Chemostratigraphic and sedimentological framework of the largest negative carbon isotopic excursion in Earth history: The Neoproterozoic Shuram Formation (Nafun Group, Oman)

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Oman records in its Neoproterozoic Nafun Group (Huqf Supergroup) an essentially complete, carbonate-rich Ediacaran succession. The Nafun Group overlies the presumed Marinoan rift-related Fiq Member (ca. 635 Ma) and ends just below the Precambrian–Cambrian boundary (542 Ma). Tectonic subsidence caused by thermal contraction following Fiq-aged rifting allowed the preservation of about 1 km of postrift stratigraphy, with no major stratigraphic breaks. The Nafun Group above the Marinoan cap carbonate (Hadash Formation) represents two siliciclastic to carbonate ‘grand cycles’, both initiated by significant transgressions: these cycles comprise the Masirah Bay/Khufai formations and the Shuram/Buah formations. The Khufai–Shuram boundary is associated with the start of a major carbon isotope perturbation. The uppermost ramp carbonates of the Khufai Formation record a smooth decrease in δ 13C from about +4‰ to values around 0, followed by two descending steps across which values plunge to a nadir of −12‰ in the overlying red siltstones and shales interbedded with thin limestones of the Shuram Formation. This fall in isotopic values is temporally rapid and coincident in both shallow and deep-water sections in the time span of a single parasequence. The δ 13C nadir is then followed by 50 million years of monotonic recovery. The ‘Shuram shift’ represents the largest δ 13C inorganic carbon negative excursion in Earth history. Although the snowball Earth theory links periods of depleted carbon isotopic ratios with periods of global glaciation, the non-glaciated context of the Shuram Formation suggests that the causal relationship between global glaciation and negative carbon isotopic excursions is non-unique. Although the precise mechanism driving this major perturbation of the carbon cycle remains enigmatic, the long-term remineralization of an isotopically depleted organic carbon reservoir in ocean water is a promising candidate.