Estimating the upper limit of the Proterozoic petrographic carbon (PC) flux

Atmospheric oxygen levels in the Proterozoic are highly debated with estimates reaching from >0.1 – 24 % PAL. A new approach to model Proterozoic atmospheric oxygen levels, based on concentrations of recycled PC in Proterozoic strata, indicates that atmospheric oxygen was potentially between 2 and 24 % PAL. However, this model lacks some precision because oxygenation rates and the Proterozoic riverine flux of petrographic carbon are poorly constrained.

This study focuses on deep marine shales of the 658 - 653 Ma-old Tapley Hill Formation (Adelaide Basin, south Australia) that were deposited during an eustatic transgression following the Sturtian glaciation. These shales contain 0.04 - 0.73 wt.% TOC and experienced burial diagenesis with temperatures of 195 – 220 °C based on Raman spectroscopic characterization. However, 14.4 - 58.7 % of the OM within these rocks is recycled graphitic PC indicating metamorphic temperatures as high as 650°C. The amount of PC increases with decreasing TOC values, resulting in total PC concentrations of 0.02 – 0.1 wt.% (average 0.05 wt.%).

Because of the likely exceptionally high post-glacial erosion rates we presume that the average of 0.05 wt.% marks an upper limit for recycling of graphitic PC in the Proterozoic. However, the diagenetic overprint in combination with petrographic similarities between graphitic and disordered OM both indicate that the actual concentration of total recycled PC is higher. Therefore, we conclude that the upper limit of the Proterozoic PC flux was similar to the upper limits found in modern marine sediments 0.1 – 0.2 wt.%.