The age and extent of the late Maastrichtian calcification stress at Bidart, France

The final 0.5 m of the Cretaceous-Paleogene (K/Pg) boundary at Bidart (France) is characterized by geochemical, taphonomic, and biotic vestiges of an ocean acidification event linked with Deccan volcanism. The larger (>150 μm) planktic foraminifera morphogroups show varying populations (absolute abundance), with lowered abundances within the Deccan benchmark (final 0.5 m) and a demographic collapse at the K/Pg boundary. Additionally, the Deccan benchmark exhibits diminished test size (p: <0.00001), wall thickness (p: 0.0005), and mixed-layer diversity (p: 0.000967), suggesting the presence of calcification stress. Nevertheless, the benthic environment presents contrasting results, with rare occurrences of size and wall thickness reduction or significant census disturbances in the benchmark, suggesting a lower degree of environmental stress. Notably, a significant increase in the relative proportion of Cibicidoides spp. (~48%; p: <0.00001), Steinsioenia spp. (~11%; p: <0.00001) and Coryphostoma spp. (~9%; p: 0.000033) is recorded at the K/Pg. Near the boundary, there is a significant decrease in the relative abundance (p: 0.001097) and diversity of infaunal benthic foraminifera (p: 0.000035). This decline signifies a collapse in productivity, aligning with a negative excursion in the carbon isotope signature at the K/Pg boundary. These results suggest that the acidification was restricted to the surface ocean and had a limited effect on benthic environment. This is consistent with the lack of extinction within the benthic community at K/Pg. A Graphic correlation of zone CF1 at Bidart with the auxiliary GSSP at Elles (Tunisia) through Hg peaks constrain the Deccan benchmark interval to the final ~58 ky of the late Maastrichtian, culminating in the K/Pg mass extinction.