Tree-ring data, isotope variations

For more than 80 years, tree-ring data have been used to make inferences about past climatic variation. In general, the characteristic most often used has been the variations in widths of the annual growth rings. However, during the past decade other properties, such as cell density (measured by x-ray densiometric techniques), relative widths of early and late wood, and isotopic composition of the cellulose have been used to infer past environmental conditions. It is the isotopic composition that is of interest here. 

On Mey 17, 1971, the Defense Advanced Research Projects Agency [15] funded our proposal that "temperature variations may be evaluated by measuring stable isotope ratios in natural data banks such as tree rings and varves". L. M. Libby had previously calculated [16] the theoretical temperature coefficients of the stable isotope fractionations in manufacture of wood from C02 and H20, finding that the coefficients are small compared with those measured in rain and snow [17]. 

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