On the necessity of organic solvent extraction for carbon isotopic analysis of α-cellulose: implications for environmental reconstructions

Abstract

α-cellulose is widely used as a target substance for isotope ratio analysis in environmental reconstructions. Its preparation includes three basic steps: organic solvent extraction, delignification and alkaline hydrolysis. Recent works have suggested omission of the first step. We have made a detailed comparison in carbon isotope ratio of α-cellulose with or without organic solvent extraction using 32 consecutive tree ring and 30 subfossil peat samples. These samples were exposed to three different chemical treatments: with organic solvent extraction as the first step (CellOE), without organic solvent extraction (CellNOE), and with organic solvent extraction as the final step (CellNOE/OE). The third treatment is used to test if organic extractives can be completely removed or if their solubility in organic solvents has been altered by delignification and alkaline hydrolysis. In tree rings and peat, δ13CC ell NOE was always significantly different from δ13CC ell OE, but the trends were not the same. In tree rings, δ13CC ell NOE was always more negative than δ13CC ell OE by −0.31 ∼ −0.01‰. In contrast, δ13CC ell NOE in peat could be more negative or more positive than δ13CC ell OE by −3.08 ∼ 0.27‰. The third chemical treatment resulted in different patterns. For tree rings, δ13CC ell NOE/OE was still more negative than δ13CC ell OE by −0.36 ∼ −0.08‰. However, the differences between δ13CC ell NOE/OE and δ13CC ell OE for peat varied in a more narrow range from −0.58 to 0.61‰, compared to the differences between δ13CC ell NOE and δ13CC ell OE. These results exposed a complex chemical evolution behaviour and an incomplete removal of lipids during delignification and alkaline hydrolysis. The mean value, long-term trend and seesaw patterns for a tree ring or peat CellNOE series were significantly different from those for a CellOE series, indicating that omission of organic solvent extraction will lead to a biased inference of past environmental conditions.