Ecological stability of late Maastrichtian benthic foraminifera amidst Deccan volcanism

The late Maastrichtian witnessed profound disruptions in biogeochemical cycles, leading to the fifth mass extinction at the Cretaceous–Paleogene (K/Pg) boundary. At Bidart section (France), the final ∼60 kyr of the Maastrichtian coincide with mercury (Hg) peaks, low magnetic susceptibility, evidence of biological stress and taphonomic alteration in planktic foraminifera, indicative of an ocean acidification event. While this event primarily appears to be a surface-ocean phenomenon, previous studies also documented a minor rise in benthic foraminiferal test fragmentation beginning 0.5 m below the K/Pg boundary, with a pronounced spike at the boundary itself. A detailed investigation of benthic foraminifera in biozone CF1 at Bidart section (France) reveals a diverse and balanced assemblage preceding the K/Pg boundary, with minimal taphonomic alterations. At the K/Pg boundary, infaunal populations diminished, diversity declined sharply, test fragmentation intensified, yet paradoxically, the absolute abundance of genera rose markedly. Preferential preservation is evident in the dominance of robust taxa ( Cibicidoides spp. , Coryphostoma spp.), while a high fragmentation index reflects strong taphonomic dissolution and time-averaging. A plausible explanation for this could be CO 2 -rich waters mixing into the ocean interior over 100–1000 years, driving dissolution during the ∼10,000-year deposition of the K/Pg boundary red clay. The stark contrast between the planktic and benthic census and morphometric data at Bidart section clearly constrains any Deccan-related calcification stress to the surface mixed layer. Lastly, the integrated planktic and benthic considerations re-emphasize a need to carefully separate taphonomic signals from true ecological stress. • Benthic foraminifera assemblage at Bidart reveal a stable, mesotrophic late Maastrichtian seafloor. • K/Pg boundary at Bidart shows signs of ecological stress and taphonomic dissolution. • Deccan-induced calcification stress was restricted to surface ocean and had minimal impact on benthic foraminifera. • Robust test ratio and fragmentation index together serve as effective taphonomic proxies.