Molecular composition of sedimentary humic acids from South West Iberian Peninsula: A multi-proxy approach

The molecular structural features of humic acid (HA) fractions isolated from recent sediments from the estuaries and continental platform along the Huelva littoral (SW Iberian Peninsula, Spain) were studied using complementary analytical tools. The approach included elemental analysis, solid state, 13C cross polarisation magic angle spinning nuclear magnetic resonance spectroscopy (13C CP-MAS NMR), pyrolysis gas chromatography–mass spectrometry (Py-GC/MS) and stable C and N isotopic composition (δ13C, δ15N). The results point to the presence of vascular plant matter contributing to all the HAs, even those far from the coastal area. A contribution of lignocellulose material was detected from 13C NMR signals at 152 ppm (O-aryl C), 55 ppm (methoxyl C) and 33 ppm (alkyl chain) and confirmed by the presence of lignin derived structures (methoxy phenols) in the pyrolysis chromatograms. Nevertheless, the HAs from the westernmost part of the continental platform had the lowest aromatic and the highest C-alkyl abundance (13C NMR spectroscopy). These data, combined with low C/N and high H/C values, a relative enrichment in δ13C and the presence in the pyrolysates of conspicuous n-alkyl nitrile and amide series, together with n-alkane/ene homologues, pointed towards a relevant marine (autochthonous) contribution to this sedimentary organic matter (OM). In contrast, HAs from sediments collected from the Tinto–Odiel River mouth and Guadiana Estuary areas revealed a major input of terrestrial OM. In general, the data support the idea that the HAs still contain valuable information about the signature of aliphatic and aromatic biomacromolecules contributing to the deposited OM.