Continental-scale patterns in modern wood cellulose δ18O:: Implications for interpreting paleo-wood cellulose δ18O

The isotopic composition of ancient wood may be a useful archive of some climatic or geochemical conditions of the past, but presently there are many uncertainties that constrain such interpretations. We sampled naturally growing, predominantly native trees in forested regions of North America and the Caribbean to evaluate the strength of the relationships among cellulose delta O-18 (delta O-18(cel)), relative humidity (RH), precipitation delta O-18 (delta O-18(opp)), and mean annual temperature (MAT) at the continental scale, and the general range of variability in delta O-18(cel),, associated with site hydrologic conditions and species differences. We found up to 470, differences among different species growing at the same site, that conifer cellulose at a site is more enriched than angiosperm cellulose by 1.5 parts per thousand (p < 0.001), and that differences in landscape position, reflecting differing access to the water table, produced differences of < 1 parts per thousand in delta O-18(cel). At the continental scale, delta 18O(cel) was strongly influenced by modeled delta O-18(ppt) (R-2 = 0.80, p < 0.001). Average summer minimum RH (RHmin) combined with delta O-18(ppt) explained more of the variability (R-2 = 0.93, p < 0.001) in delta O-18(cel) across North American and Caribbean forests. MAT and delta O-18(cel) were also strongly correlated across North America (R = 0.91 and 0.95, p < 0.001, for angiosperms and conifers, respectively). The difference between delta O-18(ppt) and delta O-18(cel) is not constant (varying from 35-44 parts per thousand) and is inversely correlated with delta O-18(ppt). The relationships among delta O-18(ppt) RHmin delta O-18(cel) and MAT established for North America and the Caribbean applied reasonably well when delta O-18(cel) was used to estimate MAT and delta O-18(ppt) in Asia, Europe, and South America, but there were important exceptions. The most accurate predictions of MAT and delta O-18(ppt) from delta O-18(cel) require RHmin. Predictions of delta O-18(ppt) and MAT made from delta O-18(cel) alone produced errors of up to 8 parts per thousand and 16 degrees C, respectively. (c) 2008 Elsevier Ltd. All rights reserved.