WHAT IS SO FORBIDDING ABOUT THE “FORBIDDEN ZONE”?

Recent reports suggest that the processes of carbonate lithification associated with the mixing of marine and meteoric waters (containing carbon from a newly evolved terrestrial biosphere) and metamorphism have altered many Neoproterozoic carbonates to the degree that they no longer retain a signature of primary depositional isotopic compositions. These processes are believed to result in strong negative correlations of carbon and oxygen isotopes such that a “Forbidden Zone” where no samples lie is encountered on cross plots of data from both Proterozoic and Phanerozoic strata. If correct, the widespread use of carbon isotope trends for Neoproterozoic correlation and the study of the carbon cycle become invalid. While the bulk of published Neoproterozoic data (predominantly from outcrop samples) conform to the bounds represented by the lithification trend, a subset of outcrop samples from our studies, unlikely to be explained by the oxidation of methane, do not conform to this trend. Furthermore, new data from pristine drill core through the Ediacaran Doushantuo Fm. in South China reveal long stratigraphic intervals with carbonates retaining near 0‰ (VPDB) oxygen isotope compositions, including horizons in the lower and middle Doushantuo characterized by strong negative carbon isotope anomalies. These observations support the view that carbon isotopes retain a memory of marine depositional conditions, rather than a mixture of terrestrial and marine carbon sources during lithification. Insofar as negative carbon isotope anomalies are recorded in both Paleoproterozoic and Mesoproterozoic successions the greening of the Earth may have occurred earlier, or later, than suggested. On the other hand, marked covariance of carbon and oxygen isotopes is noted in the post-Nantuo cap carbonate from South China and its equivalents worldwide, and of carbonates associated with the Shuram event in the upper Doushantuo Fm. and its equivalents. For the Shuram we speculate that oxidation of a large DOC pool associated with extreme warming – induced by a profound flux of CO₂ to the atmosphere – may explain the noted covariation of carbon and oxygen isotopes. In conclusion, it appears that in well preserved samples like those from the Doushantuo core, oxygen isotopes may also provide important environmental information.