A possible global drop in marine carbon isotope values to as low as −12‰ Peedee belemnite (PDB), recorded in the Ediacaran Shuram Formation of Oman, has been attributed to the non-steady-state oxidation of oceanic dissolved organic carbon (DOC) resulting from the rise in atmospheric oxygen to near modern values at the end of the Precambrian. Geologic constraints indicate that the excursion lasted between 25 and 50 m.y., requiring a DOC pool thousands of times to 10,000 times the modern inventory to conform with carbon isotope mass balance calculations for a −12‰ excursion. At the consequent rates of DOC oxidation, oceanic sulfate and oxygen in the atmosphere and oceans are exhausted on a time scale of ~800 k.y. Oxidant depletion is incompatible with independent geochemical and biological indicators that show oceanic sulfate and oxygen levels were maintained or increased during the Shuram excursion. Furthermore, a DOC-driven excursion does not explain strong covariation between the carbon and oxygen isotope record. These indicators show that negative isotope excursions recorded in the Shuram and other Ediacaran sections are unlikely to represent a global ocean signal.