Résumé
Rare-earth elements (REE) and Th concentrations on individual Devonian conodonts were measured by ICP-MS from stratigraphic sections across the Frasnian/Famennian (F/F) boundary in the Montagne Noire (Southern France) and Morocco. The present data confirm the reversed concavity of the REE spectra found by Wright et al. (1986) which is a characteristic although poorly understood feature of the ancient oceanic sediments. Within the layers corresponding to the Upper Kellwasser and the F/F events, the Ce/Ce (super ) ratios show only minor changes with respect to younger and older sediments. The magnitude of the Ce anomaly is nevertheless consistently weak over all the sections. If, as in modern oceans, the biologically mediated Ce anomaly in particles and seawater originates in the uppermost water column, the present observations suggest a global decrease in the biological productivity. Lower productivity is supported by the decreasing abundance of conodont elements during the Upper Kellwasser, but continuing carbonate deposition through this period still makes evidence quite equivoqual. As a proxy to Th/Pa fractionation in modern sediments, the Th/La fractionation is a potential tracer of the rate of horizontal exchange of seawater between deep-sea and continental margins, which may be correlated with eustatic sea-level fluctuations. During the Upper Kellwasser crisis, the Th/La ratios fall significantly in all the sections, this can reflect reduced exchanges between the deep-sea and continental margins. It is suggested that the F/F extinction event was the result of the coincidence between a rather modest transgression event and global suboxia. Abstract Copyright (1996) Elsevier, B.V.