DEEP-WATER ANOXIA DURING THE FRASNIAN-FAMENNIAN BOUNDARY EVENTS (LA SERRE, FRANCE): AN ECHO OF A TECTONICALLY-INDUCED LATE DEVONIAN OCEANIC ANOXIC EVENT?

The Late Devonian Frasnian-Famennian transition (known for the corresponding mass extinction) is well exposed in the offshore, basinal setting of the La Serre section in the Montagne Noire (France), where oxygen-deficient conditions are known to have prevailed in bottom environments. In this contribution, we study the Frasnian-Famennian transition using inorganic geochemistry (notably, major elements, redox-sensitive and/or sulfide-forming trace metals (U, V, Mo, Cd, Cu, Zn) and productivity (Ba) proxies), and the results are compared with published paleoecological or geochemical data concerning the La Serre section, other Frasnian-Famennian sections and other geological formations. The very-high enrichment in redox or productivity tracers indicates that anoxic conditions prevailed durably across the Frasnian-Famennian boundary, with euxinia possibly reaching higher parts of the water column, and that surface productivity was probably intense. Such deep-water restricted conditions coupled with high surface-water productivity are generalized in various places of Late-Devonian oceans, according to published data. Recycling cannot account alone for the intense nutrient supply required by this high productivity. Therefore, the most probable nutrient supply was the runoff from emerged lands. In this view, two facts played a major role: the Devonian development of vascular plants on lands, and the generalized episode of uplifting that stimulated rock weathering, hence long-term nutrient supply to the seas.