Contrasting sterane signatures in Neoproterozoic marine rocks of Australia before and after the Acraman asteroid impact

Abstract

Organic matter preserved in marine mudstones from three late Neoproterozoic depocentres – the Officer and Amadeus Basins and the Adelaide Fold Belt – was examined using standard microscopic and geochemical methods, including gas chromatography-mass spectrometry (GC–MS). Total organic carbon (TOC) contents of these sediments are typically low (<0.25%) and the thermal maturity (expressed as calculated vitrinite reflectance, Rc, based on measured methylphenanthrene index) is in the range 0.68–1.35%. Samples of drill core spanned the interval containing ejecta from the impact of a large chondritic asteroid at ca. 570 Ma. The remnant structure caused by the impact is located at Lake Acraman on the Gawler Craton. Palynological studies reveal a major change from the impoverished microbial and leiosphere palynoflora that survived the postulated “Snowball Earth” glaciation of Marinoan age (ca. 580 Ma) to one dominated by large complex acritarchs (acanthomorphs) following the Acraman impact. Among the benthic and planktonic inputs to these sediments, the eukaryotic steroidal biomarkers provide clear evidence of the environmental havoc wrought by the impact. C 29/C 27 sterane ratios increase from 0.7–1.3 below the ejecta horizon to 1–11 above it. This suggests that primitive cyst-forming chlorophytes with a robust reproductive cycle survived in the immediate aftermath of the impact, whereas other algae (including the leiospheroid prasinophytes) were less resilient. A corresponding fourfold drop in sterane/hopane ratios is consistent with a negative shift of 4‰ in δ 13C org at about this time, both signalling a sharp decline in marine algal productivity. On the basis of evidence from Australia and Oman, the estimated duration of this anomalous marine sterane signature is at least 20 million years. It corresponds to the opportunistic proliferation of new chlorophyte taxa in the aftermath of the Acraman event.