矿床地球化学国家重点实验室知识库

The redox condition of the ocean played an important role in the evolution of the Ediacaranbiota, but it remains largely unexplored, particularly for the deep ocean environment. In this study, we present a comprehensive study of Fe speciation and trace elements of the Doushantuo black shales from the Longe and Jinjiadong sections in South China, to investigate the redox condition of basinal environments in Ediacaran. In general, both sections show high Fe_HR/Fe_T ratios (>0.38), low Fe_Py/Fe_HR ratios (<0.7), suggesting a predominately anoxic and ferruginous condition in the deep water of Nanhua basin during the Doushantuo period. A few high Fe_Py/Fe_HR ratios (>0.7) in the Jinjiadong section indicating occasional euxinia in the basin facies. Fe_Py/Fe_HR ratios show an increase in the middle part both in the Longe and Jinjiadong sections, indicating enhanced production of H₂S via bacteria sulfate reduction (BSR) in the deep anoxic water, which was likely responsible to the upper Doushantuo negative carbon isotope excursion.

The enrichment factors (EFs) of most redox sensitive trace elements in the sediments of both sections are low (≤1), while EF-Mo shows moderately enriched (∼3–60), which is similar to the modern Cariaco Basin. Because of paleogeographic restriction or persistent ocean stratification, the trace elements renewal would be mostly obstructed in the isolated deep water, and their inventory could become depleted, which cause the low EFs of most trace elements in sediments. However, for the non-euxinic deep ocean, a “particulate shuttle” would transport Mo from shallow water to deep water through adsorption-desorption on the Mn–Fe-oxyhydroxides, which maintained moderate Mo enrichment in seawater and related sediments.