A geochemical clock in earliest Paleogene pelagic carbonates based on the impact‐induced Os isotope excursion at the Cretaceous‐Paleogene boundary

Abstract

An impact‐induced osmium (Os) isotope excursion provides a unique means of assessing the completeness of marine Cretaceous‐Paleogene (K‐Pg) boundary sections, and surmounting challenges associated with constraining the time scale of the Earth system recovery from this extreme perturbation. A model of the recovery of seawater187Os/188Os following the impact event allows independent estimates of the time elapsed since the impact, which can be directly compared to time estimates derived from biostratigraphy, magnetostratigraphy and cyclostratigraphy. This approach is tested using data from three deep ocean sites cored by the Ocean Drilling Program (ODP). Data from ODP 1262B (South Atlantic) and ODP 690C (Southern Ocean) display the expected 187Os/188Os minimum very close to the biostratigraphically defined K‐Pg boundary and yield Os‐based accumulation rate estimates similar to those obtained from magnetostratigraphy and orbital tuning. In contrast, the187Os/188Os minimum in ODP 1209C (Western Pacific) occurs ≈9 cm below the K‐Pg boundary. Low Os concentrations throughout the boundary interval and an implausibly rapid recovery to higher, pre‐impact187Os/188Os ratios provide strong evidence for a previously unrecognized gap in the K‐Pg interval of Site 1209. Results presented here provide strong empirical evidence that Os isotope data are uniquely valuable in assessing the completeness and accumulation rates of earliest Paleogene sediments from the deep sea. They are of broad interest because they have implications for astronomical tuning of the geologic time scale and illustrate that whole ocean geochemical perturbations can provide an alternative to biostratigraphy for correlation and timekeeping during abrupt biotic events.