USING MERCURY CONTENT TO EVALUATE STRATIGRAPHIC CORRELATIONS OF EDIACARAN BLACK SHALES IN THE UPPER DOUSHANTUO FORMATION

The Doushantuo Formation of South China hosts a wealth of exceptionally preserved fossils, including early metazoans and important geochemical records that detail major biogeochemical events of the Ediacaran Period, such as the Shuram Carbon Isotope Excursion. However, contextualizing the timing of these fossil occurrences and events among different sections of the Doushantuo Formation requires a robust stratigraphic framework. This is particularly important for the stratigraphic correlation of upper Doushantuo black shale units (including Member IV, Miaohe Member, and Lower Black Shale) that overlie the Shuram excursion. Member IV has been correlated with either the tripartite Lower Black Shale, Upper Dolostone, and Miaohe members or only Lower Black Shale, with the Upper Dolostone and Miaohe Member regarded as parts of the overlying Dengying Formation. Here, we use stratigraphic variations of mercury [Hg] contents to test and refine these stratigraphic correlations. Mercury abundance can provide information on regional and potentially global volcanic input and/or local redox conditions, the former of which could facilitate stratigraphic correlation. We sampled and analyzed Member IV and tripartite sequences from six sections of the Doushantuo Formation. In each of the six sections there is at least one interval of Hg enrichment in the Miaohe Member or the upper Member IV, and another interval of Hg enrichment in the Lower Black Shale, or lower portion of the Member IV. For each section, the lower enrichment is of a greater magnitude than the upper. To account for differential sedimentation rates among sections, Hg concentration is normalized with total organic carbon (TOC) content. [Hg]/TOC also shows enrichments in the lower and upper parts of Member IV, again with a larger magnitude of enrichment in the lower part. These Hg data are consistent with the conventional scheme where Member IV correlates with the Lower Black Shale, Upper Dolostone, and Miaohe members. These Hg enrichments thus offer further insights into the timeline of early metazoan evolution and geochemical events, and the co-evolution of life and environment in the Ediacaran Period.