Reconstruction of source water using the δ18O of tree ring phenylglucosazone: A potential tool in paleoclimate studies

Abstract

The oxygen isotope ratios of tree ring cellulose have a great potential as proxy for the oxygen isotope ratios of source water, which is related to climate. However, source water isotopic signatures can be masked by plant physiological and biochemical processes during cellulose synthesis. To minimize biochemical effects in the recording of source water, we modified the cellulose molecule to phenylglucosazone, which only has oxygen attached to carbon 3–6 (OC3–6) of the cellulose glucose moieties, thus eliminating the oxygen attached to carbon 2 of the cellulose glucose moieties (OC-2). Here we developed a method to use small amounts of inter and intra-annual tree ring cellulose for phenylglucosazone synthesis. Using this new method we tested if the oxygen isotope ratios of source water reconstructed from tree ring phenylglucosazone (δ18OswPG) and the observed source water (δ18Oswobs) would have a better agreement than those reconstructed from the tree ring cellulose molecule. Annual tree ring samples were obtained from Pinus sylvestris (1997–2003) (Finland) and Picea abies (1971–1992) (Switzerland) and intra-annual tree ring samples were obtained from Pinus radiata (October 2004–March 2006) (New Zealand), each near a meteorological station where precipitation and relative humidity (RH) were measured periodically. The δ18O of tree ring cellulose and tree ring phenylglucosazone for each of the three species were then used to back calculate the δ18O of source water according to a previous published empirical equation. As expected, the δ18O of tree ring phenylglucosazone was superior than cellulose in the reconstruction of source water available to the plant. Deviation between δ18OswPG and δ18Oswobs was in part correlated with variation in atmospheric relative humidity (RH) which was not observed for the cellulose molecule. We conclude that this new method can be applicable to inter and intra-annual tree ring studies and that the use of the tree ring phenylglucosazone will significantly improve the quality of paleoclimate studies.