Interpreting paleoclimate from l8 O and 2 H in plant cellulose: comparison with evidence from fossil insects and relict permafrost in southwestern Ontario

• Published in: Canadian Journal of Earth Sciences
• Main Authors: Edwards, T. W. D., Aravena, R. O., Fritz, P., Morgan, A. V.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1985
• Subjects: General Earth and Planetary Sciences; permafrost
• Online Access: http://dx.doi.org/10.1139/e85-180; http://www.nrcresearchpress.com/doi/pdf/10.1139/e85-180

The oxygen-isotope content of terrestrial plant cellulose is related to that of the source waters by a two-step process involving evapotranspirative leaf-water enrichment and equilibrium isotopic exchange between leaf water and atmospheric carbon dioxide. A combination of two models that describe these steps yields consistent agreement between measured and predicted climatic and isotopic data. Humidity is the dominant influence on variations in the cellulose 18 O enrichment relative to the source water. A good first-order approximation of the average daytime relative humidity during the growth season at a site can be based on the linear correlation that exists between humidity and cellulose enrichment, without explicit consideration of factors such as temperature, δ 18 O of atmospheric vapour, and leaf boundary-layer dynamics.The value of the combined model for paleoclimatic reconstruction has been tested using fossil wood from a late glacial site at Brampton, Ontario. Estimates of the past relative humidity were derived from the divergence between measured δ 18 O values of fossil wood cellulose and environmental water isotopic compositions inferred from the carbon-bound deuterium contents of the cellulose. Growing conditions were apparently substantially drier than those at present between about 11 500 and 8700 years BP, at a time when coniferous forests predominated in southwestern Ontario. A shift in the inferred meteoric water composition over this period suggests a gradual increase in mean annual temperature of about 2 or 3 °C, in agreement with estimates of temperature change based on paleoentomological data.