To what extent is the palaeoenvironmental signal of sedimentary organic matter modified by the root-systems of living mangroves? A quantitative assessment based on an example from Guadeloupe (French West Indies)

Mangrove swamps, being characterized by specific vegetals associated with p articular geochemical conditions, are an appropriate environment for the st udy of the preservation mechanisms of the organic matter (OM) derived from higher plants. The interpretation of geochemical and petrographic parameter s of the organic matter of mangrove sediments necessitates an assessment of the part played by living roots systems. Grain-size distributions of organ ic fractions in the surficial swamp-sediments give evidence of their physic al heterogeneity in contrast with downcore grain-size distributions which r eveal an enhanced fractionation of organic constituents with depth. Indeed, the organic carbon analyses performed on the different size fractions show ed that structured organic constituents were always predominant with respec t to humified particles. However, deep sediments under Rhizophora still pre sent heterogeneous OM while surficial sediments under Acrostichum already p resent an abundant humified organic fraction. The low amount of HCl-soluble organic matter (5 %) and its rapid decrease w ith depth show that most of the total organic matter is resistant to degrad ation, even in the upper layers. However, some HCl- resistant but HF-solubl e OM, assumed to belong to the living biomass, is present at specific level s, 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-solubl e 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 s uccessive root systems is integrated into the bulk sedimentary signal: the present system representing the last sedimentary signal with the most prese rved OM. The deeper levels reveal an ancient mangrove system with a more co hesive and humified organic material which present high organic carbon cont ents.