Conodont Sr/Ca and δ18O record seawater changes at the Frasnian–Famennian boundary
Abstract The oxygen isotopic composition (δ 18 O p ) and strontium/calcium (Sr/Ca p ) ratios have been measured in Late Devonian conodonts ( Palmatolepis sp.) from contemporaneous sections at Coumiac (France) and M'rirt (Morocco). The sequences encompass two anoxic horizons, the Lower Kellwasser (LKW) and Upper Kellwasser (UKW) events with the top of the UKW coinciding with the mass-extinction at the Frasnian–Famennian boundary. The genus Palmatolepis survived the faunal crisis but exhibited plastic responses to the environmental changes, which are recorded in the evolution of shape during the anoxic events. The present study demonstrates that shape, δ 18 O p and Sr/Ca p values of conodonts are correlated in both localities excluding diagenesis as the driving process for the measured δ 18 O p and Sr/Ca p values. The conodonts δ 18 O p and Sr/Ca p values are correlated in both localities and distributed into either a “pre-crisis” and “crisis” group. Using the relationships between the δ 18 O p and Sr/Ca p values, we estimate that the variation of the seawater Sr/Ca ratio during the Frasnian–Famennian mass extinction was of about 20% its initial value. This drastic decrease of the seawater Sr/Ca ratio is discussed in the light of the reef demise that occurred during the faunal crisis.