Sedimentary cycling and environmental change in the Late Proterozoic: Evidence from stable and radiogenic isotopes

We report C, Sr, and O isotopic as well as selected major and trace element data from Late Proterozoic (ca. 540-900 Ma) marine carbonates in three widely separated basins. The isotopic and elemental data are used to evaluate effects of post-depositional alteration of ⁸⁷Sr /⁸⁶Sr and δ¹³C. Using our present best estimates for unaltered samples, we construct a new δ¹³C-curve for 500-850 Ma marine carbonates using data in this paper and from literature sources. δ¹³C values are high (+4 to +8%) during most of the late Riphean (ca. 600-900 Ma) with brief negative excursions likely associated with glacial periods. Similarly, in the Vendian δ¹³C falls sharply (from late Riphean highs) to < -3% around the Varanger glaciation (ca. 600 Ma), and then returns to high values (+4 to +2%) remaining until the Precambrian-Cambrian boundary where the curve drops to a value of about - 1% in Lower Cambrian carbonates. Coupling of the Sr and C isotopic data is used to develop a simple model for evaluating organic carbon (C _org) burial in Late Proterozoic oceans. These calculations indicate that C _org burial rates were lower than present-day values during much of the late Riphean, at the same time that erosion rates were low. Excess O ₂ produced by the burial of C _org was likely balanced by oxidation of reduced hydrothermal fluids and weathering reactions. Near the time of the Varanger glaciation, C _org burial rates dropped but quickly recovered and reached a maximum (a factor of 2-4 greater than present day) in Vendian sediments. High C _org burial rates were probably driven by high sedimentation rates, and possibly high productivity. The high C _org burial rate likely gave rise to a large flux of O ₂; high values of δ³⁴S in Late Proterozoic marine sulfates suggest that this O ₂ flux was not balanced by increased sulfate formation. Further, the Sr-isotopic record indicates that excess O ₂ was not balanced by oxidation of submarine hydrothermal fluids. Increased oxidative weathering was probably an important sink for O ₂; nonetheless, we conclude that a significant and rapid increase in atmospheric O ₂ occurred in the Vendian. These results have important implications for environmental changes during the first appearance of an Ediacaran metazoan fauna.