A 600-year-long δ18O record from cellulose of Breonadia salicina trees, South Africa

The oxygen isotope composition in cellulose was analysed along the growth axis of two 600-year-old Breonadia salicina trees (Matumi) from subtropical South Africa, with the aim of testing the isotope variations as a regional climate proxy. The visible tree rings in B. salicina are not annual and therefore radiocarbon analysis was applied to produce an age model for the isotope record, covering the time period between 1375 and 1995 AD. Before ∼1600 AD, a co-variation is evident between the variations in δ18Ocellulose and previously published δ13Ccellulose, indicating that a common factor is responsible for both the carbon and the oxygen isotope signals in the tree cellulose. Between ∼1600 and ∼1900 AD, the correlation between the isotope series weakens, concurrently with drastic changes in growth rates and average ring production rates. Possibly, this phase was a result of extreme changes in the growth environment, affecting the cellulose production rate, the source water δ18O signal and the proportion of different types of oxygen isotope fractionation processes affecting the final isotope composition in the tree cellulose. We suggest that during the lifetime of the two B. salicina trees, different factors have governed the oxygen isotope signal in the cellulose. Both regional climate conditions and site-specific factors associated with the riparian growth environment have had an impact on the oxygen isotope variations in the two trees, clearly complicating the possibilities to utilise a δ18Ocellulose record from this tree species for paleoclimatic reconstruction.