Microbial‐mineral floc associated with nascent hydrothermal activity on CoAxial Segment, Juan de Fuca Ridge

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Abstract
Following a June 1993 seafloor eruption at CoAxial Segment, Juan de Fuca Ridge, extensive accumulation of microbial‐mineral floc was observed in association with abundant diffuse venting on new lava flows, and around less extensive new venting on older basalts. Microscopic and SEM examination revealed a diverse assemblage of microbial forms contained within a poorly organized organo‐mineral matrix. Samples more closely resembled microbial floc described from other hydrothermal areas than the layered bacterial mats observed after the eruption at 9°N on the East Pacific Rise. Iron and silica were the major compositional elements in all samples, with the latter likely important in preserving microbial remains. Organic carbon content was on the order of 3–4%. Enzyme assays indicated the presence of autotrophic CO2 fixation and potential for sulfide oxidation. Floc covered 7–14% of the seafloor on the hydrothermally active part of the new lava flow. We discuss the possibility that the CoAxial floc originated as ejecta from the sub‐seafloor hydrothermal system rather than from in situ production around vent openings.