MINERALOGY AND GEOCHEMISTRY OF NEOPROTEROZOIC BLACK SHALES FROM THE DOUSHANTUO FORMATION: CONSTRAINTS ON ORGANIC CARBON PRESERVATION

The rate of burial, and mode of cycling of organic carbon during the Neoproterozoic are of interest because these factors influence the oxidation state of the oceans/atmosphere and the size and frequency of changes in carbon and sulfur isotopic records. Therefore understanding the control of organic carbon decay in the Precambrian has potential implications for 1) determining what triggered the rise of oxygen in the Late Precambrian, and 2) interpretations of paleoclimatic and paleoenvironmental conditions based on isotopic proxies.

The Doushantuo Formation, in southern China, is a condensed, organic-rich interval deposited on a Late Neoproterozoic continental margin. Absolute ages from the top and bottom of the Doushantuo, a cap carbonate overlying glacial strata and fossils indicative of the appearance of life on land and complex organisms in the ocean show that Doushantuo Formation sediments span ~80 Ma of intense paleoenvironmental and biological change. Here we present initial results a geochemical study of Doushantuo Formation black shales. The aim of this work is to test the effect of chemical, physical and biological variables on preservation of organic carbon in marine sediments. The study involves: 1) using trace elements and iron speciation as proxies for paleoredox state of the ocean, 2) a mineralogical study, with emphasis on clay mineralogy and abundance 3) an organic geochemical investigation of the nature of organic materials. This allows us to determine if 1) the availability of oxidants (free oxygen, sulfate), 2) physical sheltering by clay minerals, or 3) changes in the chemical nature of organic carbon, determined how much organic material was preserved in these sediments.