A pulse of seafloor oxygenation at the Late Devonian Frasnian-Famennian boundary in South China

Abstract

The Frasnian-Famennian (F-F) mass extinction (~372 Ma) is regarded as one of the ‘Big Five’ mass extinctions events in Phanerozoic. The F-F boundary is characterized by two positive carbonate carbon isotope (δ13Ccarb) excursions, and is coincident with the global deposition of two layers of black shale and bituminous limestone, collectively known as the Lower and Upper Kellwasser events, referring two pulses of oceanic anoxic events that might have triggered the F-F mass extinction. The oceanic anoxia/euxinia near the F-F boundary is supported by some geochemical evidence, but other sedimentological and geochemical evidence indicates that the oceanic anoxia may not be pervasive. In this study, we report carbonate-associated iron contents (Fecarb), carbonate carbon isotopes (δ13Ccarb), sulfur isotopes of carbonate-associated sulfate (δ34SCAS) and pyrite sulfur isotopes (δ34Spyrite) data from two F-F boundary sections, the Panlong and Baisha sections, which were deposited in a shallow marine carbonate platform and an intraplatform basin environments, respectively. In the late Paleozoic, carbonate was predominantly precipitated on the seafloor, and thus may have been affected by benthic flux from sediment porewater, recording a mixture of seawater and porewater signals. In the shallow water Panlong section, the seafloor O2 fugacity calculated from Fecarb data indicates the seafloor persistently oxic. This argument is supported by high δ13Ccarb, δ34SCAS and δ34Spyrite values of the Panlong samples, reflecting a weak benthic flux and an ineffective H2S diffusion from porewater. In the intraplatform basin Baisha section, the decline of Fecarb at the F-F boundary demonstrates a transition of seafloor redox from suboxic/anoxic to oxic. The coupled positive δ13Ccarb excursion as well as generally positive shifts (though not consistently) of both δ34SCAS and δ34Spyrite suggest a diminished benthic flux and enhanced H2S oxidation. These observations indicate the seafloor oxygenation at the F-F boundary in the intraplatform basin environment. Therefore, instead of the development of oceanic anoxia, our study indicates a pulse of seafloor oxygenation at the F-F boundary in South China. A pulse of seafloor oxygenation is consistent with the occurrence of diverse brachiopod fauna at the F-F boundary in South China.