THE EMERGING RECORD OF LONG-PERIOD MODULATIONS IN CYCLOSTRATIGRAPHY

A significant number of multi-million-year long cyclic stratigraphic sequences have now been assembled from the Phanerozoic sedimentary record. The goal has been to document a continuous record of astronomical forcing of Earth’s paleoclimate through geologic time. Consequently, ever longer periodic variations in paleoclimate proxy data have come into focus. Some of these variations are orbital in origin, intimately associated with the usual astronomical forcing signals as amplitude modulators, increasingly known as “Grand Milankovitch cycles” [1,2]. The most important of these is the modulation arising from the g₂-g₅ orbital interaction of Venus and Jupiter, manifest as the dominant 405-kyr cycle in Earth’s orbital eccentricity. This cycle has been modeled with high certainty for the past 250 Ma, and is the principal tuning cycle used today in the Astronomical Time Scale. Two other principal variations originate from interactions between the orbits of Earth and Mars (g₄-g₃ and s₄-s₃), and appear as 2.4 Myr and 1.2 Myr periodic modulators of the Earth’s eccentricity and obliquity variations, respectively. These periodicities were maintained over much of the Cenozoic Era and reflect 2:1 secular resonance between the Earth and Mars orbits. Prior to ~50 Ma, modeling indicates that the two orbits transitioned through rotational states and experienced 1:1 secular resonance as the result of chaotic diffusion. Compilation of these long-period modulations from Cenozoic-Mesozoic stratigraphy will be examined to assess these emerging data for Earth-Mars orbital interactions, compared with Solar System modeling. References: 1. Olsen, PE, Grand Cycles of the Milankovitch Band, Eos Trans. AGU, 82(47), Fall Meet. Suppl., Abstract U11A-11, p. F2, 2001. 2. Olsen, PE, Implications of the Geological Determination of “Grand Cycles” of the Milankovitch Band for Behavior of the Solar System, GSA Abstracts with Programs, 40(6,) p. 282, 2008.