Organic carbon sources and transformations in mangrove sediments: A Rock-Eval pyrolysis approach

A Rock-Eval pyrolysis study was carried out on sedimentary cores and leaf and woody tissue of vascular plant species from the mangroves of French Guiana. These forests develop on moving mudbanks and have a lifetime limited to few decades before being eroded. Our main purpose was to complete the understanding of carbon cycling in this specific environment using a method that allows monitoring the depth evolution of sources and transformation of organic matter (OM) within a tight depth span. Leaf and woody tissue showed clearly distinct Rock-Eval parameters. However, these parameters strongly varied with OM decomposition, rapidly limiting their efficiency as mangrove plant tracers. The decrease in values of TPS2 (pyrolysis temperature corresponding to the maximum release of hydrocarbons) with depth clearly showed a line between a pedogenetic layer enriched in autochthonous OM and marine sediments enriched in allochthonous OM. This allochthonous OM originated from the particulate discharge of the Amazon River, which is partly deflected northwestwards by the current of the Guianas. Within the soil layer, decay processes induced a strong dehydrogenation and slight oxidation of OM, the two processes occurring simultaneously. The deeper layers were characterized by a very strong increase in OICO2 (quantity of pyrolysed CO2 relative to total organic carbon), beneath both the young and the senescent forests. This trend resulted rather from an increasing content of authigenic carbonate than from increasing concentration of oxidized allochthonous debris. Carbonate can form as a result of the more anoxic conditions prevailing in buried layers vs. the upper suboxic ones. Beneath the senescent mangrove, a greater increase in OICO2 probably reflected greater carbonate precipitation, resulting from the decomposition of higher amounts of OM.