Stromatolite from the Precambrian of Australia. (Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA)

This is a sample from the oldest known definite fossil occurrence on Planet Earth. This is a stromatolite from the Paleoarchean of Western Australia.

Stromatolites are large, layered structures built up by mats of cyanobacteria. They vary in appearance, ranging from slightly wrinkled horizontal laminations in sedimentary rocks to low mounds to prominent mounds to columnar structures and other forms. Stromatolites are most common in the Proterozoic fossil record. They are scarce today, but famous modern examples occur at Shark Bay, Western Australia. The oldest known examples are ~3.5 billion years old, from Western Australia - this is a specimen from that occurrence.

From exhibit signage:

The adjacent specimen is a laminated microbialite commonly referred to as a stromatolite. Microbialites are organo-sedimentary deposits that have accreted as a result of a benthic microbial community trapping and binding detrial sediment and/or forming the locus of mineral precipitation (Burne and Moore, 1987). This specimen comes from strata of the Dresser Formation cropping out along the North Pole Dome in the Pilbara Craton of Western Australia. Its 3.48 billion year age is well constrained by radioisotope dating of interbedded volcanic rocks. Studies of carbon and sulfur isotopes in the strata are also consistent with a biological origin.

Recent mapping (Djokic et al., 2017) has revealed that the rocks are not shallow marine as previously interpreted; but, instead are interbedded with geyserite (hot spring silica) and terrestrial sediments consistent with a depositional setting in a volcanic caldera with abundant hot springs and geysers - a suitable setting for the origin of life.

Djokic, T., M.J. Van Kranendonk, K.A. Campbell, M.R. Walters, and C.R. Ward. 2017. Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits. Nature Communications 8: 1526.

Stratigraphy: Dresser Formation, Paleoarchean, 3.48 Ga

Locality: Normay Mine, North Pole Dome, Pilbara Craton, Western Australia

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