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Organic markers as witnesses of the dynamic features of the mangrove coast in French Guyana.

Abstract : The French Guiana coastline is a highly dynamic environment, mostly composed by fine-grained sediment. This muddy-coastline results both from the proximity of the Amazon estuary and from the influence of coastal current. Indeed, this current deflects the mudbanks toward the North-West at the rate of 1km/year. As a result, erosion and accretion of mudbanks alternate on the coast. Between the zone of erosion and accretion, a tropical intertidal forest (mangrove) grows on the stable part of the mudbank. Avicenia germinans is the only species composing the studied mangrove, (managed to survive due to high capacity for adaptation to the anaerobic conditions of the sediment, notably radiating cable roots supporting vertical pneumatophores). The purpose of this study is to understand and to quantify the relationship between the organic composition of sediments, the evolution of the Avicenia-dominated mangrove forest, and therefore the dynamic feature of the coast. We also intend to determine the influence of environmental conditions on sedimentary record. For this, 12 two-meters deep cores were collected under different stages of mangrove evolution and one core in shoreface sediment as a reference. The forest age was determined thanks to Spot imageries (Vega, 2000). Moreover, we set apart 5 stages of evolution of mangrove forest: pioneer forest, mature forest, senescent forest, dead forest and "colonized" dead forest. This work includes geochemical datas from Rock-Eval and LECO CNS pyrolyzers, petrographic observations and physico-chemical parameters obtention (salinity, redox potential and pH). As expected, the content of organic matter in the sediment rises up with the mangrove age, the TOC content ranges from 1 to 18%. This trend is also underlined by the petrographic composition; i.e, an increase in ligno-cellulosic debris. However for the youngest mangrove forest, sedimentary organic matter mainly derives from algal mats with low C/Nat. (5-8), the higher plant debris are exported by the tides. In the case of the oldest mangrove (50 years old), the C/Nat. ratio rises to 19. Whatever the stage of mangrove evolution, the influence of the forest on the sediment occurs between the surface and 30cm deep. Below this depth, the characteristics of the sediment are those of the shoreface ones. The efficient decomposition by sulfate-reducing bacteria superimposed to the youth of the mangrove is responsible for this rapid decrease in organic matter content.
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Contributor : Nathalie Pothier <>
Submitted on : Wednesday, September 27, 2006 - 3:20:53 PM
Last modification on : Wednesday, September 16, 2020 - 4:51:24 PM

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  • HAL Id : hal-00101555, version 1

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Cyril Marchand, Elisabeth Lallier-Vergès, F. Baltzer. Organic markers as witnesses of the dynamic features of the mangrove coast in French Guyana.. EUG, 2001, Strasbourg, France. 1p. ⟨hal-00101555⟩

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