Abstract
Primary carbon isotope trends in marine carbonate rocks are particularly useful for chemostratigraphic correlation and paleoenvironmental reconstruction of the Neoproterozoic Earth System. Differences, however, exist between the published carbonate C-isotope profiles from the Neoproterozoic Yangtze platform and the global C-isotope record through the equivalent time interval. More C-isotope profiles are required for the Yangtze platform from where excellently preserved early animal fossils have been discovered. We studied C, O and Sr isotope compositions and Mn–Sr concentrations of two successions of the Neoproterozoic Doushantuo and Dengying Formations in Guizhou and Zhejiang Provinces, South China. Most of the 87Sr/ 86Sr ratios lie above 0.709, higher than the contemporaneous seawater values, and are likely to represent diagenetic alteration and possible leaching of clay detritus. The δ 18O profiles exhibit more fluctuation than the δ 13C profiles, indicating that oxygen isotopes are more sensitive to diagenetic effects. Both δ 13C profiles in Zhejiang and Guizhou show negative excursions to − 3 to − 4‰ in the Doushantuo cap carbonate overlying the Nantuo (Marinoan) diamictite and at the Doushantuo–Dengying boundary, and positive shifts to + 3 to + 4‰ through the rest of the Doushantuo and Dengying Formations. We suggest that oceanic overturn was responsible for both the negative δ 13C excursion within the Doushantuo cap carbonate and that at the Doushantuo–Dengying boundary. However, the mechanisms for the two overturn events are different. The former overturn occurred under conditions of high temperature and high evaporation rates after thawing of the Nantuo (end-Cryogenian) Snowball Earth that had resulted in the stagnant and stratified ocean under sea ice. The Doushantuo interval witnessed sustained high bioproductivity, elevated rates of organic matter accumulation in deep water and sediment, and progressive δ 13C stratification in the ocean, CO 2 drawdown to sediments and thus cooling, which led eventually to high latitudinal glaciation, vigorous thermohaline circulation and oceanic overturn at the end of the Doushantuo period. This latter overturn released CO 2 back to the atmosphere, thus raising the temperature and preventing further cooling. High productivity revived subsequently during the Dengying interval. More work will be needed to further constrain and interpret C-isotope evolution across the Precambrian–Cambrian boundary on the Yangtze platform.
Published Version
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