A global perturbation to the sulfur cycle during the Toarcian Oceanic Anoxic Event

The Mesozoic Era was punctuated by intervals of widespread anoxia within the ocean, termed oceanic anoxic events or OAEs. The chemostratigraphy of these intervals also contains evidence of transient perturbations to many biogeochemically important elemental cycles. Here we present high-resolution sulfur isotope data from three stratigraphic sections spanning the Toarcian Oceanic Anoxic Event (T-OAE) of the Early Jurassic. All sections show a similar increase in the sulfur isotope ratio of sulfate parallel to an overall positive excursion in carbon isotopes during the OAE interval. Based on forward box modeling, the sulfate-S isotope excursion can be generated by transiently increasing the burial rate of pyrite in marine sediments likely deposited under euxinic (i.e., anoxic and sulfidic) conditions in the water column. In addition, modeling shows that prolonged recovery of the δ34S of seawater sulfate—at least 8 Ma after the initial rise associated with the OAE—was due to the relatively long residence time of sulfate in the Jurassic ocean; estimates from our modeling put the Toarcian marine sulfate concentrations at 4 to 8 mM. The similarity of the sulfur isotope records from the North European epicontinental (or epeiric) sea and Tethyan continental margin suggests that local modification of the marine sulfur isotope signal was minimal: a point explored with isotope mixing models. Importantly, our results indicate that the sulfur isotope excursion reflects a globally significant perturbation in the sulfur cycle and that pyrite burial in the North European Epeiric Seaway alone cannot account for the excursion. This study, along with recent work from other Phanerozoic intervals of widespread marine oxygen deficiency, confirms that the sulfur cycle can be perturbed significantly by enhanced pyrite burial during periods of prolonged oceanic anoxia/euxinia.

► Positive sulfate sulfur isotope excursion in three Toarcian sections.
► Box model shows the excursion caused by enhanced pyrite burial.
► Pyrite burial in European basins alone insufficient to drive excursion.
► Epicontinental sulfate sulfur isotope records likely not affected by riverine input.