DO GEOCHEMICAL RECORDS FROM THE DOUSHANTUO FORMATION RECORD A MARINE SIGNAL?

The Doushantuo Formation in south China contains an excellent geochemical and fossil record through a large part (635-551Ma) of the Ediacaran period (635-543Ma). The Doushantuo is currently considered to be a record of an open marine setting with stable isotope and redox chemistry used to understand global ocean and atmosphere chemistry during the recovery from the Marinoan glacial period and prior to the first appearance of animal embryos that appear in the upper member of the Formation. However, insitu clays minerals and other geochemical evidence indicate a highly restricted/ alkaline lake origin for the lower Doushantuo in the Yangtze Gorges area. In contrast, black shales of the upper Doushantuo contain normal marine clay minerals and carbon isotope values that match those observed in Ediacaran sections on different continents compare to the anomalous values in the lower Doushantuo. This may indicate an upsection transition to the open ocean in the same interval that animal fossils appear. Comparisons of organic matter chemistry and distribution patterns of redox sensitive elements (Mo, Re, V and U) between mudstones and shales of the lower and upper Doushantuo Formation support the hypothesis of a change from a restricted/lacustrine to an open marine environment. These results imply that local ecological factors (e.g. a lacustrine environment) in the Doushantuo may control the stratigraphic appearance of animal fossils and not evolutionary trends responding to global controls. Our observations may also explain a ~10 per mil lateral gradient in carbon isotope values across the Yangtze Platform. Enriched carbon and oxygen isotope values (up to +4 per mil and 0 per mil respectively) are found in sections that we interpret as non-marine. Seaward sections lack clay mineral evidence of unusual water chemistry and yield negative carbon and oxygen isotope values (as low as -5 and -10 per mil). A small fractionation (~10 to 15 per mil) between sulfur isotope values of carbon associated sulfate and disseminated pyrite in the Yangtze Gorges also imply a restricted sulfur supply.