Ediacaran growth of the marine sulfate reservoir

摘要

The sulfur isotope record as preserved in sedimentary sulfate and sulfide minerals reflects the progressive oxidation of Earth's surface environment. The latest Proterozoic to early Paleozoic marine sulfur isotope record is distinguished by unusually high delta S-34 values in marine sulfates and sulfides. Geological considerations and sulfur-isotopic data indicate that the 34 S-enrichment of the marine sulfate reservoir was the result of a rapid increase in the net fractionation between sedimentary sulfate and pyrite (Delta S-34) without a compensatory decline in the fractional burial rate of pyrite (f(pyr)). A simple one-box model, in which Delta S-34 is linked nonlinearly to marine [SO4], is used to explore the response in sedimentary sulfur isotope records (delta S-34(sulf) and delta S-34(pyr)) to an increasing marine sulfate reservoir. The model results, when compared with a compilation of existing delta S-34(sulf) and delta S-34(pyr) data, suggest that the marine sulfate reservoir grew rapidly to roughly Phanerozoic levels in the middle Ediacaran Period, before the Gaskiers glaciation and prior to the onset of the Shuram-Wonoka negative delta C-13 anomaly. It is further hypothesized that pO(2) levels rose prior to the onset of this extreme perturbation to the global carbon cycle, permitting the oxidation of a previously euxinic deep ocean and appearance of the first Ediacaran fauna by ca. 575 Ma, despite the enormous demand on oxidants implied by this large magnitude and Iona duration carbon isotope anomaly. (c) 2007 Elsevier B.V. All rights reserved.