A combined TEM and NanoSIMS study of endolithic microfossils in altered seafloor basalt

The incipient alteration of basaltic glass to palagonite in recent seafloor lavas from the arctic Mohns Ridge was studied by complimentary high-spatial-resolution geochemical techniques: TEM-EDS (transmission electron microscopy energy dispersive X-ray spectroscopy) and NanoSIMS (nano-scale secondary ion mass spectrometry). Rounded to elongated pores 0.5-2μm across were found embedded in compact palagonite that have sizes and shapes comparable to microbial cells. In-situ elemental mapping revealed that the micropore rims are comparable in composition to the bulk palagonite and that some are enriched in manganese. Elevated concentrations of carbon and nitrogen were also found in some of the micropores. Hence these structures are interpreted as fossilised bacteriomorphs of endolithic microorganisms that inhabited fractures in the basaltic glass. The preferential accumulation of Mn in some of the cell encrustations suggests the mineralisation of Mn-oxidising bacteria. These data provide further evidence for the involvement of microorganisms in the colonisation and chemical alteration of recent seafloor volcanic glass and identify micro-scale Mn enrichments associated with micropores as a promising biosignature in such rocks.