Testing Late Cretaceous astronomical solutions in a 15 million year astrochronologic record from North America

Astronomically-forced insolation cycles (“Milankovitch cycles”) serve as a primary control on climate, and when preserved in the stratigraphic record, they provide a high-resolution in situ chronometer. These celestial rhythms derive from the fundamental frequencies of the Solar System, and Earth's precession constant. The fundamental frequencies of Earth and Mars presently define a secular resonance of 2:1 (2.4 Myr to 1.2 Myr), and are hypothesized to have undergone a transition between 50–100 Ma, due to the chaotic behavior of the Solar System. In this study, a 15-Myr long cyclostratigraphic record from the mid-latitude Western Interior Seaway (WIS; spanning 82–97 Ma) is assembled to quantify eccentricity amplitude modulation (AM) and obliquity AM, which could reveal the 2:1 resonance when present. Ten high-precision radioisotopic dates from the study interval permit a rigorous anchoring of the floating astrochronologies for comparison with theoretical astronomical models. The analysis is complimented by the construction of an astronomically tuned δ ¹³ C _org composite for the early Cenomanian-early Campanian, which allows a new assessment of linkages between million-year scale astronomical forcing and perturbations of the Late Cretaceous carbon cycle. Evaluation of the La2004 model reveals a switch of eccentricity modulation from 2.4-Myr cycles to a 1.2-Myr cycle from 90 to 86.5 Ma, followed by a return to a 2.4 Myr cycle in younger strata, compatible with observations from the WIS composite. These two resonance transitions are absent in the La2010d and La2011 models. Among 11 solutions calculated by Zeebe (2017), some show resonance transitions during this period, but none of them match the geological record with respect to the observed long eccentricity modulation. The results of this study also suggest that positive carbon isotope excursions are closely related to obliquity power minima, but this interpretation will require further analysis for full confirmation.