SEAFLOOR OXYGENATION IN THE LATE DEVONIAN FRASNIAN-FAMENNIAN BOUNDARY EVENT

Geochemical and sedimentological evidence indicates significant perturbations of marine carbon and sulfur cycle and the development of oceanic anoxia or euxinia in the Frasnian-Famennian (F-F) boundary (~372 Ma), suggesting the possible linkage between marine redox and the F-F mass extinction, one of the big five biological crises in Phanerozoic. In order to further explore the environmental background of the F-F mass extinction, we measured the elemental and isotopic compositions of carbonate samples collected from two sections, the Panlong and Baisha sections, which were deposited in a shallow marine carbonate platform and an intrashelf basin in South China Block. (1) Carbonate carbon isotope (δ¹³C_carb) shows a positive excursion by +3‰ in the Baisha section, while δ¹³C_carb of the Panlong section decreases by 1.6‰ followed by a 3‰ increase in the F-F boundary. (2) The Panlong section has more consistent and higher CAS sulfur isotopes (i.e. carbonate associated sulfate, δ³⁴S_CAS, Panlong: +22‰~+29‰, Baisha: +6‰~+26‰) and pyrite sulfur isotopes (δ³⁴S_pyrite, Panlong: +6‰~+26‰, Baisha: -30‰~+30‰). (3) There is a significant decrease of Fe content in carbonate (Fe_carb) in the Baisha section, while Fe_carb remains at low values in the Panlong section. (4) δ¹³C_carb shows a negative correlation with Fe_carb in the Baisha section. Because of predominant deposition in seafloor in late Paleozoic, benthic carbonate would be influenced by benthic flux from sediment porewater, which normally enriches in Fe²⁺, ¹³C-depleted HCO₃^- derived from organic carbon remineralization, and ³⁴S-depleted sulfate derived from H₂S oxidation. Thus, higher δ³⁴S_CAS and δ³⁴S_pyrite values of the Panlong section may indicate lower benthic flux, probably due to higher sedimentation rate and more oxygenated seafloor in shallow water platform, while the positive excursion of δ¹³C_carb as well as negative correlation between δ¹³C_carb and Fe_carb may indicate seafloor oxygenation near the F-F boundary. Instead of oceanic anoxia, our study indicates the seafloor oxygenation in the F-F boundary, which is consistent with emergence of diverse brachiopods fauna in the F-F boundary.