DEVELOPING AN ICHNOLOGICAL FRAMEWORK FOR BIOEROSION IN VOLCANIC GLASS

Laboratory observations have shown that microbes are capable of etching glass and textural, geochemical and microbiological data from the rock record suggest that microbial bioerosion of volcanic glass is a 3.5 billion year old process. Granular and tubular traces found in subseafloor pillow lavas represent the time integrated products of alteration by microbial consortia in addition to abiotic processes. A direct link is yet to be made however, between specific alteration morphologies and the trace making organisms. Here we outline how an ichnological approach can be used to decipher the history of bioerosion in the subseafloor and to shed light on variables that may control this bioerosion process and perhaps the tunnelling mechanism.

Two ichnogenera have been proposed for bioerosion traces in volcanic glass: Granulohyalichnus igen. and Tubulohyalichnus igen. that are found in both recent and meta-volcanic glass. The trace makers are unknown but thought to include hetero and\or chemolithotrophic microorganisms. Five morphotypes have been identified and termed ichnotaxa: (1) a granular species G. vulgaris isp.; (2) an unornamented tubular species T. simplus isp.; (3) an annulated tubular species T. annularis isp.; (4) a helicoidal tubular species T. spiralis isp.; and (5) a branched species T. stipes isp. How the microbes tunnel is not known, although cellular extensions similar to fungal hyphae observed in soils have been hypothised.

Down hole studies have mapped the abundance of bioerosion traces in volcanic glass with depth in the modern subseafloor. These have found that Granulohyalichnus igen. occurs down to 550m and is the most abundant, whereas, Tubulohyalichnus igen. comprises ≤20% of the total bioalteration with an abundance maxima at 120–130m depth. The total percentage of microbial alteration increases with permeability and the presence of celadonite, suggesting that oxygenated waters support bioerosion in the recent seafloor.

Work to integrate this type of ichnological data from the geological record with geochemical data and studies of in-situ glass alteration will get us closer to understanding the types of microorganisms involved, when and how they bioerode volcanic glass.