Models of peat accumulation assume that peat decomposition occurs mostly above the water table, with little or no decomposition once it enters the deeper, saturated, anoxic zone. Few studies have used molecular biomarkers for tracing post depositional, decomposition-related trends in peat deposits. We studied the major diagenetic changes in the phenolic constituents within a Sphagnum-dominated ombrotrophic bog deposit. The yield of lignin-derived phenols and degree of decomposition, measured using alkaline cupric oxide oxidation of bulk peat samples and their corresponding humic acids, revealed that most of the degradation takes place in the surface layers corresponding to the acrotelm. In fact, total phenolic constituents of peat samples decrease from 36.1 to 21.6 mg g−1 OC (organic carbon) over the first 36 cm, whereas in the deeper anoxic layers, phenolic constituents tend to accumulate, reaching a highest concentration of 71.0 mg g−1 OC. The diagenetic alteration of these phenolic constituents during peat accumulation involves significant demethoxylation and an increasing yield of vanillyl oxidation products despite the low redox potential. Syringyl phenols tend to be particularly resistant to diagenetic alteration and are significantly enriched within the humic acid fraction (∼twofold) with respect to the bulk peat. This, together with the higher degree of oxidation, suggests that this organic matter fraction is enriched in more resistant, but nonetheless diagenetically altered, phenolic constituents. This suggests that humic acids constitute a refractory pool of organic C with a relatively low turnover rate. Our results confirm that the major processes involved in the variation in phenolic constituents with depth are strongly related to the post depositional environment and that evaluation of diagenetic trends in phenolic constituents may provide molecular-level information on the changes that fresh biomass undergoes during early diagenesis in peatlands.
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