Absence of oxygen isotope fractionation/exchange of (hemi-) cellulose derived sugars during litter decomposition

• Published in: Organic geochemistry 2012, Vol.42 (12), p.1470-1475
• Main Authors: Zech, Michael, Werner, Roland A., Juchelka, Dieter, Kalbitz, Karsten, Buggle, Björn, Glaser, Bruno
• Format: Article
• Language: English
• Subjects: arabinose; cellulose; climate; Fagus; growth rings; hemicellulose; hygroscopicity; isotope fractionation; isotopes; lacustrine sediments; loess; mass media; mass spectrometry; organic matter; oxygen; Picea; soil; xylose
• ISSN: 0146-6380; 1873-5290
• DOI: 10.1016/j.orggeochem.2011.06.006

► In this study we investigate the δ 18O signatures of leaf litter during degradation. ► δ 18O results of bulk litter reveal systematic 18O depletion. ► δ 18O results are also obtained for hemicellulose-derived sugars using GC-Py-IRMS. ► Hemicellulose-derived sugars reveal no systematic δ 18O trends. ► Oxygen exchange reactions/fractionation can be excluded for (hemi-) cellulose sugars. Aiming at developing a novel tool for palaeoclimate research, we recently proposed a new method for determining the oxygen isotope composition of monosaccharides (Zech, M., Glaser, B., 2009. Compound-specific δ 18O analyses of neutral sugars in soils using GC-Py-IRMS: problems, possible solutions and a first application. Rapid Communications in Mass Spectrometry 23, 3522–3532). In order to answer the question whether isotope fractionation and oxygen exchange reactions during litter decomposition affect the climatically controlled δ 18O values of plant derived sugars, such as for instance xylose and arabinose from hemicelluloses, we studied the compound specific δ 18O values of five different litter species having been decomposed in a field litterbag experiment for 27 months. While δ 18O values of bulk organic matter yielded a systematic 18O depletion of 3.3‰ (pine) to 4.6‰ (spruce) accompanying total cellulose decomposition of 51% (beech) to 86% (ash), δ 18O values of individual sugars show no significant trend with time. In addition to the observed absence of isotope fractionation during decomposition, we also rule out 18O exchange reactions affecting the δ 18O signature of sugar molecules during diagenetic processes as well as during analytical procedure based on experimental findings and on theoretical biochemical mechanistic considerations. We conclude that our new method may become an analytical tool that elegantly overcomes extraction, purity and hygroscopicity problems of previous 18O cellulose methods. It furthermore has the potential to be applied to a wide range of climate archives such as tree rings, lacustrine sediments and loess palaeosol sequences.