EVIDENCE FOR VARIABLE MARINE ENVIRONMENTAL CONDITIONS FROM MERCURY CONTENT AND STABLE ISOTOPIC COMPOSITION OF THE EDIACARAN DOUSHANTUO FORMATION BLACK SHALES, SOUTH CHINA

The Doushantuo Formation of South China hosts a significant geological and paleontological record of the Ediacaran Period. Interpretation of this is complicated by ambiguous regional stratigraphic correlation and paleoenvironmental interpretation. This is especially an issue for the uppermost portion of the Doushantuo Formation (ca. 560-550 Ma), which records the recovery from the Shuram carbon isotope excursion and precedes the biological innovations of the terminal Ediacaran. In some localities, the uppermost Doushantuo Formation is represented by a single black shale unit (member IV), but in others, it is composed of a thin black shale (Lower Black Shale), a dolostone (Upper Dolostone), and a siliceous black shale (Miaohe Member). Alternatively, the latter two units have been correlated with the Dengying Formation that overlies the Doushantuo Formation. Both the member IV and the Miaohe Member have been targeted for paleoenvironmental studies, with some geochemical proxies (including redox-sensitive element enrichments and carbon and pyrite sulfur isotope variations) indicating a global oceanic oxygenation event and others suggesting locally heterogeneous and dynamic redox conditions (e.g., Fe speciation data). Here we present Hg concentration and stable isotope composition data from seven sections of uppermost Doushantuo Formation black shales around the Huangling Anticline in South China to further investigate these redox conditions though the redox and source sensitivity of Hg. These shales are characterized by high Hg/TOC ratios (up to 1478 ppb Hg/% TOC), high Hg/Al₂O₃ ratios (up to 147 ppb Hg/% Al₂O₃), and variable Hg isotope compositions (δ²⁰²Hg ranging from –2.89‰ to –0.05‰ and Δ²⁰¹Hg from –0.2‰ to 0.34‰). The differences among sections could indicate stratigraphic miscorrelation but are better interpreted in the framework of a multi-source mixing model that involves Hg derived from euxinic waters in the photic zone and other local factors (e.g., redox heterogeneity, changing terrestrial input, or basin restriction) that influenced Hg geochemistry in Ediacaran oceans. This is consistent with dynamic marine conditions during the Ediacaran Period that included intervals of photic zone euxinia along the continental margins in the context of transient ocean oxygenation.