Estimating the upper limit of Proterozoic petrographic organic carbon recycling

Sami Nabhan*, Don E. Canfield

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review

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Abstract

Atmospheric oxygen levels in the Proterozoic Eon are strongly debated, with estimates ranging from < 0.1 % – 10 % present atmospheric levels (PAL). A new approach to assess Proterozoic atmospheric oxygen levels based on the concentrations of recycled petrographic organic carbon in Proterozoic-aged strata indicates that atmospheric oxygen was potentially between 2 % and 24 % PAL. However, this model lacks some precision because the petrographic organic carbon flux in the Proterozoic is poorly constrained. This study focuses on marine siltstone and shale of the post-glacial Cryogenian Tapley Hill Formation (Adelaide Basin, south Australia) that was deposited during a eustatic transgression following the Sturtian glaciation. This strata contains concentrations of 0.041–0.754 wt% total organic carbon (TOC) and experienced burial diagenesis temperatures of 195 – 220 °C based on Raman spectroscopic characterization of disordered organic carbon. However, a portion of 12.4–58.1 % of the TOC within these rocks is of higher maturity, in part fully converted to graphite, and must therefore be recycled petrographic organic carbon of clastic origin. The measured proportion (%) of petrographic organic carbon increases with decreasing TOC concentration, resulting in petrographic organic carbon concentrations of 0.019 – 0.095 wt% (average 0.052 wt%). Because of potentially high post-glacial erosion rates, we presume that the reported petrographic organic carbon concentrations mark an upper limit on organic carbon recycling for the Proterozoic Eon. However, the diagenetic overprint of the Tapley Hill Formation in combination with petrographic similarities between graphitic and disordered organic carbon particles both indicate that the given values are a minimum estimate of petrographic organic carbon recycling and that the actual concentration of recycled organic carbon in the Tapley Hill Formation is higher. Therefore, we conclude that petrographic organic carbon recycling in the Neoproterozoic during the deposition of the Tapley Hill Formation was similar to modern marine depositional systems that display high organic carbon recycling rates. Because the weathering of petrographic organic carbon functions as a sink for atmospheric oxygen, high recycling rates during the Cryogenian period would allow for an increase in atmospheric oxygen concentrations.

Original languageEnglish
Article number107034
JournalPrecambrian Research
Volume390
Number of pages15
ISSN0301-9268
DOIs
Publication statusPublished - 15. Jun 2023

Keywords

  • Organic carbon recycling
  • Oxygen
  • Petrographic organic carbon
  • Proterozoic
  • Raman spectroscopy
  • Tapley Hill Formation

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