HIGH RESOLUTION STABLE ISOTOPE STRATIGRAPHY, THE 580 MA ACRAMAN IMPACT EVENT, AND LATE NEOPROTEROZOIC EVOLUTION

Evidence for a relationship between the ~580 Ma Acraman impact ejecta layer, a dramatic increase in the number and diversity of fossil planktonic algal species (acritarchs), and changes in the carbon isotopic composition of bulk kerogen (delta13Cker) requires further testing. The sample spacing in the original study was too wide (10-40 m), and the apparent carbon isotope variations could be due to the isotopic composition of different kerogenous components, namely algae and bacteria. These problems have been circumvented. New high-resolution sampling of seven drillholes (all but one containing the ejecta layer) which cover the affected area have improved the sample spacing to 1-5 m. The separation of acritarchs from bulk organic matter and subsequent analysis by EA-IRMS combined with close sample spacing allows a test of the Acraman impact hypothesis: that a large asteroid (~4.5 km diameter) caused an environmental catastrophe that fundamentally changed late Neoproterozoic biota.

We postulate a global dust cloud caused by the Acraman impact forced a collapse in marine primary productivity by severely restricting photosynthesis. Those planktonic species not able to cope became extinct, allowing the better adapted species to diversify in form and proliferate in number. As primary producers, acritarchs have the best-preserved carbon isotopic composition, one unbiased by kerogen of bacterial origin. Indeed, the results show that acritarch delta13C values are on average ~2 per mil heavier than bulk kerogen delta13C values. Bulk kerogen delta13C values decline at the Acraman ejecta layer by ~7 per mil before positively shifting when acritarch diversity starts to increase rapidly. Acritarch delta13C values compliment bulk kerogen delta13C values, confirming a collapse and recovery in primary productivity, as do delta15N values of bulk kerogen. In addition, large positive delta15N shifts after the impact event may indicate times of high primary productivity, coinciding with the first appearance of new acritarch species.