OCEANIC VOLCANICS AS HABITATS FOR MICROBIAL LIFE THROUGH TIME

We report biosignals from originally glassy basaltic rocks from in-situ submarine crust spanning the youngest to the oldest oceanic basins, ophiolite fragments and greenstone belts, dating to c. 3.5 Ga. Petrographic bioalteration textures in these oceanic crustal remnants differ from abiotic alteration textures. Chemical evidence and carbon isotopic data support a biogenic origin for these bioalteration textures. We present a model for their formation and discuss petrographic observations and direct radiometric dating techniques for establishing the syngenicity of these microtextures. Our compiled data suggest that the bioalteration of volcanic glass extends back to the earliest seafloor crustal fragments. It appears that bioalteration textures are only preserved in undeformed to little-deformed, low grade, metamorphic volcanic rocks, but that their chemical tracers, in particular the Delta13C signatures, are more robust and can survive even strong deformation and high-grade metamorphic conditions. We use the observations collected from well preserved, in-situ oceanic crust as a key to finding and understanding similar micro-textures from the ancient seafloor and propose a tentative model for life in Archean oceanic crust. Overall, we argue that bioalteration textures in volcanic glasses provide a valuable tracer of the deep oceanic biosphere, which constitutes one of the largest and least explored portions of the modern, and especially the ancient biosphere.