ARCHEAN HYDROTHERMAL SYSTEMS AND THE NATURE OF THE ABIOTIC/BIOTIC BOUNDARY

Stromatolitic structures that occur at two well-defined stratigraphic levels within the Archean stratigraphic succession on the 3.5Ga Pilbara Craton of Western Australia have been cited as strong evidence of an early Archean biosphere. Detailed mapping and sampling of the uppermost of these intervals, the Strelley Pool Chert, shows that it can be interpreted as a multiphase hydrothermal deposit that accumulated in response to a declining temperature regime as the hydrothermal system evolved.The stromatolitic structures were generated by direct precipitation of carbonate from hydrothermal fluids in a relatively deep marine setting where it was modulated by sea floor currents. We find no direct evidence of biogenic mediation in their construction - they are sedimentary structures.

Although the stable isotopes of carbon provide data that are consistent with a biogenic origin for the carbonates and cherts, field relations suggest that they were more likely fractionated by abiotic processes at depth in the hydrothermal system. Fractionation may have initially involved Fischer-Tropsch type processes at a depth of almost two kilometers (500 bar) and temperatures of <200oC. Further fractionation of the carbon isotopes may have resulted from de-gassing of hydrothermal fluids at c.1.1 km depth (<300 bar) in the dyke system as they migrated towards the seafloor. Sulfur occurs in two oxidation states as sulfide and sulfate. Field relations suggest that they are derived from two distinct sources and do not necessarily reflect the oxidation state of the ancient ocean and atmosphere.

This study questions the remaining evidence for life in the early Archean of Australia. The environmental setting in which life first emerged appears to have been dominated by hydrothermal processes that, because of their unusual chemistry, had the potential to emulate and perhaps synthesize life itself. These hydrothermal systems potentially overwhelm any biospheric geochemical signal and may therefore be the most difficult environments in which to draw the prebiotic-biotic boundary.