HR: 1340h
AN: B13A-0174    [Abstracts]
TI: New Mineral and Microbial Evidence of High Temperature Hydrothermal Venting Along Off-Axis Abyssal Hill Fault Scarp on the East Pacific Rise Flank at 9$\deg$27'N
AU: Ehrhardt, C
EM: cehrhardt@umail.ucsb.edu
AF: UCSB Geological Sciences, Webb Hall Building 526, Santa Barbara, CA 93106 United States
AU: Haymon, R
EM: haymon@geol.ucsb.edu
AF: UCSB Geological Sciences, Webb Hall Building 526, Santa Barbara, CA 93106 United States
AU: * Holden, T
EM: holden@bren.ucsb.edu
AF: UCSB-Bren School of Environmental Science and Management, 2400 Bren Hall, Santa Barbara, CA 93106 United States
AB: During Alvin dives in May 2002, orange-brown filamentous flocculations were sampled from an axis-facing abyssal hill fault scarp located on $\sim$0.5 Ma seafloor west of the EPR at 9$\deg$ 27'N. Many sedimentary structures thought to be products of episodic fluid expulsion were found on the top of this hill (see Haymon et al., this session). Recovered samples of flocculations from this abyssal hill have presented an opportunity to study the quantitatively significant yet rarely observed processes of off-axis hydrothermal circulation and the unknown nature of subsurface microbial communities on the ridge flanks. We report here on hydrothermal sulfide minerals and hyperthermophilic microbes identified in the orange-brown flocculations collected from the fault scarp. Powder X-ray diffractometry and Environmental Scanning Electron Microscopy (ESEM) were used to characterize mineral phases in fault scarp materials. Chalcopyrite, pyrrhotite, and pyrite constitute a significant portion of the mineral assemblage .Presence of chalcopyrite suggests that high temperature ($>$250$\deg$C) hydrothermal fluids vented from the fault scarp, and presence of pyrrhotite indicates a low oxidation state of these fluids. Molecular phylogenetic surveys are consistent withthese findings. An archaeal 16srRNA clone library reveals that a significant portion of phylotypes (54% or 39 total clones) cluster within hyperthermophilic, chemoautotrophic groups of Archaea including {\it Thermoproteales}, {\it Desulfurococcales}, {\it Thermococcales}, {\it Korarchaeota}, and {\it Methanopyrales}. The physiology and common metabolites of these groups indicate that hydrothermal fluids along the fault scarp likely exceeded 80$\deg$C and contained a variety of reduced species such as H$_{2}$S, H$_{2}$, CO$_{2}$, and CH$_{4}$. Because hydrothermal fluids emerging along off-axis fault scarps tap crustal fluid reservoirs, these scarps appear to be accessible windows into the potentially vast subsurface biosphere on ridge flanks Some of the archaeal groups identified in this study ({\it Thermoproteales}, {\it Korarchaeota}, {\it Methanopyrales}) are rarely observed at ridge crest vents or in other marine environments. This suggests that the unexplored subsurface biosphere of ridge flanks is distinct from any known marine habitat.
DE: 4840 Microbiology
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting