The expansion of reduced environments during the Ireviken Biogeochemical Event: evidence from the Altajme core drilled in Gotland, Sweden

This study investigates the Ireviken event, which occurs in the Silurian across the Llandovery-Wenlock boundary and is associated with a mass extinction, a perturbation to the global carbon cycle, and more recently, the depletion of oxygen from the global ocean. The depletion of oxygen in the ocean can be estimated by looking at the ratio of sulfur isotopes ³⁴S/³²S (δ³⁴S) in pyrite and carbonate-associated sulfate which will change based on depositional and biological processes. One biological process, known as microbial sulfate reduction, only occurs in oxygen depleted zones and will cause an increase in the ³⁴S/³²S ratio of the sulfur in the ocean because it will preferentially incorporate ³²S into its byproducts which bond with Fe to form pyrite. This pyrite will be buried and removed from the system and result in a positive δ³⁴S excursion which indicates the presence of microbial sulfate reducers and therefore oxygen depleted zones.

This study provides evidence of a δ³⁴S excursion measured in pyrite formed during the Ireviken event and indicates the local expansion of oxygen depleted zones. Near and far locations from the Altajme Core studied here, provide a similar measurement of δ³⁴S in carbonate-associated sulfate, which is thought to represent a more global signal, and indicate a similar expansion of oxygen depletion. This is important because a global expansion of oxygen depletion will allow more organic carbon preservation which is thought to be the cause of the positive excursion in carbon isotopes (¹³C/¹²C) that we see during the Ireviken. Understanding the mechanisms of this process further our understanding the Ireviken event and mass extinction.