Abstract

Mangrove swamps, being characterized by specific vegetals associated with particular geochemical conditions, are an appropriate environment for the study of the preservation mechanisms of the organic matter (OM) derived from higher plants. The interpretation of geochemical and petrographic parameters of the organic matter of mangrove sediments necessitates an assessment of the part played by living roots systems. Grain-size distributions of organic fractions in the surficial swamp-sediments give evidence of their physical heterogeneity in contrast with downcore grain-size distributions which reveal an enhanced fractionation of organic constituents with depth. Indeed, the organic carbon analyses performed on the different size fractions showed that structured organic constituents were always predominant with respect to humified particles. However, deep sediments under Rhizophora still present heterogeneous OM while surficial sediments under Acrostichum already present an abundant humified organic fraction. The low amount of Cl-soluble organic matter (5%) and its rapid decrease with depth show that most of the total organic matter is resistant to degradation, even in the upper layers. However, some HCl- resistant but HF-soluble OM, assumed to belong to the living biomass, is present at specific levels, a few dm below surface, while it is lacking in the very upper layer. The layers rich in HF-soluble OM occur at depths which are clearly correlated with the root-system of the locally predominant mangrove species. HF-soluble OM may represent up to 30% of the total organic carbon but this does not alter the general, downward-decreasing trends of vertical distribution of total organic carbon in the profiles. Through the homogenizing effect of degradation, the organic matter of the successive root systems is integrated into the bulk sedimentary signal: the present system representing the last sedimentary signal with the most preserved OM. The deeper levels reveal an ancient mangrove-system with a more cohesive and humified organic material which present high organic carbon contents.

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