Characterization of modern and waterlogged archaeological cypress (Glyptostrobus pensilis) wood: An analytical pyrolysis (Py-GC-MS and THM-GC-MS) and infrared spectroscopy (FTIR-ATR) study of within tree (radial) and decay-induced compositional variations

Abstract

A fundamental understanding of the composition of archaeological wood stored under oxygen-limited environments, such as waterlogged sediments, is needed to inform preservation conditions of remains and to evaluate the validity of sediment and tree core-derived proxies of past environmental conditions. For this purpose, we determined chemical changes along the radial direction of archaeological (∼2000 yrs old) and modern (growth period 1948 to 2018 CE) wood of the Chinese swamp cypress (Glyptostrobus pensilis), an ethnobotanically important species in subtropical China, using conventional analytical pyrolysis (pyrolysis-gas chromatography-mass spectrometry; Py-GC-MS), pyrolysis with in situ methylation (thermally assisted hydrolysis and methylation; THM-GC-MS), and Fourier-transform infrared (FTIR) spectroscopy. The results showed 1) excellent preservation of the archaeological wood expressed by significant contributions of levoglucosan to the polysaccharides fingerprint after Py-GC-MS; 2) considerable chemical differences between archaeological and modern wood, i.e. archaeological wood samples yield relatively small proportions of lignin and large proportions of polysaccharides and terpenoids; 3) major differences in chemical characteristics of sapwood (more polysaccharides) and heartwood (more lignin). Furthermore, some tested decay proxies show large differences between sap- and heartwood (and are therefore of little value), yet the 4-acetylguaiacol/total guaiacol ratio (4AG/G; Py-GC-MS) of heart- and sapwood are similar, implying that this ratio is a more suitable proxy for decay assessment. Data comparison allowed to identify discrepancies and complementarities of Py-GC-MS, THM-GC-MS and FTIR for the characterization of different wood constituents and their preservation during long-term storage under waterlogged conditions. These results expand our understanding of the chemical composition of decomposing trunks of archaeological wood in general, and of G. pensilis in particular.