EVIDENCE FOR A SUBSEAFLOOR HABITAT IN 3.5 MA RIDGE-FLANK CRUSTAL FLUIDS AT BABY BARE SEAMOUNT, EASTERN FLANK, JUAN DE FUCA RIDGE

Very little is known about the subseafloor environment within oceanic crust. Recent studies strongly suggest the existence of a subseafloor biosphere at mid-ocean ridges, and show progress in identifying subseafloor microbes and understanding their ecology. However, there is little information regarding the microbial population of old oceanic crustal fluids. Ridge-flank environments between 1 and 65 Ma constitute approximately 70% of the ocean basin area. Despite this potentially enormous habitable volume, very little is known about the presence, diversity, or distribution of microbial populations in off-axis igneous crust. This study examines a subseafloor environment in ridge-flank oceanic crust at Baby Bare Seamount, a sedimented basaltic outcrop located on 3.5 Ma crust on the eastern flanks of the Juan de Fuca Ridge. Recent geochemical and heat flow measurements suggest that Baby Bare crustal fluids originate as bottom seawater entering at Grizzly Bare, an outcrop located over 50 km away. How both entrained seawater and resident crustal microorganisms are impacted by this subsurface flow is essential for understanding and characterizing the subseafloor habitat. Using a novel extraction technique of driving two stainless steel probes several meters into the summit of Baby Bare, subcrustal fluids were obtained, and the presence of a subseafloor biosphere explored by analyzing the microbial community and geochemistry of these fluids for signatures of a subseafloor habitat. Chemical measurements show that Baby Bare fluids are enriched in ammonia with very low levels of sulfide, methane, and hydrogen, and zero magnesium, indicating that water-rock reaction at temperatures in excess of 60 °C has occurred. Cell counts at Baby Bare range from 1.8 x 10⁴ to 4.8 x 10⁵ cells ml^-1, and primary enrichments of anaerobic thermophiles, indicator organisms of a subseafloor habitat, were obtained. Phylogenetic analyses of 16S rRNA genes amplified from filtered crustal fluids indicate the presence of both bacteria and archaea that may be indigenous to the subseafloor environment, including Epsilon-proteobacteria, Thermosipho spp., and putative thermophilic Crenarchaeota. Results are compared with Axial Seamount, a young, active volcano located on the Juan de Fuca Ridge known to host a well-characterized subseafloor microbial population.