The role of mercury biomethylation during end-Devonian and OAE 2 (Cretaceous) biotic perturbations

The Late Devonian and the Middle Cretaceous are crucial periods in Earth's history and especially interesting in terms of macroevolutionary changes in marine vertebrates’ faunas at this time. The Late Devonian (Kellwasser and Hangenberg events) and mid-Cretaceous (OAE 2) events are linked to climate-controlled marine and oceanic anoxic events and biotic turnovers. These intervals are distinguished by unexpected losses in top predators represented by very characterised placoderm fishes and fish-shaped marine reptiles that were lost during these catastrophic events, respectively. During the Hangenberg crisis, they totally extincted all top predatory placoderm fishes, with the largest known predators of the time, such as Dunkleosteus. The Cenomanian-Turonian Mass Extinction is the second-order event of marine extinction and is among the best studied of any mass extinctions. This event is clearly connected with submarine volcanic-controlled climatic warming and the development of anoxic conditions in the oceans. However, one of the more important changes at this time is the total extinction of the fish-shaped or dolphin-shaped marine reptiles, ichthyosaurs, which were nektonic, very mobile, and adapted to cruising long distances, and their physiological adaptation to air breathers makes them more tolerant to oxygenation of the water column. Therefore, it remains a mystery why ichthyosaurs became extinct roughly 28 million years before the end-Cretaceous mass extinction. Both placoderms, as well as ichthyosaurs, were long-lived predators occupying the highest trophic level. Therefore, they could be more sensitive and exposed to toxic metals (such as mercury) bioaccumulation and their biomagnification in the trophic pyramid. Extensive volcanic activity during these periods should deliver huge amounts of highly toxic Hg to aquatic environments. However, the organic form of Hg with one methyl group called methylmercury (MeHg) is more toxic and dangerous to living organisms because it is almost entirely absorbed by the body and flows into the blood, and methylmercury (besides dimethylmercury) is the most toxic form of Hg. The end-Devonian and OAE 2 were characterised by the expansion of anoxic zones in marine environments. In aquatic settings, the main source of methylmercury is biomethylation of Hg by anaerobic microorganisms, such as sulfate-reducing bacteria (SRB). Consequently, the conditions during these periods might have been conducive to the biomethylation of Hg. Methylmercury has received global attention since the poisoning of thousands of people in southern Japan (Minamata) in the mid-1950s. Our aim is to address the lack of knowledge surrounding the occurrence and impact of MeHg on past ecosystems, especially in the context of macroevolutionary drastic changes in aquatic vertebrates during total extinctions, such as the placoderm at the end-Devonian and the ichthyosaurs at the end-Cenomanian. Until now, we have found MeHg in sediments representing the Kellwasser event (Germany, Thuringia) and the Hangenberg event (Poland, Uzbekistan, Austria, and Oklahoma). While in the Cretaceous, we found large mercury spikes (> 1500 ppb) in the Apennines (Italy), which are promising for the search for methylmercury. We collected more samples from these crucial intervals, which are being analysed. 

This project was financially supported by the grant of the National Science Centre in Poland (2023/49/B/ST10/00505).