PYROLYSIS OF WOOD SPECIMENS WITH AND WITHOUT MINERALS - IMPLICATIONS FOR LIGNIN DIAGENESIS

In order to evaluate lignins as geochemical tracers of paleovegetation, thermal alterations of lignins and the yields of their phenolic residues were investigated by laboratory pyrolysis experiments. Gymnosperm and angiosperm woods as well as humic acids from peats were pyrolyzed at 200-degrees-C with and without clay and other minerals. The pyrolyzed samples were oxidized with alkaline CuO to release phenolic compounds which were derivatized to their silyl ethers and quantitated by gas chromatography (GC) and GC/mass spectrometry. The yields of lignin phenolic residues after thermal degradation followed roughly first-order kinetics. Except for the accelerated degradation by acidic montmorillonite, the kinetic plots with other minerals examined were almost superimposable on the plots for the pyrolysis of wood without minerals suggesting their inability to catalyze lignin degradation. Simple aromatic C-O bond cleavage resulting in demethoxylation of lignin phenolic residues seems unlikely. The first step of the multistep reactions, involving O-C bond cleavage leading to demethylation, is probably a dominant process, after which dehydroxylation results in demethoxylation. The ratio of syringly to vanillyl (S/V) phenolic residues was thermally modified. However, the ratio remains constant at later stages of pyrolysis, indicating that the S/V ratio can, to a certain extent, be used as a qualitative index of paleovegetation and paleoclimatic indicator as well. The bonding at alpha-carbon in side chain is thermally unstable, showing an increase in phenolic acid to aldehyde ratio with pyrolysis time. This implies that acid/aldehyde (ACD/ALD) ratios in geological specimen present an integrated signal of microbial as well as thermal effects. The data obtained in the present study help define the changes in the lignin compositional trends that characterize the early stages of coalification.