A Deep-Time Dating Tool for Paleo-Applications Utilizing Obliquity and Precession Cycles: The Role of Dynamical Ellipticity and Tidal Dissipation

Abstract

Pre-Pleistocene age models used in paleoceanography and paleoclimatology often rely on the imprint of astronomically calculated cycles of eccentricity and other solar system frequencies in sedimentary records (e.g., 405, 173, and ∼100 kyr). However, use of obliquity and precession cycles (at present ∼41 and ∼20 kyr) remains challenging for these periods, mostly due to past changes in Earth's dynamical ellipticity (Ed, gravitational shape) and tidal dissipation (Td, slowdown of Earth's rotation), which affect the astronomical calculations. Here, we present a dating method for deep-time records by integrating Ed and Td into astrochronology. The key to our approach is the combination of constraints on Td (and thus indirectly on Ed) with age model optimization based on solar system frequencies, plus tuning to obliquity/precession frequencies, while varying Td and Ed. Importantly, we target deep-time intervals where Td shows significant effects but high-quality sedimentary records are available (early Cenozoic). We include a quickstart guide to our approach and make our code and pre-computed solutions freely available to users. To demonstrate the practical utility of our approach, we apply our tool to two case studies using deep-sea records from the early and middle Eocene. Our results confirm very accurate chronologies of sedimentary records from the early Eocene (∼56–54 Ma) but suggest significant improvement for the middle Eocene (∼40–39 Ma). For the early Eocene, our method provides absolute geologic ages with an estimated uncertainty of ±20–40 kyr, which is smaller than or equal to typical uncertainties from recent radiometric 40Ar/39Ar dating.