BIO-INTERACTION WITH BASALTIC GLASS AND ITS IMPORTANCE IN MAPPING THE DEPTH OF OCEANIC BIOSPHERE

During the last decade much attention has been focused on the existence of microbes contributing to the alteration of glassy rim of pillow lavas in upper oceanic crust. The first suggestion of microbial pitting of volcanic glass was made two decades ago, without any convincing mechanism explaining how microbes facilitate dissolution of glass. Later, in a study of Icelandic hyaloclastites the presence of bacteria was observed, hosted within alteration textures of basaltic glass. Based on this finding it was suggested that colonising microbes may cause local variations on pH and/or ligands that allow them to chemically "drill" into a silicate substrate, a proposal that was experimentally verified. Subsequently, similar textures were found in the fresh glassy part of pillow lavas from several DSDP/ODP drill sites into the in-situ volcanic basement with ages from Quaternary to 170 Ma, and in various ophiolites (90 Ma Troodos-Cyprus, 160 Ma Mirdita ophiolite-Albania, 440 Ma Solund-Stavfjord ophiolite-W. Norway). The microbial influence on the alteration process of basaltic glass has also been confirmed by documenting the presence of DNA and ribosomal RNA within the structures of anticipated biological origin. Further evidence of microbial fingerprints within altered glass has been demonstrated by low and high d13C values, as well as the element distribution of C and N. The upper crust displays a large variability in the relative importance of biotic to abiotic alteration, and the degree of bioalteration decreases with depth. Thus, the fraction of bioalteration of the total alteration of glass ranges from 20-90 % in the upper 300 m down to a maximum of 10 % at ~ 500 m depth. This might be due to a natural variability in the abundance of bio-altered glass or to biased sampling from low drilling recovery in young crust. The proportion of bio-altered to abiotically altered glass does not show any systematic variation with the age of crust. Thus bioalteration lasts as long as abiotic alteration does.