Lin-Hui Dong analyzed excess barium contents (Baexcess) and isotope compositions (d138Baexcess) at the State Key Laboratory of Lithospheric and Environmental Coevolution. She found a negative logarithmic correlation between Baexcess and d138Baexcess values, indicating Rayleigh-type fractionation related to barium removal from the Yangtze Block's protected basin.

The correlation between ln(Baexcess) and d138Baexcess shows an isotope fractionation of -0.22 +/- 0.03 mph , consistent with equilibrium fractionation between barite and fluid from first-principle calculations (-0.23 +/- 0.04 mph ). "Enhanced barite precipitation and subsequent sedimentation were responsible for the Ba enrichments in the lower Cambrian metalliferous black shales," Lin-Hui says.

Barite precipitation in the marine system requires a nexus of barium- and sulfate-bearing solutions. An isotopically light and homogeneous barium reservoir may have existed in the anoxic to euxinic protected basin during the Ediacaran and earliest Cambrian periods.

The sulfate likely originated from the upwelling of sulfate-rich deep seawater. As the seawater migrated onshore toward the northwest of the basin, both sulfate and barium concentrations decreased while the d138Ba value increased due to Rayleigh-type distillation associated with barite precipitation. "The spatial patterns of the Baexcess and d138Baexcess suggest the deep-ocean oxygenation at ca. 521 Ma," Wei Wei says.

A sensitivity analysis revealed that over 99% of the homogeneous barium reservoir in the protected basin had been removed around 521 million years ago. This barium removal likely extended beyond the Yangtze Block during the early Cambrian.

From the late Ediacaran to early Cambrian, progressive deep-ocean oxygenation may have increased the sulfate reservoir via sulfide oxidation and decreased the barium reservoir through barite precipitation.

Concurrently, animal diversity rapidly increased, with genus-level diversity peaking at the onset of Cambrian Stage 3. Soluble barium compounds are toxic to marine animals. "It is plausible that the removal of toxic Ba and H2S corresponding to marine oxygenation may have ultimately expanded the habitable area and paved the road for further radiation of early animals," commented Fang Huang.

Research Report:Seawater barium and sulfide removal improved marine habitability for the Cambrian Explosion of early animals

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