Preferential accumulation of selectively-preserved macromolecular substances in the humus peat deposit as seen by analytical pyrolysis and spectroscopic techniques

Abstract

A series of structural features of the three main humic fractions (humic acid, fulvic acid and humin) from different depths of a peat bog deposit in Mazago´n (Huelva, Southern Spain) were isolated and analysed by flash pyrolysis–gas chromatography–mass spectrometry, solid-state 13C-nuclear magnetic resonance and Fourier transformed infrared spectroscopy. Such techniques demonstrate that the various humic fractions were very different not only in terms of molecular weight but also in their composition and structural characteristics, showing well differentiated patterns in the relative distribution of alkyl, O-alkyl and aromatic moieties in each humic fraction. To a large extent, selectively preserved lignin accumulates in the humic acid (HA) fraction, whereas the fulvic acid (FA) consists of a colloidal carbohydrate with a substantial peptidic moiety and the humin includes the noteworthy concentration of insoluble, macromolecular polyalkyl structures. A diagnostic lignin signature in the resolution-enhanced infrared spectra of the HA points to processes of selective preservation of macromolecular substances derived from vascular plants. In general, the humic fractions were not extensive sources of chemotaxonomic descriptors of previously documented biodiversity changes along the period of the peat deposit formation. This is in harmony with a substantial homogenising effect of diagenetic transformations throughout the whole sedimentary record with peat stratigraphy recording mostly the authigenic processes of bog formation and development. Anthropogenic perturbations may be also responsible for the loss of information in the humic proxy resulting in the lack of an apparent relationship between any particular input and the composition of the humic fractions from the peat bog.