Origin and diagenesis of lignin and carbohydrates in mangrove sediments of Guadeloupe (French West Indies): Evidence for a two-step evolution of organic deposits

Abstract

The mangroves of Grande Terre Island (Guadeloupe, French West Indies) are known to store large amounts of organic carbon, and organic-rich sediments have been described to several meters depth. The purpose of the present work was to precisely determine the molecular composition (carbohydrates and lignin-derived phenols) of these organic deposits in relationship with environmental conditions. It was found that within the upper meter of the cores, geochemical data displayed the classical degradation pattern of organic matter (OM) deriving from higher plants. On the one hand, carbohydrates from mangrove tissues underwent fast decomposition, other compounds being simultaneously synthesized by bacteria. On the other hand, lignin phenols were lost rather slowly, i.e. at a similar or lower rate than TOC, their distribution with depth evidencing various lignin decomposition pathways depending on redox conditions. The position of the swamp with respect to salt and fresh water tables strongly influenced these conditions. At depth, results revealed an organic-rich layer, which was characterized by surprisingly well-preserved OM with regard to sugar and phenol compositions. We speculate that the preservation of these compounds might be explained by a rapid and permanent flooding of the mangrove stands that may have occurred more than a thousand years ago. We suggest that the geodynamic context, i.e. the recurrent seismic activity recorded in Guadeloupe, may have induced such a flooding, resulting in the preservation of this OM.