MICROBIAL ALTERATION VOLCANIC GLASS: A WINDOW INTO THE SUBSURFACE BIOSPHERE BENEATH THE SEAFLOOR

Recent ocean drilling has opened a window into the deep biosphere beneath the seafloor. Studies of in situ oceanic crust have led to the discovery of textural, geochemical, isotopic, and biomolecular evidence for fossil and active microbial alteration of basaltic glass. Bioalteration of glass from pillow basalt rims and tuffs is seen in two textures: tubular and granular. Tubular textures are characterized by micron-scale, tubular to vermicular, channel-like features extending into fresh glass. Granular textures appear as solid bands, semicircles or irregular patches of individual and/or coalesced spherical bodies with irregular protrusions into fresh glass. Within pillow basalts these textures are observed to extend away from fractures that provided the necessary pathways along which liquid water and nutrients were able to reach the microbial communities. Maximum bioalteration is observed near 300 m sub-sediment at about 70 C. Glass shards in a submarine tuff display a much greater density and variety of granular and tubular microbial alteration textures. In the tuffs these textures extend into fresh glass from a granular alteration interface rimmed by clay at the grain boundary. It is likely that glass shards in the tuffs were exposed to much higher W/R (particularly along grain boundaries) than the glassy rims on basaltic pillow lavas which are less permeable. This may make glass in these tuffs more susceptible to microbial alteration than glassy seafloor pillow basalts. Disseminated carbonate in pillow basalt glasses from in situ oceanic crust show differences in C isotope ratios from those of the adjacent crystalline cores that likely relate to microbial activity during alteration. The generally low 13-C (<–7‰) of carbonates in basaltic glass are attributed to metabolic by-products of Bacteria oxidizing dissolved organic matter from pore waters. A few positive 13-C values have been observed in glass samples from slow-spreading ridges where nearby serpentinization may be hypothesized. These 13-C enriched carbonates may result from lithotrophic Archaea producing CH4 from H2 and CO2. Although the exact metabolic requirements of the microbes responsible for the observed textures remains unknown it appears that life may exist at depth in the basaltic oceanic crust in areas accessible to warm seawater.