Ediacaran (ca. 635–541 Ma) marine carbonates capture a global δ13C carbon isotope excursion to extremely negative values (∼–12‰)—known as the Shuram excursion (SE)—that cannot be explained by conventional mass balance scenarios. Furthermore, the carbon isotopic variation of bulk organic matter (OM) does not mirror that of carbonate through the excursion, suggesting that the OM reflects a mixture of different sources. To evaluate this hypothesis, we investigated thermally immature marine sedimentary rocks that record the SE from the Sultanate of Oman. Compound-specific carbon isotopic analyses of the extractable hydrocarbons reveal low δ13C values of long-chain (>C20) n-alkanes and mid-chain monomethyl alkanes as low as −40‰. Such light signatures are rare in marine rocks of any age and provide evidence that the SE reflects a primary carbon cycle perturbation. The magnitude of the SE recorded in these organic phases is smaller than observed in carbonate and implies that the primary perturbation to dissolved inorganic carbon (DIC) was at least 5‰−7‰, and more likely 7‰−12‰, in magnitude when correcting for end-member source mixing. Due to isotopic differences in stratigraphic patterns of the different organic compounds, we propose that bulk organic carbon (both bitumen and kerogen) reflects source mixing between two distinct pools that previously masked the excursion in bulk δ13Corg measurements. OM sources were derived both from autotrophs fixing 13C-depleted DIC and from a less 13C-depleted heterotrophic microbial biomass feeding on a marine OM pool sustained by petroleum expelled from older sedimentary OM. Expulsion of these sedimentary fluids also helps explain both the duration and magnitude of the SE.

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