Carbon isotope excursions and the oxidant budget of the Ediacaran atmosphere and ocean

A possible global drop in marine carbon isotope values to aslow as -12<IMG SRC="/math/permil.gif" ALT="{per thousand}" BORDER="0"> Peedee belemnite (PDB), recorded in the EdiacaranShuram Formation of Oman, has been attributed to the non-steady-stateoxidation of oceanic dissolved organic carbon (DOC) resultingfrom the rise in atmospheric oxygen to near modern values atthe end of the Precambrian. Geologic constraints indicate thatthe excursion lasted between 25 and 50 m.y., requiring a DOCpool thousands of times to 10,000 times the modern inventoryto conform with carbon isotope mass balance calculations fora -12<IMG SRC="/math/permil.gif" ALT="{per thousand}" BORDER="0"> excursion. At the consequent rates of DOC oxidation,oceanic sulfate and oxygen in the atmosphere and oceans areexhausted on a time scale of 800 k.y. Oxidant depletion isincompatible with independent geochemical and biological indicatorsthat show oceanic sulfate and oxygen levels were maintainedor increased during the Shuram excursion. Furthermore, a DOC-drivenexcursion does not explain strong covariation between the carbonand oxygen isotope record. These indicators show that negativeisotope excursions recorded in the Shuram and other Ediacaransections are unlikely to represent a global ocean signal.