Llandovery (lower Silurian) nodular barite from the northern margin of Yangtze Block, South China, and its paleoceanographic implications

Abstract

The Llandovery epoch (Early Silurian) was marked by the interruption of widespread oceanic anoxia and associated oxygenation of seawater. The first oxygenation event appears to occur near the Rhuddanian–Aeronian stages boundary. However, ventilation of the marine environments is a dynamic process, differing in timing and magnitude from place to place, basic variations in the nature of marine circulation; e.g., open shelf versus restricted shelf. The correlation of oxygenation events from open to restricted shelf environments is of significance to enhancing our understanding of marine ventilation processes. Here, we describle nodular barite that accumulated in the open marine environment near the boundary at the northern margin of the Yangtze Block, South China, emphasizing its origin and environmental implications. Petrographic evidence suggests that the nodular barite is early diagenetic. Barium was likely derived from the dissolution of biogenic barite as suggested by the 87Sr/86Sr values of the diagenetic barite. The REE + Y pattern, low Y/Ho and high Ce*/Ce values of the barite nodules demonstrate that microbial degradation of organic matter in association with Ce oxidation controlled REE composition of the nodules. PAAS-normalized positive Eu anomalies of the studied barite, however, suggest an anoxic porewater environment perhaps related to bacterial sulfate reduction or the anaerobic oxidation of methane focused within the sulfate-methane transition zone (SMTZ) as also indicated by the minor contribution of radiogenic terrigenous Sr isotopes. We infer that high marine productivity and the associated transport of biogenic barite to the sea floor was critical to the formation of the studied barite nodules. Diagenetic barite formation and preservation in Aeronian stage deposits may indicate that circulation of open marine water masses had been much ameliorated. Our results are consistent with earlier suggestions that the restricted shelf experienced a transition from anoxic or even euxinic conditions to a more oxygenated (probably suboxic) condition during the early Aeronian. The approximately contemporaneous nature of this transition and global cooling associated with the glacial pulses indicate the critical role that global climate change near the Rhuddanian–Aeronian (R-A) boundary played in the redox evolution of the Yangtze Block, South China.