Paper No. 55-5
Presentation Time: 2:30 PM
GEOCHEMICAL SOCIETY INGERSON LECTURE: ARCHEAN (3.35 GA) LIFE ON LAND IN WESTERN AUSTRALIA
The Pilbara craton of northwestern Australia is known for what were, when reported, the oldest known microfossils and paleosols on Earth, but both interpretations are controversial. Both the microfossils and the paleosols have been considered hydrothermal artefacts: carbon films of vents and a large hydrothermal cupola, respectively. This study resampled and analyzed putative paleosols within and below the Strelley Pool Formation (3.35 Ga), at four classic locations: Strelley Pool, Steer Ridge, Trendall Ridge, and Streckfuss. The same sequence of sedimentary facies and paleosols was newly recognized near the locality for microfossils in chert of the Apex Basalt (3.46 Ga) near Marble Bar. The Apex chert did not taper, nor show phyllitic alteration, nor stockwork veining like a hydrothermal intrusion. The fossiliferous Apex chert is instead a thick (15 m) interbed of constant thickness within a sequence of pillow basalts, which form an angular unconformity capped by the same pre-Strelley paleosol and Strelley Pool Formation facies found elsewhere in East Pilbara Shire. Baritic alluvial paleosols within the Strelley Pool Formation include common microfossil spindles (cf. Eopoikilofusa) distinct from marine microfossil communities with septate filaments (Primaevifilum) of cherts in the Apex and Mt Ada Basalts. Phosphorus and iron depletion in paleosols both within and below the Strelley Pool Formation are evidence of soil communities of stable landscapes living under an atmosphere of high CO2 (2473 ± 134 ppmv or 8.8 ± 0.5 times preindustrial atmospheric level of 280 ppm) and low O2 (2181 ± 3018 ppmv or 0.01 ± 0.014 times modern). Abundant barite in alluvial paleosols and inferred rates of bedrock soil production require acid sulfate weathering by a variety of anaerobic sulfur bacteria of a terrestrial ecosystem later driven underground and out to sea by Proterozoic (2.3 Ga) aerobic carbon-cycling microbes. Sulfur-cycling microbes were widespread on land and at sea by 3.35 billion years ago.