THE EOARCHEAN- PALEOARCHEAN BOUNDARY: THE CURRENT DISCUSSION

During the 4.0 to ca. 3.6 Ga Eoarchean era, Earth had cooled down sufficiently to allow the stabilization and growth of increasing volumes of continental (-type) crust. The Archean lithology preserved from this time interval onward is characterized by typical tonalite-trondhjemite-granodiorite (TTG) suites and ultramafic to felsic volcanic igneous rocks. These rock assemblages are commonly associated with fluvial conglomerates, marine sandstones, mudstones, cherts and banded iron formation metamorphosed under low- to high-grade conditions. A low level of oxygen may have been present in the CO₂- and CH₄-rich Archean atmosphere. The oldest putative traces of life are C-isotopes and C–H–N–(P) elemental associations in the Isua Greenstone Belt, Greenland. Cherts in the Pilbara region of West Australia and in the 3.55 to 3.22 Ga Barberton Greenstone Belt, South Africa, include exceptionally preserved carbonaceous cells of prokaryotes and microbial mat fabrics of microbenthos once colonizing ancient oceans and hydrothermal systems. In the West Australian 3.48 Ga Dresser Formation, microbial mats colonizing a clastic coastal sabkha and silica hot springs formed stromatolites and microbially induced sedimentary structures (MISS). Sulfate-reducing metabolism is recorded by S-isotopes. Associated Ni suggests methanogenetic pathways, while aliphatic molecules document the presence of both Archaea and Bacteria. The already high diversity of biogenic structures and biogeochemical patterns indicates that microbial life must have been complex, forming substantial microbial films and mats with similar morphologies to those on the modern Earth. The current discussion by the ICS Subcommission on Precryogenian Stratigraphy addresses the concepts and lithological, geochemical, geochronological and paleontological characteristics that might be used for a rock record-based definition of the boundary between the Eo- and the Paleoarchean eras.