Publication

The Cretaceous/Paleogene (K/Pg) Chicxulub impact produced globally synchronous enrichments of highly siderophile elements (HSEs) and a pronounced decline in marine osmium isotope ratios (187Os/188Os), forming a key geochemical marker of the boundary. However, precise identification of the K/Pg boundary remains challenging in extensive pelagic regions dominated by deep-sea red clay, where calcareous and/or siliceous microfossils are poorly preserved. Here we identify the K/Pg boundary in a pelagic clay sequence from piston core KR13-02 PC05 recovered near Minamitorishima Island in the western Pacific Ocean using HSE concentrations and Os isotope stratigraphy, supported by ichthyolith biostratigraphy and magnetostratigraphy. A distinct Ir enrichment (up to ∼16 ppb) coincides with the most unradiogenic 187Os/188Os value (∼0.14) at ∼9.95 mbsf, defining a geochemical horizon consistent with the global signature of the Chicxulub impact. Polarity chrons spanning C29n–C30n and ichthyolith assemblages support the placement of the K/Pg boundary and provide a robust chronological framework. Above the boundary, 187Os/188Os ratios remain persistently unradiogenic over a stratigraphic interval corresponding to several hundred thousand years. This prolonged signal reflects the integrated effects of extremely low sedimentation rates, sediment reworking by bioturbation, and background extraterrestrial dust inputs. These results demonstrate that sedimentary Os isotope records in low-accumulation pelagic clay do not necessarily represent a simple archive of contemporaneous seawater composition. Instead, depositional and diagenetic processes can substantially modify post-impact isotopic signals. Our findings highlight the necessity of multi-proxy chronological control and careful geochemical screening when reconstructing oceanic responses to the K/Pg impact on microfossil-poor pelagic environments.