SINGLE FOSSIL ORGANIC CARBON ISOTOPES ILLUMINATE A MESOPROTEROZOIC ECOSYSTEM

Carbon isotopes are an essential tool for reconstructing Earth history, providing a view into the ancient carbon cycle, primary productivity, and more. The vast majority of these measurements have been done on bulk samples measuring either total inorganic or organic carbon. While these data have illuminated vast intervals of Earth’s history, they provide a time- and community-averaged view of the carbon pool. Organic carbon isotope analyses of single microfossils are one approach that can provide a window into short-term environmental variability and can also reveal ecological data about enigmatic organic fossil groups. Here, we discuss new carbon isotopic data from individual acritarch microfossils of the Mesoproterozoic Velkerri Formation of Australia obtained via nano-EA mass spectrometry. Individual fossil δ¹³C_org values range from -34.6‰ to -24.2‰ with a bulk δ¹³C_org range of -34.6‰ to -32.11‰. Thus, fossil δ¹³C_org is consistently equivalent to or heavier than the bulk value, with some samples enriched from the bulk by up to 10‰.

The purpose of this study is in part to resolve two models of eukaryotic evolution: the standard model, in which eukaryotes are restricted to oxygenated surface waters, and an alternative model, in which they are distributed throughout the water column under oxic and anoxic conditions. In a stratified ocean with a strong δ¹³C and oxygen gradient, the δ¹³C_org of individual microfossils within a given sample will be partially determined by their depth in the water column. The traditional model predicts that in a stratified, anoxic-at-depth ocean, the δ¹³C of all microfossils sampled will be greater than the δ¹³C of the bulk organic matter because the biological pump will leave the surface enriched. Under this model, microfossils should also have very similar δ¹³C values because they will have sampled the same inorganic carbon pool. Under the alternative model however, the δ¹³C of all the microfossils sampled will range from greater to less than the δ¹³C of the bulk organic carbon. Our initial data from the Velkerri Formation tentatively support the alternative model: we find fossils that are equivalent to and heavier than the bulk, potentially suggesting that eukaryotic organisms were living in anoxic portions of the Mesoproterozoic water column. If confirmed, these results would require a re-imagining of the stem-to-crown transition within the eukaryotic clade.