A broad spectrum of marine sediments, including metalliferous black shales, stratiform barite, phosphorite and sapropelite (combustible shale) was widely deposited during the Ediacaran-Cambrian transition on the Yangtze Platform. The source of metals in the metalliferous black shales and the trigger for ocean dynamics during this critical period remain controversial. We analyzed Hg isotopes in late Ediacaran to Early Cambrian carbonaceous sedimentary rocks from the basinal Sansui section, South China, and observed Hg concentrations of 102–103 ppb, which is high but within the global range of black shales. The late Ediacaran carbonaceous chert shows near-zero Δ199Hg values (−0.05–0.09‰), which can be interpreted as input of terrestrial Hg or volcanic Hg. However, the transgressive near-coastal setting supports the terrestrial origin. The Early Cambrian V rich black shales display positive Δ199Hg values (0.03–0.18‰), which are similar to those observed in polymetallic Ni-Mo-rich sulfidic black shales (0.10–0.22‰) and phosphorites (0.13–0.24‰) in the stratigraphically equivalent Maoshi and Zhijin sections, respectively. These positive Δ199Hg values suggest that Hg was of dominantly seawater origin in a restricted basin setting. The variable Δ199Hg values of the rock spectrum reflect the dynamic interplay of marine and terrestrial metal sources as a function of paleo-depositional setting within the transgressive–regressive system on the continental margin of the Ediacaran-Cambrian Yangtze platform.
Bibliographical noteFunding Information:
This paper was supported by the National Natural Science Foundation of China (No: 42163006, 41890841), Talent Base Project in Guizhou Province (No: RCJD2018-21), Cultivation Project of Guizhou University (No: 202007), and the Guizhou Province Graduate Research Fund Project (No: YJSCXJH181).
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- Early Cambrian
- Hg isotopes
- Hyperenriched metalliferous black shale
- Oceanic anoxic event