Trace elements of pyrite in the Ediacaran Doushantuo Formation reveal ancient methane release events

Abstract

Multiple records of extremely negative carbon isotopes have been reported from the Ediacaran Doushantuo Formation in South China (ca. 635–551 Ma), implying frequent methane release from seafloor sediments of the Yangtze Sea at that time. The incorporation of Ni and Co from AOM-modified biomass into pyrite has emerged as a novel indicator for ancient methane release events, while the widespread presence of pyrite in sedimentary rocks has the potential to yield a continuous long-term record of submarine methane flux. To better understand the magnitude of ancient methane release events during the Ediacaran, we measured trace-elemental compositions for pyrite and bulk rock in the Qinglinkou section and compared these results with data from the more distal (deeper-water) Wuhe section. Three episodes of methane release were identified, one each in the lower, middle, and upper parts of the Doushantuo Formation. The first two release events were observed at both Qinglinkou and Wuhe and may have been linked to destabilization of gas hydrates due to sea-level falls associated with the Marinoan and Gaskiers glaciations. In contrast, the third release event is present only within the Wuhe section, which implies that methane release may have been associated with either a potential glaciation or increased sulfate transport to offshore areas during an oceanic oxygenation event. The methanic signals revealed by pyrite trace elements are consistent with the extremely low carbon isotopic compositions found locally within the study sections. Thus, our study is significant in demonstrating the utility of pyrite trace elements as proxies for ancient methane release events. Determining the spatial extent of a given methane release event will require analysis of correlative units in multiple globally distributed basins and/or cratons. In addition, our results provide new insights into the regulation of methane releases, ocean-redox variation, atmospheric oxygen levels, and bioevolution during the Ediacaran Period.