Conodont Sr/Ca and δ18O record seawater changes at the Frasnian–Famennian boundary

• The shape, δ18Op and Sr/Cap compositions have been measured in conodonts
• The conodonts records span the Frasnian-Famennian (Fr-Fm) biotic crisis
• The shape of conodonts respond plastically to both δ18O and Sr/Ca seawater variations
• The Sr/Ca of seawater dropped by 20% at the Fr-Fm boundary
• This fits with erosion of calcitic Fr reefs and expansion of aragonitic Fm reefs.

The oxygen isotopic composition (δ18Op) and strontium/calcium (Sr/Cap) 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, δ18Op and Sr/Cap values of conodonts are correlated in both localities excluding diagenesis as the driving process for the measured δ18Op and Sr/Cap values. The conodonts δ18Op and Sr/Cap values are correlated in both localities and distributed into either a “pre-crisis” and “crisis” group. Using the relationships between the δ18Op and Sr/Cap 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.