Variations in Ce anomalies of conodonts through the Frasman/Famennian boundary of Poland (Kowala Holy Cross Mountains):: implications for the redox state of seawater and biodiversity

Rare-earth element (REE) concentrations on Upper Devonian conodonts, distinguished by very low Color Alteration Index, were measured by inductively coupled plasma mass spectrometry from a stratigraphic section across the Frasnian/Famennian (F/F) boundary in the Holy Cross Mountains (Poland). The present data show a 'hat-shaped' pattern of the REE spectra similarly to other Devonian conodont spectra already documented. However, our sample composition suggests little to moderate alteration of the original oceanic pattern through the adsorption mechanism in a context of 'weak' or 'early' diagenesis. All along the section, the magnitude of the Ce anomaly is always negative, and there is an inverse correlation between the Ce anomaly and the (La/Sm)(SN) ratio. This has been interpreted as an La enrichment due to a terrestrial input. A mathematical procedure was used in order to restore the original Ce anomalies that would reflect the seawater environment not contaminated by terrestrial inputs. No noticeable change is observed in the Frasnian with homogeneous values of Omega(Ce) model but a sharp increase is initiated just below the F/F boundary with maximum values in the lowermost Famennian. The measured (La/Sm)(SN) ratios co-vary with the Omega(Ce) model in agreement with the hypothesis of a terrigenous input. Finally, the inverse correlation between the values of Omega(Ce) model and the relative abundance of Palmatolepis during the Famennian constitutes the first evidence known of a redox control on the presence of a conodont genus that is an indicator of deep-water environment. (C) 2002 Elsevier Science B.V. All rights reserved.