Stromatolites in the ∼3400 Ma strelley pool formation, Western Australia: Examining biogenicity from the macro- to the nano-scale

The 3426-3350 Ma Strelley Pool Formation (SPF) is a silicified, dominantly sedimentary unit within the Pilbara Supergroup, Western Australia. It is found widely across the East Pilbara Terrane, and it forms a prominent marker horizon and separates the largely volcanic 3520-3427 Ma Warrawoona and 3350-3315 Ma Kelly groups. It has become one of the key formations for study by astrobiologists, following reports of some of the world's oldest stromatolites. Abundant contextural and morphological evidence has been presented over the last decade in support of a biological role in SPF stromatolite formation. This evidence is reviewed here, and additional data are presented from recent fieldwork carried out across the ∼25 km of SPF outcrops in the East Strelley greenstone belt of the East Pilbara Terrane. In addition to contextural and morphological evidence, a compelling claim for early life requires geochemical evidence for biological cycling. A potential avenue of approach to obtain such evidence for the SPF stromatolites (and other ancient examples) is discussed in the context of a pilot study in which nano-scale secondary ion mass spectrometry (NanoSIMS) was used.