P-PINI: A cosmogenic nuclide burial dating method for landscapes undergoing non-steady erosion

Existing methods of cosmogenic nuclide burial dating perform well provided that sediment sources undergo steady rates of erosion and the samples experience continuous exposure to cosmic rays. These premises exert important limitations on the applicability of the methods. And yet, high mountain sediment sources are rife with transient processes, such as non-steady erosion by glacial quarrying and/or landsliding, or temporary cosmic-ray shielding beneath glaciers and/or sediment. As well as breaching the premises of existing burial dating methods, such processes yield samples with low nuclide abundances and variable Al-26/Be-10 ratios that may foil both isochron and simple burial-age solutions. P-PINI (Particle-Pathway Inversion of Nuclide Inventories) is a new dating tool designed for dating the burial of sediments sourced from landscapes characterized by abrupt, nonsteady erosion, discontinuous exposure, and catchments with elevation-dependent Al-26/Be-10 production ratios. P-PINI merges a Monte Carlo simulator with established cosmogenic nuclide production equations to simulate millions of samples (Be-10-Al-26 inventories). The simulated samples are compared statistically with Be-10-Al-26 measured in field samples to define the most probable burial age. Here, we target three published Be-10-Al-26 datasets to demonstrate the versatility of the P-PINI model for dating fluvial and glacial sediments. (1) The first case serves as a robust validation of P-PINI. For the Pulu fluvial gravels (China), we obtain a burial age of 1.27 +/- 0.10 Ma (1 sigma), which accords with the isochron burial age and two independent chronometers reported in Zhao et al. (2016) Quaternary Geochronology 34, 75-80. The second and third cases, however, reveal marked divergence between P-PINI and isochron-derived ages. (2) For the fluvial Nenana Gravel (USA), we obtain a minimum-limiting burial age of 4.5 +/- 0.7 Ma (1s), which is compatible with unroofing of the Alaska Range starting similar to 6 Ma, while calling into question the Early Pleistocene isochron burial age presented in Sortor et al. (2021) Geology 49, 1473-1477. (3) For the Bunten Till (Switzerland), we obtain a limiting burial age of <204 ka (95th percentile range), which conforms with the classical notion of the most extensive glaciation in the northern Alpine Foreland assigned to the Riss glaciation (sensu marine isotope stage 6) contrary to the isochron burial age presented in Dieleman et al. (2022) Geosciences, 12, 39. Discrepancies between P-PINI and the isochron ages are rooted in the challenges posed by the diverse pre-burial Al-26/Be-10 ratios produced under conditions characteristic of high mountain landscapes; i.e., non-steady erosion, discontinuous cosmic-ray exposure, and elevationdependent Al-26/Be-10 production ratios in the source region, which are incompatible with the isochron method, but easily accommodated by the stochastic design of P-PINI.

Automated summary

Existing methods of cosmogenic nuclide burial dating have limitations in high mountain sediment sources due to transient erosion processes and temporary cosmic-ray shielding. P-PINI is a new dating tool that combines a Monte Carlo simulator with cosmogenic nuclide production equations to date sediments sourced from abrupt erosion and discontinuous exposure landscapes. The P-PINI model shows versatility in dating fluvial and glacial sediments, yielding burial ages that differ from isochron-derived ages.