Tuff life: Bioalteration in volcaniclastic rocks from the Ontong Java Plateau

Abstract

We report microscopic textural, geochemical, isotopic, and biomolecular evidence for microbial alteration of glass shards in a 337.7 m thick sequence of poorly sorted vitric and lithic tuffs recovered during Leg 192 of the Ocean Drilling Program on the Ontong Java Plateau. Petrographic analysis has revealed the highest density and variety of exceptionally preserved microbial alteration textures in the glass shards, when compared to previous studies of glassy pillow basalt margins from ocean crust and ophiolites. Two textural types of microbial alteration are commonly observed: tubular and granular. Tubular textures are characterized by well‐preserved, micron‐scale, tubular to vermicular, channel‐like features with both smooth and scalloped walls that commonly extend from a granular alteration interface rimmed by clay into unaltered glass. These channels are highly convoluted or twisted and in some instances bifurcate. Detailed scanning electron microscopy (SEM) images reveal the presence of delicate filaments and desiccated thin films with morphologies suggestive of a biogenic origin within the channels. Granular textures appear as solid bands, semicircles, or irregular patches of individual and/or coalesced spherical bodies with irregular protrusions into fresh glass. Microprobe X‐ray element maps show elevated levels of carbon, nitrogen, phosphorous, and potassium associated with the microbial alteration features. Bulk‐rock carbon isotope ratios of disseminated carbonates in tuffs preserving fresh glass are depleted (<−9‰), suggesting biologic fractionation. The presence of nucleic acids within the microbial alteration features has been confirmed though staining with ethidium bromide, a stain that specifically binds to double‐stranded DNA and RNA. The presence of DNA/RNA suggests that the biogenic features may be relatively recent and that microbes may be currently active.