EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Increase in ecosystem complexity aided organic carbon sequestration during the c. 2.22-2.06 Ga Lomagundi carbon isotope excursion

Frantz Ossa Ossa1,2, Stephan König1, Axel Hofmann2, Andrey Bekker2,3, Jorge E. Spangenberg4, and Ronny Schoenberg1,2
Frantz Ossa Ossa et al.
  • 1University of Tuebingen, Department of Geosciences, Germany (
  • 2University of Johannesburg, Department of Geology, South Africa
  • 3University of California Riverside, Department of Earth and Planetary Sciences, USA
  • 4University of Lausanne, Institute of Earth Surface Dynamics (IDYST), Switzerland

The first dramatic rise in atmospheric oxygen to concentrations above 10-5 present atmospheric level (PAL), known as the Great Oxidation Event (GOE), was initiated during the early Proterozoic Eon c. 2.43-2.32 billion years (Gyrs) ago [1,2].  Although atmospheric O2 concentrations are generally accepted to have remained below 1% PAL for at least 1.5 Gyrs following the GOE [3], high atmospheric O2 build up occurred during the Lomagundi carbon isotope excursion (LE) at the latest stage of the GOE [4]. The LE is the most pronounced and longest-lived carbon isotope excursion in Earth’s history that took place c. 2.22-2.06 Gyrs ago [4,5]. It reflects increased organic carbon (Corg) burial resulting from high primary productivity at the time of high phosphorous flux to the ocean associated with intense acidic chemical weathering of landmasses. However, mechanisms responsible for such high Corg sequestration are not yet fully resolved, nor has it been possible to precisely quantify the magnitude and expansion of oxygenation within the coeval atmosphere-ocean system.

Here, we studied diagenetic concretions of pyrite and carbonate and their host black shales of the Francevillian Group, southeast Gabon, deposited during the LE. Light sulfur (δ34S ‰, VCDT) and carbon (δ13Ccarb ‰, VPDB) isotope ratios indicate that both pyrite and carbonate formed in sediments through microbial sulfate reduction and Corg remineralization, respectively. Selenium isotopic ratios (δ82/76Se ‰, NIST3149) of the pyrite concretions and their host shales are dominated by highly negative values as low as –3‰, which is strong evidence for partial reduction of selenium oxyanions (SeOx2-) in the sediment below an oxygenated seawater column. Collectively, the data suggests an oxygenated water column in the Francevillian Basin with a large SeOx2- reservoir that continuously resupplied these electron acceptors to the sediment and prevented their quantitative reduction. The studied black shales host putative, fossilized large colonial multicellular organisms that had the ability to laterally and vertically migrate within the sediments [6]. We propose that bioturbation by these organisms allowed an increased flux of electron acceptors (e.g., O2, NO3, SeOx2-, SO4-) into the sediments and pushed the microbial sulfate reduction and methanogenesis zones downward. As a consequence, CH4 and H2S generated in these zones were re-oxidized in more oxic upper levels of the sediments, which prevented them from escaping to the water column. An increase in ecosystem complexity thus likely aided Corg sequestration to the sediments and O2 accumulation in the atmosphere-ocean system during the LE.


[1] Bekker et al. (2004), Nature, 427, 117–120. [2] Holland (2006), Philos. Trans. R. Soc. B 361, 903–91. [3] Colwyn et al. (2014), Geobiology, DOI: 10.1111/gbi.12360. [4] Karhu and Holland (1996), Geology, 24, 867–870. [5] Bekker (2014), Encyclopedia of Astrobiology, Springer-Verlag, 1–6. [6] El Albani et al. (2019), Proc. Natl. Acad. Sci. USA, 116, 3431–3436.

How to cite: Ossa Ossa, F., König, S., Hofmann, A., Bekker, A., Spangenberg, J. E., and Schoenberg, R.: Increase in ecosystem complexity aided organic carbon sequestration during the c. 2.22-2.06 Ga Lomagundi carbon isotope excursion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10828,, 2021.


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