Extraction of α-cellulose from mummified wood for stable isotopic analysis

Abstract

Stable isotope ratios of carbon (δ13C) and oxygen (δ18O) from tree ring cellulose can provide valuable paleoclimatic information at annual and subannual resolution, from time periods long before instrumental climate records. Recently, mummified (non-permineralized) wood was discovered within Canadian Subarctic kimberlites, inviting paleoclimatic investigations of the time in which the trees grew. In the mummified wood, polysaccharides (hemicellulose, α-cellulose) are frequently preferentially degraded, in the low-silica anaerobic burial environment of the kimberlites, resulting in a lignin-rich material. However, some samples from the Ekati Panda kimberlite pipe (ca. 53.3Ma) contain remnant cellulose, demonstrating the extraordinary preservation potential of kimberlites. Preservation of α-cellulose is important because it allows for the construction of stable isotopic proxy records of deep-time paleoclimates at annual to subannual resolution. Established α-cellulose extraction methods [i.e., using a 17.5% sodium hydroxide (NaOH) solution] were unsuitable for this material because all holocelluloses were dissolved. Therefore, we tested variants of two cellulose extraction methods [i.e., Brendel et al. (2000) and Jayme-Wise (Leavitt and Danzer 1993, Loader et al. 1997)] to optimize a procedure for extraction of a consistent yield of mummified wood cellulose for stable isotopic analysis. Stable carbon (δ13C) and oxygen (δ18O) isotopes were measured from cellulose from each extraction method, as well as Extractive-Free Wood and unextracted mummified wood. vitrinite reflectance, used to assess thermal alteration of the unextracted material to estimate post-burial temperatures within the kimberlite, suggested low post-burial peak temperatures (Tpeak=60°C). We detected no mineral contaminants (i.e., iron oxides) in the material using Energy-Dispersive X-ray Spectroscopy (EDX). Despite low cellulose yield (<5%), the Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR) spectra of mummified cellulose samples strongly resembled modern α-cellulose. All cellulose treatments were similar in stable isotope values and ATR-FTIR peaks, but significantly different from unextracted wood and Extractive-Free Wood. As, reported by other sources (Anchukaitis et al. 2008, Brookman and Whittaker 2012), use of the Brendel method may cause cellulose acetylation, introducing an FTIR peak near 1720cm−1, thus complicating hemicellulose detection at a peak near 1725cm−1. For this reason, the Jayme-Wise method is recommended for detection of hemicelluloses in mummified wood. If hemicelluloses are not present in holocellulose, due to groundwater hydrolysis, the NaOH step may be omitted or reduced in concentration and still produce α-cellulose.