Oxygen Isotope Bias Arising from the Drying Process of Nanocellulose: Implications for Studies Using Subfossil Wood

The oxygen isotopes (δ¹⁸O) from tree rings in living and subfossil wood provide valuable climate records across different periods, yet their climatic interpretation fundamentally depends on the accuracy of the isotopic measurements. While homogenization procedures are commonly employed in laboratories to ensure measurement precision, subfossil wood samples present unique challenges due to diagenetic effects─specifically, the shortening of cellulose chains. These shortened cellulose chains can reach nanoscale dimensions. For such nanocellulose, hornification induced by hydrogen bonds during drying processes may result in incomplete water removal and subsequent δ¹⁸O heterogeneity. Through comparative analysis of oven-dried and freeze-dried samples, we reveal that freeze-drying minimizes hornification, prevents interfiber hydrogen bond formation, and maintains isotopic homogeneity. These findings not only enhance the accuracy of future cellulose δ¹⁸O analyses but also reinforce the reliability of subfossil cellulose δ¹⁸O as a robust proxy for paleoclimate reconstruction. Our study establishes a critical methodological framework for improving isotopic fidelity in cellulose-based climate archives.