USING SINGLE-FOSSIL ORGANIC CARBON ISOTOPES TO ILLUSTRATE THE PALEOECOLOGY AND DEPOSITIONAL CONDITIONS OF THE MESOPROTEROZOIC VELKERRI FORMATION OF AUSTRALIA

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. Considerable work has been done on the Roper Group on both this fossil record as well as geochemical proxies, especially those relating to redox. Analyses of carbon isotopes within these strata have primarily focused on measuring the organic or inorganic carbon from bulk samples. While this has provided us with new information concerning broad biogeochemical trends, the results are also time- and community-averaged.

In this study, we look instead at carbon isotope measurements of individual microfossils and bulk organic carbon from the Roper Group Velkerri Formation to obtain a more short-term view of the conditions of the water column during the deposition of sediments as well as information on fossil paleoecology. We obtained carbon isotopic data from individual leiosphere microfossils via nano-EA mass spectrometry, and found a range of average δ13C_org per horizon from -30.07‰ to -24.03‰ compared to the bulk rock δ13C_org, which ranges between -34.6‰ and -32.11‰. In the standard model of eukaryotic evolution, eukaryotes were limited to oxygenated surface waters in a stratified ocean with strong δ13C and oxygen gradients. Although conditions like these would leave the microfossils’ δ13C_org values partially determined by their location in the water column, the δ13C of all microfossils sampled would be greater than the δ13C of the bulk organic matter because the biological pump would leave the surface δ13C-enriched. Our research appears to illustrate that fossil δ13C_org values are consistently equivalent to or heavier than the bulk rock, with some samples enriched from the bulk by up to 10‰, which means our data support the hypothesis that eukaryotic organisms were likely restricted to surface portions of the Mesoproterozoic water column where oxic conditions were more likely.