent Field in 1984 (i.e., 

 137 of 2,920) contain more than 300 hexactinellid sponges per 

 frame; of these photos having elevated sponge densities, however, 

 only 17.5 percent include any low temperature hydrothermal 

 nontronite sediment. Preliminary results show that maximum 

 sponge densities occur beyond this seemingly barren sediment 

 zone, at least 250 m from the vent source, and then drop to 

 average caldera-wide values as more typical benthic living 

 conditions are encountered 700 to 1,000 m from the locus of high 

 temperature venting. The local density of other megafauna 

 identified in photographs can also be described in terms of 

 distance from venting centers. 



Nonparametric statistical analysis of information gathered 

 from photographic data indicate that other factors (e.g., 

 community dynamics, local geology, substrate, sediment type and 

 thickness, morphology, and surface texture of lava flows) must be 

 important controls on organism distributions as well. Additional 

 data from Alvin dives and camera tows taken within Ashes in 1985 

 and 1986 (courtesy Marine Resources Research Division, NOAA/PMEL, 

 Newport, OR) are currently being added to this data base to fine- 

 tune initial observations. The excellent navigation and 



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