PLATFORM TO BASIN d13C VARIATIONS OF THE DOUSHANTUO FORMATION OF SOUTH CHINA: ISOTOPIC GRADIENT OR DIAGENESIS?

The Neoproterozoic Doushantuo Formation (~600 to ~580 Ma?) of south China has attracted considerable interest for well-preserved early animal fossils and large positive-negative d¹³C shifts. Detailed isotopic analysis of two platform to basin transects, however, reveals strong d¹³C variability and trends between sections both adjacent and across the shelf to basin transects. Such isotopic heterogeneity may reflect 1) strongly diachronous development of sedimentary facies and d¹³C values leading to incomplete stratigraphic record at corresponding sample localities, 2) large (up to 10 per mil, PDB) isotopic gradient or lateral d¹³C variation not representative of the whole ocean signal, and 3) pervasive diagenetic alteration that has modified the primary d¹³C values.

Physical stratigraphic study seems not to support large stratigraphic hiatus or truncation applicable for the inconsistent d¹³C trends. Strong spatial d¹³C variations of some intervals between closely spaced sections no more than a few kilometers apart are difficult to be interpreted as resulting from a greater Neoprotrozoic isotopic gradient. In contrast, the similar isotopic values between tectonic veins and their host rocks, and the identical d¹³C values between carbonate rocks and interbedded shaly layers in which carbonate components are apparently formed from later fluids favor an interpretation that the d¹³C values of the Doushantuo Formation have been substantially modified by diagenetic fluids, even in the dolomicrite that has been considered as least susceptible to diagenetic alteration. However, such diagenetic overprints cannot be eliminated by application of existing geochemical screens and compels a reevaluation of the Neoproterozoic carbon isotopic record.