© Science China Press and Springer-Verlag GmbH Germany 2017 earth.scichina.com link.springer.com SCIENCE CHINA Earth Sciences SPECIAL TOPIC: Carbon cycling in the China Seas • RESEARCH PAPER • https://doi.org/10.1007/s11430-017-9137-1 Quantitative model evaluation of organic carbon oxidation hypotheses for the Ediacaran Shuram carbon isotopic excursion SHI Wei 1 , LI Chao 1* & Thomas J. ALGEO 1,2,3 1 State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China; 2 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China; 3 Department of Geology, University of Cincinnati, Cincinnati OH 45221-0013, USA Received September 10, 2017; accepted November 8, 2017; published online November 21, 2017 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. Keywords Carbon isotope, Shuram Excursion, Dissolved organic carbon, Atmospheric oxygen, Oceanic oxidation Citation: Shi W, Li C, Algeo T J. 2017. Quantitative model evaluation of organic carbon oxidation hypotheses for the Ediacaran Shuram carbon isotopic excursion. Science China Earth Sciences, 60, https://doi.org/10.1007/s11430-017-9137-1 1. Introduction The largest negative carbonate carbon isotope (δ 13 Ccarb) ex- cursion in Earth history (to −12‰) is present in the Edi- acaran stratigraphic succession (635–541 Ma) that overlies Neoproterozoic glacial diamictites. This event, termed the Shuram Excursion (SE) or Doushantuo Negative Carbon iso- tope Excursion (DOUNCE), lasted for ~25–50 million years at ~600–550 Ma (Grotzinger et al., 2011; Le Guerroué et al., 2006; Lu et al., 2013)(Figure 1). An unusual feature of the *Corresponding author (email: chaoli@cug.edu.cn) SE is that the carbon isotope compositions of organic matter (δ 13 Corg) and the δ 13 Ccarb were decoupled, which deviates from the pattern of δ 13 Corg-δ 13 Ccarb coupling due to derivation from a common dissolved inorganic carbon (DIC; HCO3 – /CO3 2– ) reservoir that characterizes the Phanerozoic. This decoupling may have been linked to the existence of a large dissolved or- ganic carbon (DOC) reservoir (>100–1000 × modern level) in the Ediacaran deep ocean with a residence time >10 kyr (Rothman et al., 2003). In this scenario, oxidation of this DOC reservoir generated large amounts of 12 C-enriched CO2, resulting in the large decrease in the δ 13 C of oceanic DIC that was recorded by the SE (Rothman et al., 2003; Fike et al., 2006; Jiang et al., 2007, 2011; McFadden et al., 2008; Wang