Inventory of sedimentary organic matter in modern wetland (Marais Vernier, Normandy, France) as source-indicative tools to study Holocene alluvial deposits (Lower Seine Valley, France).
Résumé
Palynofacies' analysis was developed for petroleum exploration, but it is now commonly used to differentiate the major constituents of the sedimentary organic matter (SOM) in Postglacial deposits. Our work aimed at studying the SOM distribution in a present wetland of the Seine estuary (Vernier Marsh) in order to provide comparative elements to interpret the composition of SOM in the Lower Seine Valley alluvial infilling. In the Vernier Marsh, the distribution of major constituents is heterogeneous. Thus, for each depositional environment studied (soils, drainage ditches, channels, ponds), the composition of SOM has been characterized by using two ternary diagrams ("Amorphous OM/Preserved Phytoclasts/Transformed Phytoclasts" and "Opaque/Amorphous/Gelified Particles"; OP/AP/GP). This approach allowed us to define four optical indices that can be used to estimate the relative abundances of principal organic fractions in mixed deposits based on the origin of SOM (aquatic/terrestrial), its source areas (vegetation, soils, river), and controls of early diagenetic changes (aerobic/anaerobic conditions, advanced oxidation). "Amorphous OM contents" increase from terrestrial environments to fluvial deposits and evaluate the abundance of planktonic contribution. "Preserved / transformed phytoclasts ratio" decreases from sites of production to those of deposition indicating the degree of degradation of terrestrial materials. "OP contents" allow one to define fluvial (high values) and palustrine (low values) environments. "GP / AP ratio" allows one to discern soil (low values) and plant (high values) supplied areas. Finally, these optical compositional indices are measured in Holocene deposits and used to complete palaeoenvironmental reconstructions.
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