USING GEOCHEMICAL TOOLS TO STUDY EDIACARAN MACROORGANISMS FROM THE KHATYSPYT FORMATION, OLENEK UPLIFT, SIBERIA

Ediacaran macroorganisms provide important insights into the early evolution of multicellular animals. Traditionally, investigations of Ediacaran macroorganisms have focused on morphological characterization. More recently, the integration of geochemical proxy data with the paleontological data has become increasingly important to improve our understanding of the paleoecology and taphonomy of Ediacaran macrofossils. Here we present some preliminary geochemical data from our study of Ediacaran macrofossils in the Khatyspyt Formation of northern Siberia. Our geochemical analysis was focused on Aspidella-type discoidal holdfasts preserved as casts within carbonate nodules. These nodules were probably formed as a result of authigenic cementation, perhaps facilitated by microbial degradation of the holdfasts through microbial sulfate reduction. To test this hypothesis, we analyzed the organic carbon and pyrite sulfur isotopes of the nodules and surrounding carbonate matrix. Organic carbon isotope values range from -36.95‰ to -31.55‰, and reveal no significant differences among different parts of the holdfasts (i.e., holdfast center, holdfast margin, and attached stalk when preserved) or between holdfasts and the matrix. Pyrite sulfur isotope values are generally very high (32.0 – 41.9‰) and holdfasts have isotopically lighter pyrite than does the matrix (35.2–40.0‰ for holdfasts; 37.4–41.9‰ for matrix; p ~ 0.01, one-tailed Student’s t-test). Considering that, due to isotopic fractionation associated with microbial sulfate reduction, pore-water sulfate typically becomes increasingly enriched in ³⁴S below the water-sediment interface, our data indicate that the holdfasts were lithified at shallower depths in the sediment column than the sedimentary matrix. Thus, the available geochemical data support the hypothesis that the holdfasts were fossilized through early authigenic processes facilitated by MSR. Additional geochemical data are required to test whether the presence of microbial symbionts (e.g., sulfur oxidizing bacteria) in the holdfasts may have caused the different pyrite sulfur isotopic compositions between the holdfasts and the sedimentary matrix.