Abstract: J. Fryda, B. Frydova (2025)

The Silurian Period was characterized by repeated extinctions, carbon cycle fluctuations, and significant climatic shifts. Among the most notable events was the Homerian double-peaked carbon isotope excursion associated with the Mulde and Lundgreni extinctions, both linked to a period of global cooling. The Lundgreni Event was marked by a significant decline in graptolite diversity, while the Mulde Event was characterized by a reduction in conodont diversity. In this study, we present new data that characterize marine palaeoredox conditions in the Prague Basin, peri-Gondwanan terrane, based on high-resolution sampling of a section representing an offshore setting. Our data are derived from the Kosov section, which is known for the most detailed quantitative biostratigraphical record currently available from peri-Gondwana. High-resolution geochemical sampling of graptolite shales from the Cyrtograptus lundgreni Biozone to the Colonograptus ludensis-gerhardi Biozone, provides clear evidence of abrupt changes in marine palaeoredox conditions. The redox-sensitive trace metal data demonstrate that the stratigraphical interval corresponding to the upper Cyrtograptus lundgreni Biozone to the end of Colonograptus praedeubeli-deubeli Biozone represents a period of substantial oxygenation in offshore settings. A comparison of high-resolution chemostratigraphical and biostratigraphical records reveals that this increased oxygenation of offshore environments, which significantly reduced the habitable areas for graptolites, played a key role in the decline of graptolite biodiversity during the Lundgreni Event. This oxygenation correlates with a global sea-level drop, likely triggered by glaciation, and also with the Homerian double-peaked delta 13C anomaly. In conclusion, the analysis of new high-resolution chemostratigraphical records across the Homerian Lundgreni Event, combined with a high-resolution quantitative graptolite biostratigraphical record from the same section, provides insights into the potential causes of this global marine ecosystem crisis and fills an important gap in our current global dataset for this interval of the Silurian.

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High-resolution records of the mid-Homerian (Silurian) marine chemistry evolution and graptolite biodiversity across the Lundgreni Event reveal what nearly killed the graptolites