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

Although tree-ring stable carbon (δ13C) and oxygen (δ18O) 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 δ13C and 2,237 δ18O 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 δ13C values increase by ∼0.035‰ per 10 years of tree age, and therefore require detrending, which is not the case for δ18O that only contains minor changes related to age. This study provides independent evidence for the unique paleoclimatic value of stable δ18O 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. [ABSTRACT FROM AUTHOR]