The oxygen isotope value of whole wood, α-cellulose, and holocellulose in modern and fossil wood

Abstract

The oxygen isotope value (δ18O) of α-cellulose extracted from modern and fossil wood has been used widely to reconstruct climate throughout the Quaternary and deep-time. Substantial effort has been made towards developing more efficient and cost-effective methods for obtaining α-cellulose from whole wood, though some research groups advocate for measuring δ18O directly on whole wood due to a constant offset between the δ18O of whole wood and α-cellulose. The utility of alternative substrates to α-cellulose (e.g., whole wood or holocellulose) for paleoclimate reconstruction using fossil wood, however, has not been evaluated. Here we present 48 new δ18O measurements on mummified (subfossil) wood ranging in age from Eocene to Miocene. Whereas the δ18O value of whole wood and α-cellulose is only weakly related in these subfossils, there is a strong linear correlation between the δ18O value of α-cellulose and holocellulose. We augmented this dataset with 1546 new and published oxygen isotope pairs on whole wood, α-cellulose, and holocellulose from modern wood (Quaternary to present) to compare with our pre-Quaternary fossil samples. In contrast to the fossil wood, we found strong correlations between α-cellulose and whole wood and α-cellulose and holocellulose in modern samples. This finding suggests that all three substrates (whole wood, cellulose and holocellulose) derive oxygen isotopes from a common source (i.e., meteoric water) on a global scale, and that processing of α-cellulose may not be required for modern wood in some settings. Because average holocellulose yields in fossil wood were approximately double that of α-cellulose (22.2 ± 12.8% versus 11.6 ± 10.3%), δ18O analysis on holocellulose rather than α-cellulose should produce reliable environmental data while reducing time and sample material required.