MERCURY CONTENT AND STABLE ISOTOPE COMPOSITION FROM EDIACARAN DOUSHANTUO FORMATION BLACK SHALES SUGGEST DYNAMIC ENVIRONMENTAL CONDITIONS INCLUDING PHOTIC ZONE EUXINIA AND DYNAMIC MERCURY SOURCES

The Ediacaran Doushantuo Formation (DST) of South China hosts a significant geological and paleontological record. The uppermost DST (~560-550 Ma) is of particular interest as it contains the recovery from the Shuram carbon isotope excursion and precedes the biological innovations and extinctions of the terminal Ediacaran stage. In some localities the uppermost DST 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 correlated to the overlying Dengying Formation). The member IV and the Miaohe Member have been targeted for paleoenvironmental studies, with data from some geochemical proxies (including redox-sensitive elements and carbon & pyrite sulfur isotopes) used as evidence of a global oceanic oxygenation event whereas others (e.g., Fe speciation data) are suggestive of locally heterogeneous and dynamic redox conditions. Here we present mercury (Hg) concentrations and stable isotope compositions from seven sections of DST black shales around the Huangling Anticline to investigate these conditions via the redox and source sensitivity of Hg. Mercury has multiple stable isotopes and exhibits both mass dependent fractionation (reported as δ²⁰²Hg) and mass independent fractionation (reported as Δ²⁰¹Hg) which can record details about the Hg sources and the chemical processes Hg is undergoing in depositional systems. The DST shales are characterized by high Hg/TOC ratios (< 1478 ppb Hg/% TOC), and high Hg/Al ratios (< 277 ppb Hg/% Al), indicating intervals of increased mercury deposition. The Hg isotope compositions vary, with δ²⁰²Hg ranging from –2.89‰ to –0.05‰ and Δ²⁰¹Hg from –0.2‰ to 0.34‰. There are differences in Hg geochemistry among sections that could indicate stratigraphic miscorrelation but are better interpreted as the result of dynamic mixing of Hg derived from euxinic waters in the photic zone with that derived from other local sources and/or processes (e.g., redox heterogeneity, terrestrial input, atmospherically deposited mercury, or basin restriction). This is consistent with dynamic marine conditions at the local scale during the Ediacaran Period that included intervals of photic zone euxinia along the continental margins in the context of overall transient ocean oxygenation.