BIOGENIC ORIGIN FOR ORGANIC MATTER IN CHERT/JASPER AND SHALES OF THE 3.46 GA TOWERS FORMATION, PILBARA, WESTERN AUSTRALIA

The 3.46 Ga Marble Bar chert/jasper beds (~70 m thick), which occur ~1 km stratigraphically below the Apex chert where Schopf (1993) reported "cyanobacteria-like" microfossils, were the first target of NASA's Astrobiology Drilling Project. We have extensively investigated the petrological, mineralogical, and geochemical characteristics (e.g., fluid inclusions; major and trace elements; REEs; S and C isotopes) of the drill cores. Our results suggest that the chert/jasper beds are mostly composed of silica, hematite, and siderite that nucleated when episodic discharges of submarine hydrothermal fluid (T > 150°C) mixed with oxygenated deep (>400m) ocean water and settled near the vent sites. Organic C-rich beds (~mm thick), which occur with abundant clastic minerals (clays) at ~10 cm intervals in the chert/jasper beds, appear to represent remnants of microbial mats that developed near the vents during quiescent periods of submarine hydrothermal activity. Enrichments of uranium were found in organic C-rich bands, suggesting that the geochemical cycle of U at 3.46 Ga was the same as today.

Pyrite- and organic C-rich black shale beds (~5 to ~30 m thick) intersected in another drill hole a few kilometers below the Marble Bar Chert. The S- and C- contents, their isotopic values, and the morphology of pyrite crystals in these black shales suggest that the organic C represents remnants of planktonic and benthic organisms that lived in locally-developed anoxic basins, and that the pyrite crystals were formed by both bacterial sulfate reduction and hydrothermal processes.

Thus, diverse communities of microorganisms had evolved by ~3.5 billion years ago in the oceans.