Sedimentary Cycling and Environmental-Change in the Late Proterozoic - Evidence from Stable and Radiogenic Isotopes

TitleSedimentary Cycling and Environmental-Change in the Late Proterozoic - Evidence from Stable and Radiogenic Isotopes
Publication TypeJournal Article
Year of Publication1992
AuthorsDerry, LA, Kaufman, AJ, Jacobsen, SB
JournalGeochimica Et Cosmochimica Acta
Date PublishedMar
Accession NumberWOS:A1992HJ96300033

We report C, Sr, and O isotopic as well as selected major and trace element data from Late Proterozoic (ca. 900-540 Ma) marine carbonates in three widely separated basins. The isotopic and elemental data are used to evaluate effects of post-depositional alteration of Sr-87/Sr-86 and delta-C-13. Using our present best estimates for unaltered samples, we construct a new delta-C-13-curve for 850 500 Ma marine carbonates using data in this paper and from literature sources. Delta-C-13 values are high (+4 to +8 parts per thousand) during most of the late Riphean (ca. 900-600 Ma) with brief negative excursions likely associated with glacial periods. Similarly, in the Vendian delta-C-13 falls sharply (from late Riphean highs) to < -3 parts per thousand around the Varanger glaciation (ca. 600 Ma), and then returns to high values (+4 to +2 parts per thousand) remaining until the Precambrian-Cambrian boundary where the curve drops to a value of about -1 parts per thousand 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 O2 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 O2; high values of delta-S-34 in Late Proterozoic marine sulfates suggest that this O2 flux was not balanced by increased sulfate formation. Further, the Sr-isotopic record indicates that excess O2 was not balanced by oxidation of submarine hydrothermal fluids. Increased oxidative weathering was probably an important sink for O2; nonetheless, we conclude that a significant and rapid increase in atmospheric O2 occurred in the Vendian. These results have important implications for environmental changes during the first appearance of an Ediacaran metazoan fauna.