SIMS SULFUR ISOTOPE ANALYSIS OF PYRITES IN CHERT NODULES OF THE EDIACARAN DOUSHANTUO FORMATION: CONSTRAINTS ON EXEPTIONAL FOSSIL PRESERVATION

Dolostones and mudstones in the lower Ediacaran Doushantuo Formation of the Yangtze Gorges area (South China) contain abundant cm-sized chert nodules with exceptionally preserved microfossils, including cyanobacteria, multicellular algae, spiny acritarchs, and animal resting eggs and embryos. Petrographic observations suggest that these chert nodules were formed during early diagenesis prior to compaction, but the geochemical processes responsible for silicification are not well understood. This study aims at reconstructing the paragenetic sequence and providing geochemical constraints on the role of bacterial sulfate reduction in authigenic mineralization and fossil preservation.

The nodules are pervasively silicified, with minor amounts of micritic and detrital components. They typically have a microbial mat fragment in the center, a pyrite rim, and a blocky calcite rim. Petrographic analysis indicates that the blocky calcite rim is of late diagenetic origin, postdating the pyrite rim and the silica core. The pyrite rim appears to have grown centripetally, representing a reaction front that was determined by the relative diffusion rates of H₂S (possibly from degradation of mat fragments in the center of nodules by sulfate reduction bacteria) and Fe²⁺ (from surrounding argillaceous dolomicrite matrix). SIMS sulfur isotope analysis of individual pyrite crystals in the pyrite rims show highly positive values and insignificant differences between small (~10 µm in size, in outer rim) and large crystals (~500 µm in size, in inner rim). The isotopically heavy and homogenous pyrites are evidence for rapid bacterial sulfate reduction in a diagenetic environment with limited sulfate availability. We hypothesize that the Doushantuo chert nodules nucleated on microbial mat fragments, which were degraded by rapid bacterial sulfate reduction, generating outward-diffusing H₂S that met inward diffusing Fe²⁺ to precipitate the pyrite rims. Sulfate reduction may have also altered the local pH values, driving the dissolution of primary micritic carbonate and simultaneous precipitation of amorphous silica responsible for fossil preservation.