Abstract
The carbon isotopic ratios of marine carbonate rocks from the latest Precambrian to possibly earliest Cambrian were studied to evaluate possible changes in organic carbon burial and hence in the carbon cycle associated with the evolution of higher life forms. Samples were taken from carbonate units within the Huqf Group of Oman, which was deposited during the interval from approximately 560 to 540 Ma. The data show an initial period of positive δ 13 C values, around + 4‰ PDB, followed by a sharp decrease, over a few meters of section, of about 8‰ in carbon isotopic values, to inferred oceanic carbon values of appoximately − 4‰ PDB. The δ 13 C values then more slowly increase to between 0 and + 2‰. This isotopic pattern may be correlated across Oman over a distance of 800 km
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