Absolute Age of the Cretaceous-Paleogene Boundary at the Precessional Scale

The absolute age of the Cretaceous/Paleogene (K/P) boundary has recently undergone a re-evaluation through integration of new chronostratigraphy, high-precision geochronology and astrochronology. Results now point to a boundary age of 65.957 Ma with an uncertainty that could be as low as +/-0.01 myr (1). There are, however, doubts about the astrochonological estimates, which are based on matching sediment cycle modulations to orbital eccentricity on the Paleogene side of the boundary. Low sediment accumulation and/or disturbance immediately above the boundary in the studied sections introduces uncertainty into the matching that could be as large as +/-0.4 myrs. To address this problem, we assessed cycling from the Maastrichtian side of the boundary, where sedimentation rates are generally much higher and clearly resolve precession-scale cyclicity. Comparison of cyclostratigraphic records (carbonate content, magnetic susceptibility, luminosity, gamma log, FMI, Fe concentration) from multiple sites (ODP Legs 191B, 207, 208; DSDP Sites 516F, 525A, 528; Zumaya) to the La2004 precession index (PI) (2) shows a consistent match between the amplitude modulations of the PI and the recorded precession cycles leading up to the boundary, provisionally set to 66 Ma. An unusual set of low-amplitude precession cycles between 66.45-66.35 Ma in the PI can be recognized in all of the examined records. Anchoring analogous low-amplitude recorded cycles to this interval, and counting the succeeding cycles upward results in an age for the K/P boundary that is between 66.05-66.15 Ma, only slightly older than (1). We discuss the uncertainties in our bottom-up estimates, and assess the utility of this proposed precession-based scaling in providing precise correlation horizons for other events (impact, extinctions, magnetic reversals) associated with this famous stratigraphic interval. References: 1. Kuiper et al., 2008, Science, 320:500; 2. Laskar et al., 2004, Astron. Astrophys. 428: 261.