Orbitally-forced meridional shifts of the westerlies modulated the hydroclimate of northeast China during the late Eocene

The mid-latitude westerlies are a fundamental component of the global climate system. There has been extensive research about how meridional shifts of the westerlies influenced the wet-dry paleoclimate regimes of midlatitude Asia during the Quaternary. However, how the westerlies operated in much older geological times, such as the late Eocene, particularly on an orbital time scale, is not fully understood. Here, we conducted detailed cyclostratigraphic and palynological analyses on a late Eocene (41.2-37.8 Ma) mudstone-shale sedimentary sequence from the Fushun Basin in northeast China, which uniquely records orbitally-forced meridional shifts of the westerlies. Cyclostratigraphic analyses reveal that the mudstone-shale rhythms exhibit a prominent short eccentricity cycle, with a modest expression of obliquity cycles and a weak expression of precession cycles. The palynological results show that the vegetation experienced frequent alternations between semi-desert and swamp forest, and the climate exhibited periodic oscillations between warm-dry and cold-wet climates. We propose that a combination of eccentricity-modulated low-latitude summer insolation and obliquity-forced high-latitude incipient ice sheet and meridional temperature gradient variations drove cyclical vegetation and hydroclimate fluctuations in northeast China during the late Eocene by regulating the intensity and displacement of the westerlies and the subtropical high. Our results imply that continued poleward movement of the westerlies and the subtropical high will lead to increased aridification along the southern margin of the westerlies under a continuous global warming scenario.