Isotopes in tree-rings: Development and application of a rapid preparative online equilibration method for the determination of D/H ratios of nonexchangeable hydrogen in tree-ring cellulose

Filot, Marc (2006). Isotopes in tree-rings: Development and application of a rapid preparative online equilibration method for the determination of D/H ratios of nonexchangeable hydrogen in tree-ring cellulose (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The work of this thesis is embedded in the European ISONET project, which aims to reconstruct the climate variability of the European climate of the last 400 years using a stable isotope network from tree-ring cellulose. It was proposed to measure carbon and oxygen isotopes for the whole period and hydrogen isotopes for the last one hundred years.
Within ISONET a rapid on-line equilibration technique for the determination of the deuterium to hydrogen ratio of nonexchangeable hydrogen in cellulose was successfully developed and tested. The new method is base on the rapid equilibration between water vapour hydrogen and the exchangeable hydrogen in cellulose. The therefore constructed equilibration chamber can be easily coupled to widely used high temperature conversion units coupled to a continuous flow isotope ratio mass spectrometer. The values measured with the new equilibration are in excellent agreement with values obtained with the conventional nitration technique, where the exchangeable hydrogen is chemically replaced by a hydrogen free group, namely NO2. The uncertainty of the method is about 4‰ resulting from the standard deviation of the isotopic measurement of equilibrated samples and the uncertainty due to variable exchangeability. Lowering the crystallinity of the cellulose will most probably lead to improved uncertainties due to easier access of sites of hydrogen exchange. The required sample amount could be reduced from 20mg to ideally 0.5mg with the new method. Furthermore a 100-fold process time reduction could be obtained and the use of hazardous chemicals can be avoided. The equilibration was tested for glucose and glucose derivates, but exchangeability proved to be insufficient due to the high crystallinity of the samples. The benefits of the developed continuous flow equilibration chamber led to an application for a provisional patent.
A lower temperature limit for the quantitative high temperature reduction of organic substances for isotope ratio analysis was determined to 1425°C, which is crucial for accurate measurements. At lower conversion temperatures the element of interest is split in different pools, which lead to isotope fractionations for all involved isotope species.
The equilibration method was successfully used to determine the deuterium to hydrogen ratio of the nonexchangeable hydrogen of cellulose from 6 different tree-ring chronologies covering the last one hundred years. Three chronologies stem from the Iberian Peninsula, two from the south alpine region of Switzerland and one from the southern Italy. Furthermore the oxygen isotopic composition was determined for the last 400 years for the Spanish sites using conventional continuous flow high temperature conversion coupled to a stable isotope ratio mass spectrometer.
The correlation between the oxygen isotope chronologies of the Spanish sites is poor, thus indicating that each site records site specific or regionally environmental conditions. This is not further surprising since the sites are very distant. The same is observed for all hydrogen isotope series measured at the Division of Climate and Environmental Physics. The hydrogen and oxygen isotope chronologies show also little common signal, although these isotopes both stem from source water, i.e. precipitation and groundwater.
For the Swiss sites the isotope chronologies were correlated to meteorological parameter of the nearby weather station as well as to the nearest station of the Global Network of Isotopes in Precipitation (GNIP). The correlations clearly show that the recorded isotopic signal depends on more than one environmental parameter. Thus a mechanistic model is applied to study the incorporation of the various environmental signals into tree-ring cellulose.

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