A NEW MODERN ANALOG FOR THE EOCENE ARCTIC FOREST FROM OXYGEN ISOTOPES IN CELLULOSE-DERIVED PHENYLGLUCOSAZONE

Several environmental parameters have been reconstructed for the Arctic forests of the Eocene, including mean annual temperature (MAT), mean annual precipitation (MAP), and relative humidity (RH). Based on these characteristics, a variety of locations have been proposed as modern analogs, including the Pacific Northwest, southeastern U.S., eastern Asia, and Chile. Here we report analyses of oxygen isotopes within a specific compound derived from fossil cellulose, phenylglucosazone (P-G), which lacks the oxygen atom attached to the second carbon of the cellulose-glucose moieties. Exclusion of this highly-exchangeable oxygen atom allows for improved estimates of the oxygen isotope composition of meteoric water (δ¹⁸O_MW) supplied to the forests. Using a large modern dataset, we show that deviation of actual δ¹⁸O_MW from the Dansgaard-predicted value is a function of seasonality, specifically the month-to-month distribution of precipitation. Using MAT estimates for the middle Eocene and values for δ¹⁸O_MW determined from P-G, we calculated that precipitation delivery to the Arctic forests of the Eocene was highly seasonal, with at least 30% of the annual precipitation falling in the wettest month. Our results, together with estimates of MAP and RH, highlight the Korean peninsula within eastern Asia as the best modern analog for the climate that supported these vast forests north of the Arctic Circle.