Quantitative model evaluation of organic carbon oxidation hypotheses for the Ediacaran Shuram carbon isotopic excursion

Abstract

The largest global carbon-cycle perturbation in Earth history was recorded in the Ediacaran—a persistent negative shift in the global marine dissolved inorganic carbon (DIC) reservoir that lasted for ~25–50 million years, with a nadir of –12‰ (i.e., the Shuram Excursion, or SE). This event is considered to have been a result of full or partial oxidation of a large dissolved organic carbon (DOC) reservoir, which, if correct, provides evidence for massive DOC storage in the Ediacaran ocean owing to an intensive microbial carbon pump (MCP). However, this scenario was recently challenged by new hypotheses that relate the SE to oxidization of recycled continentally derived organic carbon or hydrocarbons from marine seeps. In order to test these competing hypotheses, this paper numerically simulates changes in global carbon cycle fluxes and isotopic compositions during the SE, revealing that: (1) given oxygen levels in the Ediacaran atmosphere-ocean of ≤40% PAL, the recycled continental organic carbon hypothesis and the full oxidation of oceanic DOC reservoir hypothesis are challenged by the atmospheric oxygen availability which would have been depleted in 4 and 6 million years, respectively; (2) the marine-seep hydrocarbon oxidation hypothesis is challenged by the exceedingly large hydrocarbon fluxes required to sustain the SE for >25 Myr; and (3) the heterogeneous (partial) DOC oxidation hypothesis is quantitatively able to account for the SE because the total amount of oxidants needed for partial oxidation (<50%) of the global DOC reservoir could have been met.