D/H COMPOSITION OF N-ALKANES FROM TREE LEAVES: EVALUATION OF ENVIRONMENTAL CONTROLS

D/H composition of terrestrial plant lipids is a new tool that has the potential to provide information about paleohydrological conditions in the Holocene. To assess the robustness of the link between dD values of leaf wax lipids and environmental parameters we analyzed n-alkanes from 150 oak leaves in California and 70 ohia leaves in Hawaii. Samples were collected from trees throughout strong climatic gradients. Here we present data from a suite of oak samples from California and compare our results with preliminary measurements performed on ohia leaves. The nC₂₉-alkane isotopic data from oak samples in California show two major trends. First, samples taken close to the coast are D-enriched relative to those in the Sierra Nevada, averaging – 174‰ and – 185‰, respectively. Second, samples from northwestern California are D-depleted in comparison with those from the southeast, averaging – 195‰ and – 170‰. In order to investigate the source of these differences we compared dD values of nC₂₉ in oaks with dD values of precipitation and other environmental parameters in California, such as the amount of precipitation, temperature, and altitude. Principal Component Analysis shows that D/H composition of precipitation is linked to D/H of nC₂₉ stronger than any other parameter. The dD values of nC₂₉ alkanes along the coast – Sierra Nevada transect show a good correlation with dD of precipitation: R² = 0.54. However, the relationship between these two variables along the northwest – southeast transect is less robust: R² = 0.33. Qualitative assessment of the relationship between dD of nC₂₉ and the relative humidity suggests a negligible effect of this parameter along the coast - Sierra Nevada transect, since the coastal sites with a high relative humidity are characterized by D-enrichment. The importance of relative humidity on the distribution of dD values of nC₂₉ along the northwest – southeast transect, however, is likely to be a major factor, because samples from the relatively wet northwestern California are generally D-depleted in comparison with those in the drier southeast. The results of this study will provide an opportunity to compare the strength of these relationships between physiographically different plant species: continental oaks growing in California and ohia indigenous to an isolated system of Hawaiian Islands.