Analysis of delta O-18 in tree rings: Wood-cellulose comparison and methoddependent sensitivity

During recent years, new on-line methods in mass spectrometry have been dev eloped for measuring delta(18)O in organic material. They allow a much high er sample throughput than off-line methods with the result that sample prep aration becomes the time-limiting factor. Therefore we tested whether analy sis of tree ring samples can be made on whole wood instead of pure cellulos e, which until now was commonly used in almost all O-18 tree ring studies. Measurements with an on-line method based on pyrolysis in an elemental anal yzer show that the tree ring delta(18)O time series of wood and cellulose f rom an oak of the Swiss "Mittelland" are similar (r(2) = 0.65). However, th ere are significant differences, and some climatic information may be lost if bulk wood is analyzed instead of cellulose. This can partly be balanced by an increased sample throughput, resulting in the averaging of more data. Further, we improved an off-line method (pyrolysis in a nickel tube follow ed by catalytic CO to CO2 conversion on nickel powder) by adding a CO2 trap to enhance the CO to CO2 conversion. The best reproducibility associated w ith this method is better than 0.1 parts per thousand. We also found a stro ng memory effect linked with this method, causing a dampening of the signal of 30-40%. Therefore published climatic interpretation of delta(18)O data measured using similar methods may require revision.