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Abstract Rainfall amount and intensity are increasing under anthropogenic climate change, but many instrument records span less than a century. The oxygen isotopic composition of tree‐ring cellulose (δ¹⁸O_cell) reflects local source water, climate, and tree physiology. The patterns of δ¹⁸O_cellwithin tree‐rings has the potential to extend pre‐instrument climate records with subannual resolution, but the influences on intra‐ring δ¹⁸O_cellprofiles are unexplored in many settings. In this study, high‐resolution δ¹⁸O_cellprofiles were analyzed on three longleaf pine trees growing in a native savanna in Louisiana, United States. The time series covers a wide range of rainfall conditions from 2001 to 2008 C.E. with a total of 421 δ¹⁸O_cellanalyses. The δ¹⁸O_cellvalues for individual years are well correlated with each other both within and between trees (r = 0.71–0.78). We used principal components analysis andk‐means clustering to differentiate δ¹⁸O_cellprofiles into two groupings: symmetrical δ¹⁸O_cellprofiles versus asymmetrical profiles that have depressed latewood δ¹⁸O_cellvalues. The slope of latewood δ¹⁸O_cellprofiles and mean δ¹⁸O_cellvalues of latewood tissue correlate with total June‐November precipitation. We hypothesize that poorly drained soils in the study area mediate the influence of any individual storm event: in dry years,¹⁸O‐depleted signals from convective storms are moderated by subsequent evaporative enrichment of standing water, whereas in wet years, increased humidity and frequent re‐supply of¹⁸O‐depleted water overrides evaporative enrichment effects, resulting in low δ¹⁸O_cellof latewood. These results suggest that δ¹⁸O_cellproxies for tropical storm occurrence need to account for soil conditions at the site of tree growth. 

Abstract 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 (P_s) than winter (P_w). Seasonal precipitation estimates were calculated using Monte Carlo resampling, resulting in medianP_s = 1,042 (95% C.I. = 628–1,517) andP_w = 235 (95% C.I. = 50–578), which are indistinguishable from the instrument record at a nearby weather station, whereP_s = 977 (95% C.I. = 662–1,434) andP_w = 292 (95% C.I. = 165–515), and from proxy application on two modern trees near the fossil site, whereP_s = 1,058 (95% C.I. = 617–1,558) andP_w = 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.