THE ACRAMAN IMPACT AND ITS RELATION TO THE SNOWBALL EARTH AND BIOTIC DIVERSIFICATION

The c.580 (Ediacaran) Acraman impact event formed a collapse crater ~90 km in diameter, scattered ejecta across 1000 km of Australia, and may have produced a dust cloud large enough to block out sunlight and cause a biotic crisis. Range charts, based on >2000 samples from >30 drillholes across Australia, indicate that the event had a profound effect on Neoproterozoic marine phytoplankton and produced a marked negative excursion in the organic carbon isotope curve.

Pre-glacial assemblages (especially c.850-700 Ma) consist of coccoid and filamentous cyanobacteria, fragments of benthic microbial mats, and a leiosphere-dominated shelfal plankton. Most are conservative species with global Mesoproterozoic–early Neoproterozoic records. Short-ranging, ornamented taxa (e.g. Cerebrosphaera) are important marker fossils. Assemblages are sparse during and between the Sturtian and Marinoan glaciations (c.700-600 Ma). Samples just above the Marinoan glaciation are barren, but leiospheres and benthic mats quickly re-established and proliferated as sea level rose. They persist through several hundred metres of drill core. Post-glacial species are the same as pre-glacial ones, except that an impoverished biota is further depleted. This challenges Snowball Earth predictions. There is no post-glacial recolonisation by rapidly diversifying species, and taxa did not evolve from extremophiles from hot-spring refugia.

There is a striking change ~20 m.y later, when >50 species of large, acanthomorph acritarch species appear. Diversification is rapid, and four zones have been erected. The Acraman ejecta layer coincides with the first appearance of the acanthomorphs, and with a negative δ13C organic excursion. The Ediacaran acritarch diversification may be a recovery event following a bolide impact. Diversification coincides with a rapid positive δ13Corg excursion and it did not occur until the second post-glacial marine excursion. It appears unrelated to sedimentology or sequence stratigraphy. There are species in common with Svalbard, Norway, Siberia, and China, indicating a potential for global correlation, but so far, ranges have not been determined outside Australia. In Australia, the complex acanthomorphs disappear at about the time of the first appearance of bilaterians in the Ediacara fauna.