δ34SCAS and δ18OCAS records during the Frasnian-Famennian (Late Devonian) transition and their bearing on mass extinction models

Many proposed extinction mechanisms for the Late Devonian (Frasnian-Famennian boundary - FFB) mass extinction event include the spread of anoxia in bottom waters during the two Kellwasser anoxic events, which have in turn been linked to changes in continental weathering, volcanic/hydrothermal fluxes, sea-level and climate change. Although there is strong evidence for bottom water anoxia in epicontinental seas at tropical/equatorial palaeolatitudes, the global extent of these changes is poorly understood, largely due to the lack of an oceanic and high-latitude record. In this study, marine δ34Ssulphate and δ18Osulphate records, derived from the analysis of carbonate-associated sulphate (CAS), were constructed from FFB sections in Belgium and Poland in order to examine the effect of the postulated changes in ocean redox conditions on the global sulphur cycle. The results reveal no overall changes in seawater δ34Ssulphate and δ18Osulphate over the FFB interval suggesting that any changes in sulphur cycling, namely relating to rates of pyrite burial and weathering, were not significant on a global scale. This supports the idea that the Kellwasser events were not oceanic phenomena and that they were restricted to epicontinental seas at low latitudes. The exception to the stability of the FFB δ34Ssulphate record is a brief negative excursion during the mass extinction interval in the linguiformis Zone, albeit based on one datapoint at this level in two different sections. This may be attributable to short, sharp marine regression and an increased flux of light sulphur from weathered pyrite.