THE EARLY ARCHEAN RECORD OF LARGE ASTEROIDAL IMPACTS ON EARTH

Initial evidence for very large impacts on the early Earth (Lowe and Byerly, Geology, 1986) was field and petrographic data from several unusual sedimentary layers in South Africa and Australia. Particles within these layers had melt droplet shapes and internal textures, and the layers often represented unusually energetic events in otherwise low energy depositional environments. Large iridium anomalies, in beds 30 cm thick, indicated that the bolides were up to 50 km in diameter (Lowe et al., Science, 1989). Ni-rich spinel and impact layer compositions confirmed the large size of the bolides, that they were likely chondritic, and that the target was dominantly basalt (Byerly and Lowe, GCA, 1994). Recently, chromium isotopic anomalies were used to confirm an extraterrestrial component in these layers, to confirm the bolides were indeed of 20-50 km sizes, and to type one as a CV and another as untyped member of the carbonaceous chondrites (Shukolyukov et al., Impacts and the Early Earth, 2000; and Kyte et al., Geology, 2003). Byerly and others (2002, Science) reported precise U-Pb isotopic ages for the oldest impacts in South Africa and Australia, both at 3470 +/- 2 Ma, suggesting these were indeed global events. A second cluster of at least three large impact events was recognized in South Africa at 3260-3240 Ma (Byerly et al., Precamb. Res. 1996; and Lowe et al., Astrobiology, 2003). All these impacts were large, comparable to those of Moon’s Late Impact Cataclysm (Tera et al., EPSL, 1974; and Ryder, Eos, 1990) at circa 3850 Ma, and likely had a profound influence on the course of physical and biological evolution on early Earth. Interestingly, the Lunar surface also records impact spikes at about 3470 and 3200 Ma (Culler et al., Science, 2000; and Glikson, J. Geodyn., 2001). Inner solar system evolution may be, in part, the product of several major and discrete events or event clusters, and not that of a continuous decline in impact activity over time.