A summertime rainy season in the Arctic forests of the Eocene

The discovery of exceptionally well-preserved fossil wood revealed that extensive forests existed north of the Arctic Circle during the Eocene (ca. 45-55 Ma). Subsequent paleobotanical studies led researchers to suggest eastern Asia as a modern analog, based on the distribution of nearest living relatives. During the last decade, proxy-based reconstructions of mean annual paleoprecipitation, productivity, and relative humidity have led workers to characterize the climate of the Arctic forests as similar to today's temperate forests of the Pacific Northwest. Using a new model, we reconstructed the seasonal tinting of paleoprecipitation from high-resolution intra-ring carbon isotope measurements of fossil wood. We showed that the Eocene Arctic forests experienced, on average, 3.1 times more precipitation during summer than winter, entirely dissimilar to the Pacific Northwest where summer precipitation is only one-half to one-sixth of the winter precipitation. This new result shows that although mean annual climate conditions may have been similar to the mean annual conditions the Pacific Northwest, consideration of seasonality implies that the temperate forests of eastern Asia represent the best overall modern analog for the Eocene Arctic forests.