| Summary: | We measured the d18O of cellulose (d18Ocel) extracted from fossil wood collected at 9 sites in the northern and southern hemispheres as a potential source of information about precipitation d18O (d18Oppt) in the past and paleotemperatures. The samples had been buried in fluvial sediments for periods of time ranging from ca. 45 million to 250 years. At the oldest localities (high latitude, Eocene- through Pliocene-age sites in Canada and Russia), mean annual temperature (MAT) esti-mates derived from the modern relationship between MAT and d18Ocel are 6–16!C lower than the MAT estimates derived from other biological proxies. Estimates of Pleistocene and Holocene mean annual temperatures are close to the modern val-ues at those sites. These results are consistent with other recent findings that the MAT/d18Oppt relationship across North America was not constant throughout the Cenozoic. Paleo-d18Oppt estimates derived from fossil cellulose and the modern North American relationship between d18Ocel and d 18Oppt are within the current annual range of d 18Oppt values at all loca-tions. The middle Eocene d18Oppt we determined from arctic cellulose samples (!21.9&) is consistent with river water d18O determined in two other studies (!19.1 & to!22&). These findings provide some evidence that a precipitation d18O signal may be retained in wood cellulose during millions of years of burial, and that latitudinal patterns in d18Oppt may
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