Major sulfur cycle perturbations in the Panthalassic Ocean across the Pliensbachian-Toarcian boundary and the Toarcian Oceanic Anoxic Event

Abstract

The early Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) was characterized by marine deoxygenation and the burial of organic-rich sediments at numerous localities worldwide. However, the extent of marine anoxia and its impact on the sulfur cycle during the T-OAE is currently poorly understood. Here, stable sulfur isotopes of reduced metal-bound sulfur (δ 34 S pyrite ) and pyrite sulfur concentrations (S PY ) have been analyzed across the Pliensbachian-Toarcian boundary (Pl-To) and the T-OAE from the Sakahogi and Sakuraguchi-dani sections (Japan), which were deposited in the deep and shallow Panthalassic Ocean, respectively. Our data reveal marked positive δ 34 S pyrite excursions of >10‰ across both the Pl-To and the T-OAE at Sakahogi, coincident with increases in S PY , and a positive excursion of >20‰ at the onset of the T-OAE at Sakuraguchi-dani. Whilst the development of deep-water anoxic/euxinic conditions could have resulted in an enhanced burial of pyrite, and also partly contributed to the positive excursion of δ 34 S pyrite , variations in δ 34 S pyrite at Sakahogi were most likely controlled by elevated export production and/or preservation. On the shallow shelf generally low and highly variable S PY and the positive shift in δ 34 S pyrite were likely attributable mainly to elevated sedimentation rates, with redox playing only a minor role in controlling pyrite abundance. Our discovery of a positive δ 34 S pyrite excursion across the Pl-To at Sakahogi indicates a hitherto unrecognized perturbation to the deep-water sulfur cycle, potentially associated with increased seafloor organic matter flux and pyrite burial at this time, consistent with a transient interval of anoxia. • First pyrite S-isotope (δ 34 S pyrite ) data across the Toarcian OAE (T-OAE) • Positive pyrite S-isotope excursions across T-OAE from deep and shallow Panthalassa • Excursions mainly attributable to local depositional conditions. • Deep-water sulfur isotope excursion across Pliensbachian-Toarcian boundary