Lignin selective preservation

Selective preservation theories have been questioned by several authors (O Brien and Stout, 1978 Nadelhoffer and Fry, 1988 Melillo et al., 1989) based on the fact that 813C values generally increase with depth in soils, whereas lignin and fatty acids... 

In addition to lignin, certain components of the leaf cuticles of vascular plants are also selectively preserved. Cutin, a polyester-like biopolymer that is a major component of modem cuticles, is apparently biodegraded during peatification (Nip et al., 1986 Tegelaar, 1990), but cutan, a minor component of the leaf cuticle, is more readily preserved in peats (Tegelaar et al., 1989). The chemical stmcture of cutan is still subject to debate, but may consist of polymethy-lenic stmctures ester-bonded to cellulose or to an aromatic core (Tegelaar et al., 1989 Nip et al., 1989 McKinney et al., 1996). Whatever its chemical stmcture, cutan is sufficiently resistant to survive peatification. 

On the basis of these studies on woody tissues, it seems that lignin from vascular plants can be selectively preserved compared to biologically degradable polysaccharides when buried. The same can be expected for the lignin in humin from peat the spectra shown in Figure 2 consistently demonstrate this selective preservation with increasing depth. 

A similar selective preservation was observed in peat as discussed earlier where an additional component, lignin, was also preserved selectively. However, the major component of humin from Everglades peat was the paraffinic component that also appeared to be selectively preserved relative to the polysaccharides. It is interesting to note the similarity between the spectra of delignified humin at the 15-16 cm interval in peat (Fig. 5) and that of the algal sapropel from Mangrove Lake at the 272-290 cm interval. The similarity between these two spectra infers that similar structural entities are present in these two depositional environments, and it is probable that the two similar structural components are from a common source, namely, algal and microbial remains. 

In peat, humin is also composed of the three structural entities mentioned above. The anaerobic nature of peat precludes the extensive decomposition that occurs in aerobic soils, and biomolecules are likely to be better preserved. Carbohydrates are major components of humin in near-surface intervals but are decomposed and lost with depth in the peat. Lignin and the paraffinic structures are selectively preserved with depth. When the humin of peat is delignified, the paraffinic structures remain. These components are likely to be derived from nonvascular plant contributors to the peat, namely, algae. 

In an attempt to delineate the degree of preservation of lignin in pre-Tertiary coal, we examined numerous coalified wood samples ranging in age from Carboniferous to Cretaceous. The samples were initially screened by solid-state l C nuclear magnetic resonance to detect the possible presence of methoxyl carbon. Once such carbons were detected, the samples were subjected to analytical pyrolysis to determine the relative yields of methoxyphenols which would provide an indication of the state of preservation of the lignin-derived structu units. We report here on the identification of lignin-derived methoxyphenols in the coalified wood samples selected for analytical pyrolysis. 

Microbiological Degradation. From a microbiological standpoint, selective lignin preservation is to be expected. No microorganisms are known... 

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