Untangling the biotic stress in the late Maastrichtian Deccan-benchmark interval of Bidart (France)

The late Maastrichtian witnessed substantial surges in Deccan volcanism, prompting the hypothesis that these voluminous pulses may have instigated repeated episodes of ocean acidification during this period. The Cretaceous-Palaeogene (K/Pg) boundary at Bidart (France) is preceded by a ∼0.5 m thick interval with geochemical and taphonomic vestiges of an ocean acidification event linked with Deccan volcanism. New planktic foraminifera census and morphometric data now confirm biotic stress conditions related to acidification in the Deccan benchmark interval. The absolute abundance data of larger (>150 μm) heavily calcified planktic morphogroups show fluctuating populations throughout zone CF1 (spanning the final ∼225 ky), lowest peaks within the Deccan benchmark, and a demographic collapse (>90%) at the K/Pg boundary. The analyzed species are generally reduced in size, with thinner test walls in this ∼0.5 m interval, indicating the likelihood of calcification stress as a contributor to the overall biotic stress. At the K/Pg boundary, maximum biotic stress is recorded in all the tested faunal proxies. A preliminary graphic correlation of zone CF1 at Bidart with the auxiliary GSSP at Elles (Tunisia) constrains the Deccan benchmark interval of high biotic stress to the final ∼58 ky of the late Maastrichtian, culminating in the K/Pg mass extinction. The volcanogenic Hg peaks coincident with faunal and taphonomic evidence of ocean acidification strengthen the Deccan-related ocean acidification hypothesis.