Mercury isotopes reflect variable metal sources as a function of paleo-depositional setting in the Ediacaran-Cambrian Ocean, South China

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.