FOSSIL MICROBES IN LATE PROTEROZOIC FIBROUS CALCITE VEINS FROM ARKAROOLA, SOUTH AUSTRALIA

Extremophile microbes are currently known to inhabit the surface and upper kilometres of the Earth's crust. Although deep microbial life may date back to the beginning of life on Earth, fossil evidence for deep life remains virtually non-existent. A main problem is the difficulty to find and recognise micron-sized fossil remains in the vast volume of rock.

We analysed 585 My old antitaxial fibrous calcite veins that formed at about 4-6 km depth in dark carbonaceous shales, found near Arkaroola, northern Flinders Ranges, South Australia [1]. The veins contain micron-sized biomorph structures, such as coccus- and globular structures, indications of cell division (wall bands, meridian constriction) and thin (0.1 µm) fibres with knots. The combined morphological evidence indicates that these constitute fossil microbes. These microbes must have lived during vein growth and, therefore, possibly played a role in the formation of the veins. The microbes bear some resemblance to the modern-day hyperthermal Archaea, such as Pyrodictium, which thrive at temperatures of about 80-120°C and have been found in hot springs and oil wells. Living by sulphur-metabolism, these Archaea typically alter carbonate solubility, leading to calcite precipitation, as well as minor pyrite as a by-product. Minor sulphides were found inside the veins. XRF and LA-ICPMS analyses of the veins and their immediate wall rock showed mobilisation of Ca, Fe, Mg, Na, Mn, P, S, Sr, Y, Sc, Pb, Zn, U, Cu, and Mo, some of which may indicate microbial activity.

The fossil microbes from Arkaroola indicate that life had penetrated deeper rocks at least at the end of the Proterozoic. If there is a link between the distinct antitaxial fibrous texture of the veins and microbial activity, this type of veins may help in the search for more, and possibly older evidence for deep microbial life in the geological record.

[1] Elburg et al. 2002. The origin of fibrous veins: constrains from geochemistry. Geol. Soc, London, Spec. Publ. 200, 103-118.