CARBON ISOTOPIC RECORD OF THE LATEST PROTEROZOIC FROM OMAN

The carbon isotopic ratios of marine carbonate rocks from the latest P recambrian to possibly earliest Cambrian were studied to evaluate poss ible changes in organic carbon burial and hence in the carbon cycle as sociated with the evolution of higher life forms. Samples were taken f rom carbonate units within the Huqf Group of Oman, which was deposited during the interval from approximately 560 to 540 Ma. The data show a n initial period of positive deltaC-13 values, around +4 parts per tho usand PDB, followed by a sharp decrease, over a few meters of section, of about 8 parts per thousand in carbon isotopic values, to inferred oceanic carbon values of appoximately -4 parts per thousand PDB. The d eltaC-13 values then more slowly increase to between 0 and +2 parts pe r thousand. This isotopic pattern may be correlated across Oman over a distance of 800 km. Similar changes have been observed in Namibia, Gr eenland and Siberia. The changes are thought to be primarily an origin al oceanic signal, and not a diagenetic one. The data indicate a sudde n significant reduction in organic carbon burial rates in the late Ven dian, followed by a slow return to more normal conditions by the end o f the Proterozoic. These large changes in the carbon cycle shortly bef ore the Precambrian/Cambrian boundary may have triggered or favored th e radiation of metazoan life.