Tumbiana Formation (Pilbara, Western Australia)
The Tumbiana Formation is a 2,724 ± 5 Ma old, well-preserved series of calcareous and micaceous sandstones interlayered with volcanogenic siltstone in the Fortescue Group of the Pilbara craton of Western Australia. The sediments were deposited in a shallow marine or lacustrine environment associated with the influx of riverine freshwater from the continent. The Tumbiana Formation has been the focus of most astrobiological studies for its content of well-developed, genuine biogenic stromatolites and possible filamentous microfossils. Carbon and sulfur isotopes show large variations at a fine, centimeter to meter scale. Very depleted δ13C values down to −60 ‰ suggest carbon cycling by various anaerobic or aerobic methane pathways, particularly aerobic or anaerobic methanotrophy.