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Archaean asteroid impacts, banded iron formations and sulphur MIF-S anomalies.

Andrew Glikson

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

The origin of the abrupt end-Archaean - early Proterozoic (c. 2.45 Ga) transition from an oxygen-poor to an oxygeneting atmosphere remains one of the outstanding problems in early Earth history. A change in mass-independent fractionation of sulphur isotopes (MIF-S) (δ33S) about 2.45 Ga is commonly interpreted in terms of the termination of UV-triggered chemical reactions under oxygen-poor atmosphere conditions. Late-Archaean (~2.7–2.5 Ga) and mid-Archaean (~3.2 Ga) sequences in the Pilbara craton (Western Australia) and Kaapvaal Craton (South Africa), in which MIF-S data were measured, contain asteroid impact ejecta units dated as 2.48, 2.56, 2.63, 3.24, 3.26 and 3.47 Ga-old. The scale of the impacts is estimated by mass balance calculations based on Iridium and 53C/52Cr isotopic anomalies and on impact spherule (microkrystites. see figure) size distribution, suggesting projectiles several tens of kilometsers in diameter. In view of an incomplete preservation of impact ejecta units, the above represents a minimum rate of the Archaean impact flux. High UV flux due to low ozone levels in the Archaean atmosphere may have been enhanced by large impacts, accentuating MIF-S anomalies. The appearance of iron-rich sediments above late and mid-Archaean impact ejecta units may be related either to to microbial oxidation of ferrous iron or, alternatively, photochemical oxidation of ferrous to ferric iron. Similar MIF-S anomalies may have been associated with Proterozoic and Phanerozoic impacts, although to date little evidence exists in this regard. Detailed sampling and isotopic analyses across the impact ejecta fallout units are required in order to test possible relationships between Archaean impacts and MIF-S anomalies.