LOCALLY VARIABLE RESPONSES TO A GLOBAL CARBON-CYCLE DISTURBANCE: ENVIRONMENTAL PERTURBATIONS DURING THE FRASNIAN–FAMENNIAN MASS EXTINCTION (Invited Presentation)

The Frasnian–Famennian (FF) transition during the Late Devonian (~371.9 Ma) was marked by one of the ‘Big Five’ mass extinctions of the Phanerozoic Eon. The ultimate trigger of the extinction remains debated, but was likely linked to profound environmental disturbances prior to and during the event, including abrupt climate changes (in particular, a global cooling pulse), intensified continental weathering rates, perturbed nutrient cycling, and widespread marine anoxia. These phenomena were linked in a complicated cause-and-effect chain that constituted a global-scale climate and carbon-cycle perturbation. This last is recorded as a pronounced positive carbon-isotope (δ¹³C) excursion across the FF boundary in stratigraphic archives of the extinction worldwide. However, whilst this disturbance to the carbon cycle was unquestionably global in nature, it remains unclear to what extent the plethora of environmental perturbations associated with it also occurred globally, or whether they were localized around certain geographical regions.

This study investigates the marine conditions of three sites, the H-32 core (Iowa, USA), the Kowala quarry (Poland), and the Sinsin section (Belgium) using a combination of sedimentary carbon- and nitrogen- (δ¹⁵N) isotope ratios, Rock-Eval information, and organic-biomarker compositions. Importantly, these three sites also feature robust cyclostratigraphic age models, allowing the timing and rates of recorded environmental changes to be compared, both across the studied sites and with two further locales in North America and China that have been previously studied for δ¹⁵N. The results show that whilst most areas experienced environmental degradation during the FF extinction, the exact nature of those changes, and timing of their onset, varied greatly across the globe. Such a geographical disparity in environmental responses is consistent with other major climate crises from Earth’s history, such as the Mesozoic oceanic anoxic events, which were superficially similar in many ways to the FF extinction. Further work is needed to establish the paleoenvironmental controls that influenced how different regions responded to the global climate changes during the extinction, and the impact of these heterogeneous disturbances on the global biosphere at that time.