The Frasnian–Famennian transition of the Late Devonian was one of the most critical intervals in the Phanerozoic. Sulphur isotopic pairs of carbonate-associated sulphate and pyrite sulphide from coeval sections in South China and Poland reveal frequent perturbations of sulphur cycling during this time interval. These data suggest a sudden oceanic overturn during a rapid sea-level fall probably induced by jerky block tilting in the latest Frasnian. This event was followed by long-lasting photic-zone euxinia during a rapid sea-level rise in the earliest Famennian. Large increases in continental nutrient fluxes, and subsequent primary productivity and organic burial, could have greatly enhanced bacterial sulphate reduction, producing excessive sulphide through the water columns owing to iron depletion. Subsequently, rapid ventilation of oceanic basins occurred, during which direct aerobic oxidation of sulphide into sulphate predominated in bottom waters and even surface sediments with minimal fractionation. This oxygenation was probably induced by intensive climatic cooling and/or large-scale sea-level fall. The temporal coincidence of two extinction phases with the oceanic overturn and succeeding photic-zone euxinia suggests that these extreme oceanic events played an important role in the severe biotic crisis. Furthermore, photic-zone euxinia coupled with subsequent climatic cooling may have delayed post-extinction recovery of some taxa.
