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

Extending climatic series back to the first millennium using isotopic tree-ring chronologies in northern regions represents a challenge because trees seldom reach the adequate age. Fossil trees deposited in boreal lakes could serve such an endeavor provided that dating the tree-ring series is possible and that the isotopic ratios of sub-fossil wood are preserved. From two boreal lakes in north-eastern Canada, we collected segments of Picea mariana (black spruce) stems with different degrees of wood textural preservation and covering the last millennium. Our main objectives are to assess the selection criteria for wood textures suitable for isotopic reconstruction, and verify the reliability of sub-fossil cellulose carbon and oxygen isotopic series for climatic reconstruction in boreal regions. The isotopic differences between cellulose and lignin obtained for 48 ring pairs of a living tree (Δδ13Cc–l = 3.7 ± 0.3‰; Δδ18Oc–l = 13 ± 1) and 46 contemporaneous pairs from a sub-fossil stem (3.5 ± 0.3 and 12.9 ± 0.9‰ in the same order), are systematic and matching. For the selection of all sub-fossil samples, we have visually identified three main classes of wood textures for which the degree of alteration is confirmed by secondary electron microscopy: well preserved, slightly altered and highly altered. Slightly altered stem segments have cellulose proportion showing a relative decrease reaching 35%, but Δδ13Cc–l and Δδ18Oc–l values within the range of living trees. Non-altered sub-fossil stems covering the 14th and 11th centuries show systematic and coherent Δδ13Cc–l (3.5 ± 0.2; 3.8 ± 0.4‰) and Δδ18Oc–l results (13.9 ± 0.7; 13.5 ± 0.5‰). Highly altered wood shows a decrease in both cellulose proportion and δ18Ocellulose values, but apparently preserved δ13C ratios. This research shows that it is possible to visually identify the degree of wood preservation and preselect sub-fossil segments holding reliable isotope-ratios, and to use sub-fossil stems collected from boreal lake floors to reconstruct climate over the last millennia.