Relationships between environmental conditions and the diagenetic evolution of organic matter derived from higher plants in a modern mangrove swamp system (Guadeloupe, French West Indies)

The sedimentary organic matter content of a series of 2-m-deep cores was investigated along a sea-land transect at five sites representing five species of mangrove and salt-marsh vegetation. The vegetation was settled at the same place for at least 1000 years and therefore sedimentary organic matter deriving from the decay of higher plants has accumulated in large amounts. The global morphological and chemical preservation of the organic matter attested by various sedimentary parameters, including the proportions of cellulose-bearing tissues, the TOC contents, the C/N ratios, the total sulphur contents and the δ13C compositions, reveals that the sedimentary organic matter is less degraded when it has been under the influence of the marine water table for a long time. This is indicated by a good correlation between the salinities of interstitial waters, the redox potentials measured in the wet sediments and the chemical composition of the organic matter. Moreover, the concentration of organic sulphur shows a positive correlation with the hydrogen index (HI), only under the marine water-table influence, suggesting the very local presence of sulphur–HC compounds.