Investigation of age trends in tree-ring stable carbon and oxygen isotopes from northern Fennoscandia over the past millennium

Although tree-ring stable carbon (delta C-13) and oxygen (delta O-18) isotopes are increasingly used for climate reconstructions, it remains unclear whether isotopic ratios from the two chemical elements and different tree species exhibit age-related trends that require removal prior to any paleoclimatic interpretation. Here, we present 2,355 delta C-13 and 2,237 delta O-18 decadal measurements of living and relict Scots pines (Pinus sylvestris L.) from northern Fennoscandia to investigate the occurrence of isotope-specific age trends at both the individual tree and chronology level between 941 and 2010 CE, together with total-ring width and maximum density data. We show that delta C-13 values increase by similar to 0.035 parts per thousand per 10 years of tree age, and therefore require detrending, which is not the case for delta O-18 that only contains minor changes related to age. This study provides independent evidence for the unique paleoclimatic value of stable delta O-18 isotopic ratios from the cellulose of living and relict pine wood to reconstruct high-to low-frequency climate variability. Conversely, caution is advised when information from diverse tree-ring parameters, species and regions is combined in multi-proxy climate reconstructions.