INTERPRETING OXYGEN ISOTOPE RATIOS IN FOSSIL WOOD CELLULOSE

We sampled modern trees in forested regions of North America and the Caribbean to evaluate the strength of relationships among cellulose δ¹⁸O (δ¹⁸O_cel), relative humidity, precipitation δ¹⁸O (δ¹⁸O_ppt) and mean annual temperature (MAT) at the continental scale, and the general range of variability in δ¹⁸O_cel associated with site hydrologic conditions and species differences. Additionally, we measured the δ¹⁸O of cellulose extracted from fossil wood collected at 9 sites in the northern and southern hemispheres as a potential source of information about paleo-δ¹⁸O_ppt and paleo-temperatures. Samples ranged from 45 million to 250 years old, and have δ¹⁸O_cel values within the range of modern values. We 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 burial.

At the oldest localities, MAT estimates derived from the modern relationship between MAT and δ¹⁸O_cel are 6-16°C lower than MAT estimates derived from other biological proxies. Estimates of Pleistocene and Holocene MAT's 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-δ¹⁸O_ppt estimates derived from fossil cellulose and the modern relationship between δ¹⁸O_cel and δ¹⁸O_ppt are within the current annual range of δ¹⁸O_ppt values at all locations. Paleo-δ¹⁸O_ppt determined from δ¹⁸O_cel 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 δ¹⁸O signal may be retained in wood cellulose over millions of years, and that latitudinal patterns in δ¹⁸O_ppt may not have changed much during the past 45 million years.