Predominant orbital forcing on Asian hydroclimate during the Miocene Climatic Optimum

Abstract The Miocene Climatic Optimum (MCO), spanning from ca. 17 Ma to 14 Ma, marks a pivotal interval in global climate history with elevated greenhouse gas levels and a 3–7 °C increase in global temperatures, disrupting the long-standing Cenozoic cooling trend. However, the dynamics that drove the MCO in the continental realm are not well understood. The driving forces may have conditioned regional scale climate phenomena, such as shifts from an arid to humid environment with a significant temperature rise due to elevated greenhouse gases. Terrestrial outcrops in eastern Asia are particularly valuable for studying the MCO's continental impacts due to their exceptional preservation of orbital-scale sedimentary cycles and sensitivity to monsoonal dynamics, which are pivotal for disentangling hydroclimate-orbital linkages. Notably, the hydrological circulation and orbital influence during the MCO in eastern Asia are still understudied. Employing magneto-cyclostratigraphic chronology, we analyzed samples from a 120-m-thick outcrop section in inland Asia, revealing detailed, orbital-scale, terrestrial responses to the MCO. Based on cyclostratigraphic analysis, our findings emphasize the pronounced influence of two orbital cycles: the dominant 405-k.y. eccentricity and the subtler 173-k.y obliquity band. Based on the magnetic susceptibility, Fe content, and Rb/Sr ratio of rock samples, we concluded that eccentricity and obliquity climate control shifted the summer movement of the Intertropical Convergence Zone in East Asia, impacting northward moisture transport. Significantly, we identified six drought events based on variations in magnetic susceptibility, geochemical proxies (i.e., Fe content and Rb/Sr ratio), and correlation with global phenomena, such as distinct δ13C positive excursions (such as the Monterey Excursion), Antarctic cooling phases (Mi-events), global sea surface temperature changes, and sea-level fluctuations. Our results provide insights into climate variations on both regional and global scales, with implications for past and potential future scenarios.