Improving the climate signal of tree-ring blue intensity of sub-fossil wood using hydrogen peroxide: An example from the gulf of Alaska, USA

Blue intensity (BI) in tree-rings is a proxy for maximum latewood density (MXD) and has been widely used to reconstruct annual growing-season temperatures. BI records are more easily produced than MXD because their measurement requires fewer resources, however, there remain obstacles with samples that have color differences not related to wood density. Color differences between stained sub-fossil logs and unstained living trees in particular can cause systematic shifts in BI chronologies and thus limit paleoclimate reconstructions. Here we report on a promising treatment for sub-fossil wood destaining using hydrogen peroxide (H₂O₂) on mountain hemlock (Tsuga mertensiana) logs that were overrun by glaciers and preserved in sediments for several centuries from sites along the Gulf of Alaska (GOA). We found that the removal of this taphonomic wood staining is best accomplished using a 24-h bath in 12 % hydrogen peroxide (H₂O₂) at room temperature. The bleaching may have been achieved through the oxidative de-chelation of the Fe species from the Fe-tannate complex which is thought to be the source of wood color stain. BI measurements made before and after bleaching of 60 samples spanning 682 years indicate that the destaining leads to an improved inter-correlation of BI indices. This included a 26 % higher correlation for delta BI, 19 % higher for latewood and 7 % greater for earlywood BI. The standard deviation for delta BI measurement also increased after the treatment (0.071–0.090). We further tested the potential improvement of the climate signal after soaking by comparing an independent temperature reconstruction based on ring-widths through the 1050–1350 CE interval, which showed an improved climate signal for all BI parameters in terms of the variance of temperature explained after soaking. This includes a 19 % higher improvement for latewood BI, 5 % higher for delta BI and 12.5 % higher with the post-treatment earlywood BI. Furthermore, EPS and Rbar chronology statistics were significantly improved and the improvement is relatively insensitive to the standardization used. This work documents how H₂O₂ destaining can improve the development of mountain hemlock BI tree-ring records and that it could potentially help in similar BI chronologies that include stained sub-fossil samples.