Deep-water anoxia over the Frasnian-Famennian boundary (La Serre, S France) : a tectonically-induced oceanic anoxic event ?

The Frasnian–Famennian (F‐F) transition is well exposed in the basinal setting of La Serre (Montagne Noire, France), where oxygen‐deficient conditions prevailed in bottom environments. In this paper, we study the F‐F transition using inorganic geochemistry (notably, major elements, redox‐sensitive and/or sulphide‐forming trace metals, and productivity proxies). The results are compared with published data concerning the La Serre section, other F‐F sections and other geological formations also deposited in basinal settings. The very high enrichment in redox and productivity tracers indicates that anoxic conditions prevailed across the F‐F boundary, and that surface productivity was intense. Such deep‐water restricted conditions coupled with high productivity occurred in various places in Late Devonian oceans. Recycling alone cannot account for the intense nutrient supply and therefore the most probable source was via land runoff. We discuss the Eovariscan episode of uplift that stimulated rock weathering, and hence long‐term nutrient supply to the seas.