A Unified Framework for the 100-kyr, 41-kyr, 26-kyr, and 15-kyr Cycles in Earth's Glacial Cycles: A Hypothesis Based on Asymmetric Dual Half-Cycle Synthesis and Lunar Torque Modulation

The recurring ~100-kyr, 41-kyr, 26-kyr, 15-kyr, and millennial-scale cycles observed in Quaternary ice core and deep-sea sediment records have long puzzled paleoclimatologists. This paper proposes a novel unified hypothesis that integrates these cycles into an asymmetric dual half-cycle framework generated by the nonlinear coupling of obliquity (41 kyr) and axial precession (precession). The core idea is that the oscillation of Earth’s obliquity is naturally divided into two unequal half-cycles: an outer-inclination half-cycle (mean → maximum → mean) of approximately 26 kyr, and an inner-inclination half-cycle (mean → minimum → mean) of approximately 15 kyr. Crucially, the so-called 100-kyr glacial cycle is not a direct response to eccentricity but rather a nonlinear beat-frequency synthesis of 2 × 41 kyr + 26 kyr (or 15 kyr), yielding a range of 97–108 kyr, which precisely matches the observed geological record. Furthermore, we argue that the ~1470-year Dansgaard–Oeschger (D-O) events embedded within the 15-kyr short half-cycle, and the ~2900-year and ~4400-year quasi-periodicities within the 26-kyr long half-cycle, are higher-order harmonic responses excited by these asymmetric half-cycles—likely modulated physically by long-term variations in lunar orbital torque affecting precessional forcing. This hypothesis offers a self-consistent and testable alternative explanation for two enduring puzzles in Milankovitch theory: the “100-kyr problem” and the origin of D-O events. Keywords: obliquity, precession, asymmetric half-cycles, lunar torque modulation, Dansgaard–Oeschger events, eccentricity, 100-kyr cycle, beat-frequency synthesis