Obliquity and precession forcing of the amplitude of millennial-scale East Asian monsoon variability during the late Miocene.

Asian monsoon systems influence billions of people and understanding past monsoon variability and dynamics is instructive in predicting future trajectories. Recent studies have demonstrated that insolation, plus interrelated changes in ice-sheet extent and volume, particularly in the Northern Hemisphere (NH), have controlled the magnitude of millennial-scale East Asian summer monsoon (EASM) variability during the Quaternary. However, it is unclear how the EASM varied at both orbital and millennial timescales, and whether orbital-scale variations impacted millennial-scale variations, during intervals lacking large permanent NH ice sheets at times when CO levels were close to the present-day value of approximately 400 ppm. Here, we present high-resolution (∼1-kyr) dry-wet variation records from late Miocene eolian sediments in the Jianzha Basin of the northeastern Tibetan Plateau, spanning an interval when CO levels were persistently close to the modern level and the NH lacked large permanent ice sheets. Our results reveal orbital-scale forcing interwoven with millennial cycles. In contrast to dominant precession and eccentricity forcing of the EASM at orbital scales, the amplitude variations of millennial-scale EASM cycles exhibit strong obliquity (and its modulating cycle) forcing and weak precession forcing. This pattern is different from the pattern observed in either the Quaternary or early Miocene, which we attribute to the effects of different boundary conditions.