On the origin of precession and obliquity cycles within the Mesoproterozoic Hakatai Shale (Grand Canyon)

Astronomical insolation curves calculated for the top of Earth’s atmosphere show that the variations in (summer) insolation received at low and intermediate latitudes are dominated by (eccentricity-modulated) precession, while obliquity becomes important only at high latitudes. Nonetheless, empirical and modelling studies have shown that obliquity can exert a significant control on lower-latitude (paleo)climate, via its influence on cross-equatorial/meridional temperature gradients or the equatorward transfer of high-latitude (e.g. glacial) signals. Here we explore the origin of regular mudstone-carbonate alternations within the Mesoproterozoic Hakatai Shale of the Grand Canyon, whose cyclostratigraphy points to the combined influence of climatic precession and obliquity forcing on an ancient sabkha-playa system that was situated at subtropical paleolatitudes. We also present new age constraints for the Hakatai based on CA-ID-TIMS U-Pb zircon dating. The results of lithofacies, major and trace element analysis and a lateral stratigraphic correlation of the patterns across 65 km (from Tapeats Creek to Red Canyon) reveal a stronger contribution of obliquity relative to precession at the more landward (continental) vs shoreward sites. We hypothesize that this change in obliquity power over a relatively short distance is explained by a stronger sensitivity (and nonlinear response) to obliquity-paced sea level variations, which determined the supply of marine alkalinity to the coastal mudflat and the formation of carbonate-rich beds in addition to precession, influencing regional paleohydrology, in situ carbonate production/precipitation and storm supply. Variations in high-latitude (continental) ice volume and low-latitude monsoonal circulation may thus both have been operative during the early assembly phase of Rodinia in response to astronomical-induced insolation changes.