INTERPRETING SINGLE-FOSSIL AND BULK ORGANIC CARBON ISOTOPES FROM THE MESOPROTEROZOIC VELKERRI FORMATION OF AUSTRALIA TO BETTER UNDERSTAND THE ENVIRONMENTAL CONTEXT OF EARLY EUKARYOTIC EVOLUTION

The Mesoproterozoic Roper Group of Australia contains some of the oldest definitive eukaryotic fossils and thus is an important window into the early evolution of eukaryotes. In this study, we look at δ¹³C_org of 100 individual microfossils as well as bulk δ¹³C_org through the Roper Group Velkerri Formation to obtain a short-term view of the conditions of the water column during the deposition of sediments as well as information on fossil paleoecology. We analyzed δ¹³C_org from individual microfossils via nano-EA mass spectrometry, and found a range of average δ¹³C_fossil per horizon from -31.7‰ to -24.03‰ compared to the bulk rock δ¹³C_org, which ranges from -35.22‰ to -27.74‰. In the standard model of eukaryotic evolution, Mesoproterozoic eukaryotes were limited to oxygenated surface waters in a stratified ocean with strong δ¹³C and oxygen gradients. Such conditions would leave the microfossils’ δ¹³C_org values partially determined by their location in the water column. Because the biological pump would have left the surface ¹³C-enriched, the δ¹³C of organisms living near the surface would be greater than the δ¹³C of the bulk organic matter.

Our results reinforce this standard model with the exception of 2 horizons where some δ¹³C_fossil are less than the bulk rock values. Results from these two horizons potentially indicate that the fossils’ habitats were located out of the surface ocean, perhaps in anoxic portions of the water column. However, the majority of our data supports the hypothesis that eukaryotic organisms were living in surficial and partially oxygenated waters. Though our data could potentially result from such conditions as N-limitation or partial mixing of the water column, we rely heavily on the veracity of bulk redox proxies in our interpretation of our results. Specifically, trends in our mean bulk and fossil δ¹³C appear to correlate with changing redox conditions as documented by Nguyen et al. (2019), and lighter δ¹³C_org values associated with periods of euxinia may represent relatively higher contributions by methanogenic carbon metabolisms relative to sulfate reduction. If the Velkerri Formation was not a consistently stratified, anoxic-at-depth basin, then our data may instead be tracking changes in the water column δ¹³C gradient recorded by organisms living in the surface ocean.