Evaluating the integrity of C and O isotopes in sub-fossil wood from boreal lakes

► Cellulose and lignin were extracted from fossil stems sampled in boreal lakes. ► Carbon and oxygen isotopes of preserved and slightly altered wood are reliable. ► Only highly altered wood has cellulose with non-reliable oxygen isotopes. ► Visual pre-selection of fossil wood segments holding reliable isotope ratios is easy. ► Reconstruction of millennial climatic series is now possible for all boreal regions.