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

Abstract

Abstract. 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, we used seven chemical destaining reagents to treat samples of subfossil black spruce (Picea mariana (Mill.) B.S.P.) trees from two lakes in the eastern Canadian boreal forest. We subsequently compared latewood BI (LBI) and delta BI (DBI) time series along with conventional maximum latewood density (MXD) measured from the stained and destained samples. Results showed that the stain of our samples is most likely caused by postsampling oxidation of dissolved ferrous iron in lake sediments that penetrated into wood. Three reagents (ascorbic acid, sodium ascorbate, and sodium dithionite all mixed with ethylenediaminetetraacetic 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 temperature reconstruction due to incomplete destaining. On the contrary, the simple mathematical delta correction, DBI, was unaffected by the Fe stain and showed very similar results compared to MXD data (r>0.82) from annual to centennial timescales over the past ∼360 years. This study underlines the difficulty of completely destaining lake subfossil samples while confirming the robustness of the DBI approach. DBI data measured from stained LSTs can be used to perform robust millennial temperature reconstructions.

Disentangling the Evidence of Milankovitch Forcing From Tree-Ring and Sedimentary Records

“…The question then arises as to whether some of the procedures can be taken as artificial factors generating the difference between the mid-Holocene temperature reconstructions from BI and MXD data. Indeed, in addition to pitfalls in MXD data, the BI records also have been shown to contain potential biases due to taphonomy (i.e., factors related mostly to post-mortem conditions in the sedimentary environment) that may then lead to offsets in BI-based temperature estimates between living and subfossil trees (Wang et al, 2020). Here, we used no living trees for the production of the BI data, thus avoiding this principal divergence.…”

“…Here, we used no living trees for the production of the BI data, thus avoiding this principal divergence. However, the question may remain whether the longer preservation of geologically older subfossil trees in waterlogged conditions could gradually intensify similar effects for BI values from taphonomic processes, such as those suggested to have been caused by dissolved iron penetrating into the wood preserved in sedimentary archives (Wang et al, 2020), and thus potentially leading to at least partly non-climatic bias in temperature estimates over the Northgrippian and Meghalayan stages. We note that this hypothesis is not supported by the fact that the temperature decline we obtained was not stable over the full study period but experienced changes with a stronger slope over the Common Era, in comparison with earlier periods (Figure 4F), and we find no reason to suggest stepwise penetration of dissolved components into the wood over millennial scales.…”

“…Additional hardships of deriving multimillennial climatic trends are represented by diversifying methodologies developed recently to adjust the latewood BI data by earlywood BI values (Björklund et al, 2014;Björklund et al, 2015). While the adjustment has since been shown to remove discoloration bias due to subaerial versus sedimentary origin (Wang et al, 2020), the applicability of the method to operate on millennial timescales has not, to our knowledge, been rigorously tested. Contrary to the well-known sensitivity of latewood density to summer temperatures, the density of earlywood seems to reflect spring temperatures (Helama et al, 2012).…”

Tropical volcanoes synchronize eastern Canada with Northern Hemisphere millennial temperature variability

“…To ensure data homogeneity 18 , 47 , we sampled living trees from the lakeshore forests of corresponding lakes where subfossils were collected. New MXD data from these sites were measured from 1–2 radii of each sample using the X-ray densitometric technique (see Wang et al 48 for details). The dating of millennial chronologies at these three sites was validated using a subfossil wood sample showing a globally coherent cosmogenic 14 C signature at 774 CE 49 .…”

A 2233-year tree-ring chronology of subarctic black spruce (Picea mariana): growth forms response to long-term climate change

“…Light rings are found in trees and stunted individuals, and their formation process is similar in dry sites (woodlands) and in wetlands (Arseneault and Payette 1998). While wood pieces buried in peat or in lake sediments have a darker color than living trees (Wilson et al 2019) due to oxidation of dissolved ferrous iron (Wang et al 2020), light rings can nevertheless be easily detected in buried trees and shrubs.…”