Interpreting oxygen isotope ratios in fossil wood cellulose

The isotopic composition of ancient wood may be a useful archive of past climatic or geochemical conditions, but many uncertainties constrain such interpretations. Some of these uncertainties relate to the preservation of cellulose, while others involve the strength and applicability of empirical relationships linking isotope ratios in cellulose to specific environmental variables. I sampled modern trees in forested regions of North American and the Caribbean to evaluate the strength of relationships among cellulose δ 18O (δ18Ocel), relative humidity, precipitation δ18O(δ18Oppt ) and mean annual temperature (MAT) at the continental scale, and the general range of variability in δ18Ocel associated with site hydrologic conditions and species differences. I measured the δ 18O of cellulose extracted from fossil wood collected at 9 sites in the northern and southern hemispheres as a potential source of information about paleo-δ18Oppt and paleo-temperatures. Samples ranged from ca. 45 million to 250 years old, and have δ 18Ocel values within the range of modern values. I used X-ray diffraction crystallography to compare the crystalline structure of ancient and modern cellulose. Results indicated differences in cellulose crystallinity related to the concentration of NaOH used in the extraction and the presence of contaminant minerals acquired during buried. At the oldest localities, MAT estimates derived from the modern relationship between MAT and δ18Ocel are 6-16°C lower than MAT estimates derived from other biological proxies. Estimates of Pleistocene and Holocene MAT\u27s are close to modern values at those sites. These results are consistent with other findings that the MAT/δ18O ppt relationship across North America was not constant throughout the Cenozoic. Paleo-δ18Oppt estimates derived from fossil cellulose and the modern relationship between δ18 Ocel and δ18Oppt are within the current annual range of δ18Oppt values at all locations. Paleo-δ18Oppt determined from δ18Ocel for Axel Heiberg Island during the Eocene is consistent with values determined from fossil teeth, fish scales and fresh water mollusks for nearby Ellesmere Island in two other studies. These findings provide some evidence that a precipitation δ18 O signal may be retained in wood cellulose over millions of years, and that latitudinal patterns in δ18Oppt may not have changed much during the past 45 million years