SPATIAL ISOTOPE VARIATIONS OF THE EDIACARAN SUCCESSION IN SOUTH CHINA: IMPLICATIONS FOR THE CHEMOCLINE INSTABILITY ACROSS DEPOSITIONAL ENVIRONMENTS

An integrated sedimentological and isotope study in the Ediacaran (635–542 Ma) Yangtze platform in South China reveal considerable spatial variations in δ¹³C at different paleogeographic settings. In relatively deep-water environments such as restricted shelf lagoons, carbon isotope values show negative δ¹³C shifts in response to regressive portions of the depositional sequences, while in shelf margins where shallow-water deposits dominate, isotope values display frequent positive-to-negative shifts. Such 'facies-dependent' isotope variations may be interpreted as diagenetic overprints. However, distinctive isotope values from the Ediacaran strata contrast with those from the overlying Paleozoic strata in similar faices of the same sections, suggesting that even diagenetic modification has been significant, pervasive negative δ¹³C shifts may record signature from unique Ediacaran chemical environments favorable for generating ¹³C-depleted alkalinity. Under this assumption, spatial variations in δ¹³C may record the spatial instability of chemoclines in the Ediacaran basins where, below the chemocline, strong sulfate reduction led to remineralization of organic carbon and generation of ¹³C-depleted alkalinity. Isotope variations may reflect a mixed signature of ¹³C-depleted carbon produced below the chemocline and photosynthetically generated carbon from the surface ocean. In regions where a relatively stable chemocline was maintained, only major oxidation events led to isotope shifts; while in platform margins where the chemocline may fluctuate in response to smaller-scale sea-level changes or upwelling, frequent positive-negative δ¹³C shifts were observed. The study calls for the need of a detailed investigation of the late Neoproterozoic isotope anomalies in a sedimentological context to reveal the origin of regional and global carbon isotope variability.