EXCEPTIONALLY LARGE NEOARCHEAN SPHEROIDAL MICROFOSSILS FROM SOUTH AFRICA: POSSIBLE CONTRIBUTORS TO THE GOE

Fossil evidence of oxygenic photosynthesis is quite sparse and ambiguous in geologic units from the Archean, but this was the time when oxygen production by photoautotrophs must have begun leading ultimately to the Great Oxidation Event (GOE) of the Paleoproterozoic. This study presents evidence for large spheroidal planktonic microorganisms preserved in a Neoarchean carbonate platform. Microfossils reported here were located in a black chert unit from the upper part of 2.52 Ga Gamohaan Formation of the Kaapvaal Craton, South Africa, near the town of Kuruman. The fossils studied range in size from ~60 to 300 µm in diameter (median: 125 µm) and are preserved as solitary (non-colonial) kerogenous spheroids compressed perpendicularly to bedding within a finely laminated black chert unit. They display folding patterns typical of compressed flexible-walled spheroids, similar to many Proterozoic acritarchs and some spheroidal fossils from the Mesoarchean. This compression is likely the result of both loss of turgor pressure within the microorganism upon death and compaction of the sediments prior to lithification. The latter cause is evidenced by draping of laminations around the fossils. Specimens were imaged using confocal laser scanning microscopy to confirm their three-dimensional morphology and their kerogenous nature was determined by use of Raman spectroscopy. This morphological, geochemical, and taphonomic cascade of evidence indicates that the structures preserved in this unit had a biogenic origin. The solitary nature and originally spherical morphology of the fossils suggests they were planktonic organisms that settled into the deep basin where the host sediments were deposited. Based on their size and likely solitary planktonic nature, it is possible that these fossils represent the remains of oxygen-producing cyanobacteria or other stem group oxygenic photoautotrophs, the organisms most likely responsible for the GOE that culminated in an orders-of-magnitude increase in free oxygen in the Earth’s atmosphere. This interpretation is consistent with a large number of geochemical and isotopic studies that provide evidence for at least localized ocean oxygenation during the Neoarchean.