Sulfur isotope evidence for low and fluctuating sulfate levels in the Late Devonian ocean and the potential link with the mass extinction event

Abstract

High amplitude positive carbon isotope excursions in the Late Devonian, the punctata and Kellwasser events, reflect major perturbations in the global carbon cycle that have been attributed to increased continental weathering and subsequent ocean eutrophication. Despite the comparable carbon isotope anomalies, however, a major extinction has been reported only for the Kellwasser Events, while the punctata Event is marked by low extinction intensity. This study presents multiple sulfur isotope records of carbonate-associated sulfate (CAS) and pyrite from Late Devonian sections in the Great Basin, USA, in order to document changes in the coupled (or decoupled) geochemical cycles of carbon and sulfur during the punctata and Upper Kellwasser events. A positive sulfur isotope shift in both CAS and pyrite accompanies the onset of the punctata Event, but to a larger extent in the latter. As a result, the sulfur isotope offset between CAS and pyrite (Δ[superscript 34]S[subscript CAS-py]) dropped to less than 10‰. In the middle of the punctata Event, a sharp negative δ[superscript 34]S[subscript CAS] excursion and negative Δ[superscript 34]S[subscript CAS-py] values coincide with the Alamo impact. Unlike the rapid δ[superscript 34]S[subscript py] and δ[superscript 34]S[subscript CAS] oscillations associated with the punctata Event, the Upper Kellwasser was a period of relative stability, except for a brief δ[superscript 34]S[subscript CAS] drop before the event. Paired sulfur isotope data, aided by a simple box model, suggest that the geochemical cycle of sulfur may have been partly responsible for the contrasting biological responses that define these events. High stratigraphic δ[superscript 34]S[subscript py] and δ[superscript 34]S[subscript CAS] variability, coupled with strong reservoir effect, demonstrates a relatively small oceanic sulfate pool existed during the punctata Event. Further, the Alamo impact likely triggered the rapid oxidation of microbially-produced sulfide within this event. The expansion of sulfidic bottom water thus may have been impeded during the punctata Event. In contrast, the lack of a positive shift in δ[superscript 34]S[subscript CAS] and sizable Δ[superscript 34]S[subscript CAS-py] values (>15‰) throughout the Upper Kellwasser Event imply higher relative sulfate levels. A larger seawater sulfate reservoir may have promoted the development of sulfidic bottom waters in the eutrophic epicontinental seas, increasing biological stress and potentially contributing to the mass extinction.