Chapitre B2b Composition isotopique du strontium à la limite Campanien-Maastrichtien; étude à Tercis les Bains (Landes, France) et comparaison avec d'autres bassins

Summary The characterisation of the Campanian-Maastrichtian deposits of the main section at Tercis les Bains has been undertaken using the 87Sr/86Sr isotopic ratio. This ratio is known to vary through time in the open oceanic system. The gross variation has been documented from deep-sea sediments; the ratio significantly and continuously increases during the Late Cretaceous. Carbonate components from Tercis have been rejected as potential recorders of the initial strontium isotopic ratio due to the recrystallisation processes observed in these components. Instead, phosphate has been selected; it is present all along the succession in the form of fish scales and shards. Carefully selected separates of phosphatic remains from 25 levels were prepared for isotopic analyses. It soon appeared (after measurement of half the hand-picked separates) that the isotopic ratios (between 0.70779 and 0.70786 for the analysed separates) saved no information on the original oceanic compositions but on an isotopic equilibrium of the Tercis limestone with 87Sr-enriched waters. A precisely determined record of the strontium isotopic composition is known from NW Germany and is considered here in connection with a biostratigraphic (macrofossil) record and with an approximately determined mean rate of deposition. Taking as a working hypothesis that the stage boundary in Tercis is approximately contempora-neous with the first occurrence of the species B. lanceolata in NW Europe, and that the depositional rates are correctly determined in both series at Kronsmoor and at Tercis, it is possible to suggest what the Sr isotopic ratios were in the oceanic water at the time of deposition of the Tercis limestone.