ISOTOPE GEOCHEMISTRY OF 2.7 GA CHERT AND CARBONATE, TUMBIANA FORMATION, AUSTRALIA: IMPLICATIONS FOR LATE ARCHEAN CLIMATIC TEMPERATURES AND EARLY EVOLUTION OF LIFE

The most ¹⁸O depleted carbonates and cherts in the geologic record are found in the 2.7 Ga Tumbiana Formation in Western Australia. δ¹⁸O for carbonate averages about -17.5‰ (literature and new data) and ranges from -26.6‰ to -8.3‰ (PDB). Chert ranges from +11.6‰ to +15‰ (SMOW). Although extraordinarily low in ¹⁸O, chert in this unit has the highest δD (-31‰ to -22‰, SMOW) measured for chert of any age. No known metamorphic or late diagenetic alteration process can account for the low δ¹⁸O except for shallow hydrothermal alteration in the presence of meteoric waters. However, there are no cross-cutting veins, deformation, metamorphic minerals, or any other geologic evidence of hydrothermal alteration in the rocks examined. Indeed, the Tumbiana Formation is one of the least altered Archean sedimentary units known. High δD in the cherts precludes low¹⁸O glacial melt waters as diagenetic fluids. Therefore, the simplest explanation is that the Tumbiana Formation was deposited and stabilized in a late Archean lake, consistent with some (but not all) available sedimentologic interpretations. Even with low ¹⁸O lake waters, however, the δ¹⁸O values are so low that temperatures in excess of 50°C are required. δ₁₃C is relatively uniform near 0.0 ‰ but ranges between -4.0‰ to +1.9‰ (PDB), suggesting equilibration of lake waters with atmospheric CO₂ and absence of significant input of terrestrial photosynthetic C. Abundant and pervasive stromatolites, organic material, and at least one good microfossil occurrence suggest that life was thriving in this environment. Contemporaneous and older Archean marine units described so far do not show comparable evidence of such biological intensity. Taken together, the existing data suggest that the previously inferred high temperatures for the early Archean (Knauth and Lowe, 2003) persisted to at least 2.7 Ga and are consistent with the hypothesis (Knauth, 2005) that early evolution was most active in non-marine environments.