Micro-bioerosion in volcanic glass: Extending the ichnofossil record to Archaean basaltic crust

Microbial bioerosion of volcanic glass produces conspicuous ichnofossils in oceanic crusts that are a valuable tracer of sub-surface microorganisms. Two morphologically distinct granular and tubular ichnofossils are produced. The 'Granular form' consists of individual or coalescing, spherical bodies with average diameters of ~0.4 μm. The 'Tubular form' are straight, sometimes branched, to curving and spiralled tubes with average diameters of 1-2 μm and lengths of up to ~200 μm. A biogenic origin for these structures is confirmed by: the concentration of DNA that binds to biological stains in recent examples; enrichments in C, N and P along their margins in both recent and ancient examples; and systematic C isotope shifts measured upon disseminated carbonate in the surrounding glass. The constructing microorganisms are thought to include heterotrophs and chemolithoautotrophs that may utilise Fe and Mn from basaltic glass as electron donors and derive carbon sources and electron acceptors from circulating fluids. These microbial ichnofossils are found at depths of up to 550 metres in the oceanic crust in the glassy rims of pillow basalts and interpillow breccias. A diverse spectrum of ichnofabrics is created by overlapping phases of granular and tubular bioerosion; banded abiotic dissolution; and the precipitation of phyllosilicates, zeolites and iron-oxy-hydroxides. The resulting ichnofabrics have been documented from in situ oceanic crust spanning the youngest to the oldest oceanic basins (0 to 170 Ma). Their geological record extends to include meta-volcanic glass in oceanic crustal fragments from Phanerozoic to Proterozoic ophiolites. Examples infilled by the mineral titanite (CaTiSiO₄) have also been found in Palaeo- to Mesoarchaean pillow basalts from the Barberton Greenstone Belt of South Africa and the East Pilbara Terrane of Western Australia. Direct ²⁰⁶Pb/ ²³⁸U radiometric dating of the Australia examples has confirmed their Archaean age and thus they represent the oldest candidate ichnofossils on Earth.