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

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 (C_org) 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 C_org 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 (δ¹³C_carb ‰, VPDB) isotope ratios indicate that both pyrite and carbonate formed in sediments through microbial sulfate reduction and C_org remineralization, respectively. Selenium isotopic ratios (δ^82/76Se ‰, 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 (SeO_x^2-) in the sediment below an oxygenated seawater column. Collectively, the data suggests an oxygenated water column in the Francevillian Basin with a large SeO_x^2- 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^–, SeO_x^2-, 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 C_org 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.