A FIRST ORDER EVALUATION OF NEOPROTEROZOIC CARBON ISOTOPE DATA WITHIN A PHYSICAL STRATIGRAPHIC CONTEXT

Neoproterozoic carbon isotope excursions have been widely used for stratigraphic correlations and for interpretations of the ancient oceanic events. Our 864 samples, collected from multiple sections within a sequence stratigraphic framework of the terminal Proterozoic (595-543 Ma) Infra Krol Formation and Krol Group of the Lesser Himalaya, India show wide d 13C range from -10 to +9‰ (PDB), and d 13C scatter of some intervals cannot be eliminated by application of existing geochemical screens. This leads to two alternative interpretations. If the physical surfaces are reliably interpreted across the carbonate platform, then the inconsistency of d 13C values for 'coeval' rocks within apparently coeval strata must relate to some combination of diagenesis and facies variations. Because diagenesis commonly produces more negative rather than positive d 13C overprints in carbonate rocks, the most positive values of the 'correlatable' units should be nearest to the original signal of the ancient seawater. The composite d 13C record for the Krol under this assumption shows a pattern that compares well with the published d 13C records from other terminal Proterozoic successions, but the magnitude of excursions is smaller. An alternative interpretation is that the inconsistency of d 13C values implies stratigraphic hiatuses that are subtle and not recognizable in our physical stratigraphy. The composite d 13C curve constructed under this assumption results in more positive-negative pairs of larger magnitude than observed in other terminal Proterozoic successions, perhaps suggesting that the d 13C record from the carbonate-rich Krol is more complete. In either case, doubt is cast on the reliability of correlation schemes that are based on counting the number of d 13C excursions from single sections and compels a reevaluation of the Neoproterozoic carbon isotopic record.