LAUSR

The onset, characteristics, and drivers of paleo‐monsoon conditions in East Asia remain a topic of heated debate. Records from the Eocene suggest pronounced rainfall seasonality consistent with monsoon rainfall across China, likely driven by migrations of the Inter‐Tropical Convergence Zone. Model simulations indicate that modern‐like monsoon circulation of China was established by the early Miocene at the latest, but uncertainty remains due to a paucity of proxy records from the Oligocene. Here, we provide the first annually resolved, quantitative estimates of seasonal precipitation from East Asia during the Oligocene, based upon intra‐annual variation in carbon isotopes across growth rings of exquisitely preserved fossil wood from southern China. We find a clear pattern of consistent, summer‐dominated precipitation with ∼4.5 times more precipitation in summer (Ps) than winter (Pw). Seasonal precipitation estimates were calculated using Monte Carlo resampling, resulting in median Ps = 1,042 (95% C.I. = 628–1,517) and Pw = 235 (95% C.I. = 50–578), which are indistinguishable from the instrument record at a nearby weather station, where Ps = 977 (95% C.I. = 662–1,434) and Pw = 292 (95% C.I. = 165–515), and from proxy application on two modern trees near the fossil site, where Ps = 1,058 (95% C.I. = 617–1,558) and Pw = 188 (95% C.I. = 31–583). These data demonstrate that by the late Oligocene, precipitation patterns in East Asia had similar strength and seasonality to modern conditions, which suggests the presence of an East Asian Monsoon‐style system prior to the Neogene.