Comparison of two methods for the analysis of lignin in marine sediments: CuO oxidation versus tetramethylammonium hydroxide (TMAH) thermochemolysis

A comparison was made between tetramethylammonium hydroxide (TMAH) thermochemolysis and CuO oxidation methods for lignin characterization as a means of assessing terrestrial input to marine sediments. Surface marine sediments from the Louisiana shelf just outside the outflow of the Mississippi River were analyzed. The overall lignin yields and the compound ratios used as plant source proxies were found to be considerably different depending on the method used. The CuO technique afforded higher amounts of lignin phenols and in a simpler compound distribution. Additionally, comparison of the molecular source indicators using the liberated syringyl, cinnamyl and vanillyl phenols showed that the two techniques suggest different plant types contributing to the sediments; however, both techniques exhibited similar trends in oxidation state proxies. This source discrepancy is a function of the different chemical mechanisms of lignin chemolysis, with extractable tannins, polyphenols and cinnamyl-based products being emphasized by the TMAH method. Also, source comparison is hampered by a poorly developed database of plant source-to-lignin chemistry relationship for the TMAH method. One important benefit of the CuO technique is that it allows analysis of up to 5 times the sediment mass compared to the TMAH method, which is an important consideration for sediments with low terrestrial organic carbon content where lignin yield may be near detection limits. In general, the CuO technique proved more suited for lignin-based plant source assessment than the conventional (i.e., not 13C-labeled) TMAH thermochemolysis method for those marine sediments containing low plant input.