Sequential fatty acid analysis of a peat core covering the last two millennia (Tritrivakely lake, Madagascar): Diagenesis appraisal and consequences for palaeoenvironmental reconstruction

Seven samples from a 1 m long peat core, taken from the centre of the Tritrivakely maar lake (Madagascar) and covering the last 2300 yr, were analysed for their fatty acid (FA) content. Solvent extraction, followed by acid hydrolysis and saponification allowed successive release of “free”, H+-labile and OH−-labile bound FAs. Downcore variations in the concentration of individual compounds released by the different treatments allowed distinction between the following three major types of input: (i) primary production; (ii) secondary production (that most certainly forms as a result of microbial degradation of primary products) immediately underlying the sub-surface; (iii) a notable bacterial stock that appears later and slowly at depth. The latter input is revealed by the regularly increasing concentration of several H+-labile compounds with depth. Sedimentary horizons that were sites for good preservation conditions are indicated by high relative amounts of nearly all the compounds, especially those inherited from the primary as well as from the secondary producers (including the primary consumers). In contrast, depth intervals that have been the site for intensive diagenetic alteration contain few compounds, including those inherited from late consumers.