CARBON ISOTOPIC EVIDENCE OF PHOTOAUTOTROPHY IN AN OPEN MARINE ECOSYSTEM PRECEDING THE GREAT OXYGENATION EVENT

Surface environments during the late Archean experienced a rapid increase in global oxygen production, prior to the Great Oxygenation Event at ~2.3 Ga. Earth’s early atmosphere was essentially anoxic, however there is evidence supporting partially oxygenated surface conditions before 2.5 Ga. Early development of oxygenic photosynthesis was likely limited to small, nutrient-rich, shallow marine niches, and it has been suggested that a widespread expansion of oceanic environments dominated by oxygen production must have preceded the global oxygenation of the atmosphere. Carbon isotopic and other geochemical analyses of planktonic microfossils from this time may provide additional evidence to support that interpretation.

Samples of finely laminated black cherts from the Gamohaan Formation, in the Kaapvaal Craton of South Africa (~2.52 Ga), contain abundant spherical and sub-spherical microstructures, ranging in size from ~60 to 300 mm in diameter. These large organic microfossils are preserved as compressed structures between individual bedding planes of fine-grained sediments and show taphonomic features such as folding, wrinkling, and degradation of organic material, supporting a planktonic and likely photosynthetic origin. Stable carbon isotope analyses of the bulk organic matter show mean values that are somewhat lighter than average for prokaryotic photosynthetic fractionation. Additional carbon isotope analyses of a small number of selected microfossils from an acid macerate provide supporting evidence for their photoautotrophic nature, and help to distinguish the δ¹³C_org values of the fossils from those of the background kerogen.

These microstructures are relatively large and complex, and could therefore represent an evolutionary stem group of photosynthetic prokaryotes, or perhaps early eukaryotes, that contributed to the oxygenation of the atmosphere from the open ocean. Future work will include the study of micron-scale spatial heterogeneity of carbon isotope values by using secondary ion mass spectrometry (SIMS) to compare in situ carbon isotope compositions of individual microfossils with those of autochthonous organic matter formed in the deep-water sediments to help resolve this question.