Astrobiology Workshop, Macquarie University July 12-13 2001

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Did the c.580 Ma Acraman Impact Event trigger terminal Proterozoic biodiversification?

Kathleen Grey (Geological Survey of Western Ausatralia)

The c.580 Ma Acraman Structure is one of the ten largest and best-documented impact structures in the geological record and meets all recognition criteria for large bolide impacts. The impact site (Lake Acraman, Gawler Craton, South Australia), displays typical cratering morphology, geophysical characteristics, shock metamorphic features (devitrified melt particles, shatter cones, and shocked quartz-crystals), and is associated with microspherules, an iridium anomaly, evidence of a tsunami, and a debris layer. A collapse crater ~90 km in diameter indicates impact by a 4.8 km-diameter earth-crossing chondrite. The associated ejecta layer is known from 12 outcrops >200 km E of the impact site, in drillhole WWD 1 near Lake Torrens, and from drillholes >500 km W of the impact structure (Lake Maurice West 1, Observatory Hill 1, and as redeposited ejecta in Munta 1). The ejecta blanket is consistent in extent with patterns for 80-90 km diameter lunar craters, such as Tycho and Copernicus.

The ejecta horizon is a significant synchronous marker layer that provides an appropriate datum for plotting relative distributions of planktonic acritarch species observed during biostratigraphic studies of the terminal Proterozoic of Australia. As data compilation progressed, a remarkable coincidence between the first appearance of the acanthomorphs (spiny acritarchs), a negative delta13Corg excursion, and the Acraman impact ejecta layer became apparent. Observed biotic changes are radical ones. An older leiosphere-dominated palynoflora is abruptly succeeded by a complex acanthomorph-dominated palynoflora. There is a rapid increase in abundance, size, morphological complexity, and taxonomic diversity and these changes indicate a major evolutionary radiation in the green algae. The actual or presumed position of the transition matches a negative delta13Corg excursion that coincides with the Acraman impact layer, and indicates rapid depletion of organic carbon followed by a steady rise. Preliminary results suggest the Ediacarian acritarch diversification may be a recovery event following a bolide impact, which may also have triggered other significant biotic changes.

Whether the events are related or purely coincidental requires further investigation. Evidence for a relationship between the impact event and the observed changes in acritarch assemblages is still circumstantial, but the effects of a large bolide impact supplies a plausible explanation for dramatic biotic changes that are otherwise difficult to explain. Available evidence suggests that the Acraman event was large enough to have substantially reduced light intensity for days or months globally, restricting photosynthesis. A model is proposed for further testing to show how the Acraman Impact Event might have triggered changes in the admixture of the global biota at about 580 Ma.