第七届青年地学论坛

   The early Cambrian marine redox states, especially of the deep ocean, are highly debated, hampering the understanding of the relationship between the biological evolution and oceanic oxygen level. Here, we present Ba_excess contents and δ¹³⁸Ba_excess of the early Cambrian (ca. 521 Ma) metalliferous and barren black shales from the Yangtze Block. The logarithmic negative correlation between the Ba_excess and δ¹³⁸Ba_excess and extremely high δ¹³⁸Ba_excess (up to 1.15‰) suggest a Rayleigh-type Ba isotope fractionation of -0.22±0.03‰ in a closed system. This fractionation occurred during enhanced barite precipitation by mixing between the Ba-bearing anoxic/euxinic seawater in a protected basin and sulfate-bearing oxic seawater, which progressively scavenged light Ba isotopes from the water masses. In additional, we observed lateral gradients of increasing Ba_excess and deceasing δ¹³⁸Ba_excess with offshore distance. Combining the earlier occurrence of stratiform barite ores in the deeper environment, we suggest that the enhanced barite precipitation and subsequent sedimentation during the early Cambrian (ca. 521 Ma) was tightly linked to upwelling of the oxic pelagic seawater corresponding to global oceanic oxygenation.

Metalliferous black shale,Early Cambrian,South China,Heavy Ba isotopes,Global oceanic oxygenation