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. The stain of wood samples from lake subfossil trees(LSTs) is challenging the wide application of the blue intensity (BI)technique for millennial dendroclimatic reconstructions. In this study, weused seven chemical destaining reagents to treat samples of subfossil blackspruce (Picea mariana (Mill.) B.S.P.) trees from two lakes in the eastern Canadian borealforest. We subsequently compared latewood BI (LBI) and delta BI (DBI) timeseries along with conventional maximum latewood density (MXD) measured fromthe stained and destained samples. Results showed that the stain of oursamples is most likely caused by postsampling oxidation of dissolvedferrous iron in lake sediments that penetrated into wood. Three reagents(ascorbic acid, sodium ascorbate, and sodium dithionite all mixed withethylenediaminetetraacetic acid) could remove >90 % of Fe.However, even for the best chemical protocol, a discrepancy of about +2 ∘ C compared to MXD data remained in the LBI-based temperaturereconstruction due to incomplete destaining. On the contrary, the simplemathematical delta correction, DBI, was unaffected by the Fe stain and showedvery similar results compared to MXD data ( r>0.82 ) from annualto centennial timescales over the past ∼360  years. This studyunderlines the difficulty of completely destaining lake subfossil samples while confirming the robustness of the DBI approach. DBI data measured fromstained LSTs can be used to perform robust millennial temperaturereconstructions.

Chemical destaining and the delta correction for blue intensity measurements of stained lake subfossil trees