WET SUMMERS IN THE EOCENE ARCTIC QUANTIFIED FROM HIGH-RESOLUTION CARBON ISOTOPE MEASUREMENTS ACROSS FOSSIL TREE-RINGS

Discovery of exceptionally well-preserved fossil wood revealed that vast forests existed north of the Arctic Circle during the Eocene (~45-55 Ma). Recent estimates of mean annual paleoprecipitation, productivity, and relative humidity have led workers to characterize these forests as similar to today’s temperate forests of the Pacific Northwest. Above the Arctic Circle, the extreme photoperiod limits photosynthesis to the summer season; freshwater availability during summer months was therefore required to support photosynthetic organisms in both terrestrial and marine realms. However, in the Pacific Northwest, summer precipitation is only one-half to one-sixth of the winter precipitation. Using a novel model for seasonal precipitation derived from high-resolution carbon isotope measurements within tree rings, we show that across the Canadian Arctic the Eocene forests experienced, on average, 3.1 times more precipitation during summer than winter, entirely opposite the distribution of precipitation in the Pacific Northwest. Thus, seasonal precipitation focused in summer months made possible the existence of forests north of the Arctic Circle and also the vast Azolla communities of the Arctic Ocean, which require freshwater to bloom. Our results establish that precipitation in the Eocene Arctic was disproportionally delivered to photosynthetic plants for use during the summer season of continuous light, and we suggest the temperate forests of East Asia are the best modern analog for the Eocene Arctic forests.