Organic matter sources and early diagenetic degradation in a tropical peaty marsh (Tritrivakely, Madagascar). Implications for environmental reconstruction during the Sub-Atlantic

Peat samples from a one metre core and living Cyperaceae, collected in Trit rivakely marsh in Madagascar, were studied to determine the organic matter (OM) composition and extent of OM degradation in this core. The study was c arried out combining light microscopy observations, bulk analyses, infra-re d spectroscopy, hydrolyses of sugars, oxidation of lignin and pyrolyses. In the surface peat, organic matter derived from Cyperaceae undergoes extensi ve degradation of its basic cell wall components, morphologically revealed by destructuration of plant tissues and their transformation into reddish a morphous organic matter occurring in large amounts all along the core. Two ratios (cinnamic units/lignin and xylose + arabinose/total sugars) were det ermined as markers of Cyperaceae. It appeared that the vegetation of the ma rsh remained probably unchanged during the considered accumulation period, i.e. the last 2300 years B.P. Rhamnose, mannose and non-cellulosic glucose probably have a common origin and are mostly derived from bacteria. In cont rast, galactose is likely to be a marker of algal source, especially of the diatoms that occur only in the upper part of the core (0-ca. 50 cm). Acid/ aldehyde ratios of syringic and vanillic monomers (index of lignin oxidativ e depolymerisation) and mannose+ rhamnose+ non-cellulosic glucose/total sug ars ratios (reflecting bacterial degradation of hemicelluloses) are positiv ely correlated, and can thus be considered as markers of microbial degradat ion of the Cyperaceae tissues. The n-alkane/n-alk-1-ene doublets that domin ate the pyrolysates of hydrolysed peat samples reflect the contribution of B. braunii algaenan and higher plant suberans, and of condensed lipids most ly derived from higher plants and microalgae. The upper part of the core is characterised by a greater dilution of Cyperaceae-derived compounds by org anic matter from microalgae when compared with deeper samples, as recorded by pear bulk features, hydrolysable sugars, lignin oxidation products and p yrolysis products. Two accumulation periods can thus be distinguished in th e core: a peaty phase between 2300 years B.P. and ca. 1500 years B.P. (low watertable and strongly limited microalgal growth); a waterlogged marsh, fr om ca. 1500 years B.P. to the present time, in which a higher water table w as longer lasting with a substantial algal production. The environmental va riation thus recorded could correspond to a regional climatic change occurr ing around 1500 years B.P. (C) 2000 Elsevier Science Ltd. All rights reserv ed.