Pyrite morphology and redox fluctuations recorded in the Ediacaran Doushantuo Formation

Recent geochemical studies of the Doushantuo Formation (ca. 635–551Ma) in South China suggested that the Ediacaran ocean was strongly stratified, with an oxic surface layer above a euxinic wedge that was sandwiched within ferruginous deep waters. This ocean redox model, however, was derived largely from the data obtained from stratigraphic sections in the Yangtze Gorges area that, according to recent paleogeographic reconstruction, were deposited in a restricted intrashelf lagoon. In order to test the redox conditions in open-ocean, deep-water environments, we have conducted a detailed morphological analysis of authigenic pyrites from the upper to lower slope sections of the Doushantuo Formation. In analogy to modern euxinic basins such as the Black Sea, framboidal pyrites with smaller and less variable size distribution are taken as evidence for sulfide precipitation in euxinic water column, while larger and more variable framboidal and euhedral pyrites are formed or diagenetically altered in sediments with an overlying oxic/dysoxic water column. Except for a few brief intervals, pyrites from the Doushantuo Formation in upper slope sections (Siduping and Taoying sections) are mainly of early diagenetic origin and do not record water column euxinia during deposition. In contrast, pyrites from the Doushantuo Formation in the lower slope section (Wuhe section) are dominated by fine-grained framboids indicative of pervasive water column euxinia below chemocline. In both upper and lower slope sections, temporal changes in genetic pyrite types occur at centimeter to decameter scales, suggesting frequent chemocline fluctuations. The overall decrease in abundance of framboidal pyrites toward the upper Doushantuo Formation in upper slope sections suggests increasing water column oxygenation and deepening of the chemocline. Macroalgae and metazoan fossils are found mainly from shale intervals without syngenetic pyrites in upper slope sections, indicating the sensitivity of macroscopic eukaryotes to the ocean chemocline. The redox fluctuations recorded by the Doushantuo pyrites compels for comprehensive geochemical data in deep-water successions to further test the existing paleoceanographic models of the Ediacaran ocean. ► Framboidal pyrite size distribution can be used as indicator for water euxinia. ► Deep water euxinia is recorded in lower slope facies of Doushantuo Formation. ► Submeter scale redox fluctuations existed during the Doushantuo deposition. ► Redox