MERCURY CONTENT AND STABLE ISOTOPE DATA AS A TOOL FOR STRATIGRAPHIC CORRELATION AND ENVIRONMENTAL INTERPRETATION: A CASE STUDY OF EDIACARAN BLACK SHALES OF THE DOUSHANTUO FORMATION, SOUTH CHINA

Key environmental changes and evolutionary events during the Ediacaran Period set the stage for the Cambrian radiation. The Doushantuo Formation of South China hosts an important record of the fossil and geochemical history of these events. Proper interpretation of this record requires stratigraphic correlation among disparate sections of the Doushantuo Formation, especially with regards to the black shale units. The member IV black shale at the top of some Doushantuo sections in the Huangling Anticline has been stratigraphically correlated to either (1) the lower black shale, upper dolostone, and Miaohe Member black shale sequence in other sections or (2) just the lower black shale, meaning the upper dolostone and Miaohe Member correlate to the younger Dengying Formation. These competing correlations impact the interpretations of the age and duration of major geochemical events in the Doushantuo Formation. We intend to test these potential correlations by supplementing existing lithostratigraphy and carbon isotope chemostratigraphic data with mercury (Hg) content and isotopic compositions. Hg has strong affinity with organic matter and sulfides and is thus often enriched in black shales. With these data we intend to test these correlations by assuming horizons with elevated Hg content and distinct Hg isotope compositions represent correlatable horizons that reflect events like volcanism, regional scale changes in redox conditions, or changes in the weathering input of Hg. In the seven sections we measured, we identified at least one stratigraphic horizon with elevated Hg concentration in the Miaohe Member and the upper member IV, along with another horizon of elevated Hg concentration in the lower member IV and the lower black shale. This pattern persists when Hg contents are normalized by total organic carbon content (to account for differential sedimentation rates and primary productivity). These results are consistent with the correlation of the member IV to the lower black shale through Miaohe Member sequence. Additionally, Hg isotope ratios in the most complete Doushantuo section at Jiulongwan, when supplemented with other recently published Doushantuo Hg isotope data, imply they reflect changing sediment input and oceanic redox conditions during the deposition of the black shale intervals.