A MID-TONIAN INCREASE IN EUKARYOTIC DIVERSITY AGAINST THE BACKDROP OF THE BITTER SPRINGS CARBON ISOTOPIC ANOMALY

Paleontological and molecular clock studies suggest major eukaryotic diversifications occurred in the early Neoproterozoic associated (at least temporally) with the Bitter Springs carbon isotopic stage (~811-789 Ma), an interval of globally recorded and synchronous negative excursions in δ¹³C values that imply a global carbon cycle disruption. Coupled geochemical and paleontological datasets from across the carbon isotopic anomaly offer a basis for testing the temporal relationship between eukaryotic diversification and carbon cycle change. Here we investigate this apparent coincident rise of eukaryotic diversity and early Neoproterozoic carbon cycle disruption by targeting Bitter Springs correlative successions of Officer Basin, and Adelaide Rift Complex, Australia for paleontological and geochemical analyses.

The Bitter Springs carbon isotopic anomaly was detected in the Coominaree Dolomite of the Manya-5 drillcore in eastern Officer Basin in work by Hill and Walter (2000) and is supported here with a higher resolution dataset. Preliminary paleontological study of Manya-5 shales and mudstones reveals a well-preserved assemblage of abundant and diverse organic-walled microfossils deposited before and during the anomaly interval. Specifically, the occurrences of certain taxa whose first appearances had been associated with the Bitter Springs interval have been found to predate the onset of the anomaly. This suggests the broadly observed increase in species richness likely preceded the Bitter Springs carbon cycle disruption and, possibly, that the previously noted pattern of a ~800 Ma diversity peak may be a result of sampling bias.