Geochemical study of the Lac du Bouchet (Hte-Loire, France) Part II: water-sediments-organic matter interactions during the last 2500 years.

The early diagenesis of the superficial sediment of the Lac du Bouchet has been studied by analysis of interstitial water and solid phase. 14C dating of the sediment gives 2500±350 years at a depth of 80 cm below the sediment-water Interface (SWI). Solid phase is made of 19 to 49% clays (Mainly kaolinite), detrital minerals (quartz, feldspars, ferromagnesian), Diatoms frustules and organic matter (ligno-cellulosic debris, pollens, amorphous grey matter). The main part of organic matter (OM) mineralisation takes place in the first 10 to 20 centimetres under the SWI, but continues at a deeper level 3 to 4 times more slowly. Methanogenesis reaction is the dominant degradation process (90 to 93% in the upper section of the mud, up to 97% below), followed by sulphate reduction (4% in spring and 7% in autumn, only in the top section) and iron reduction (3 to 2%). A formula is proposed for OM undergoing oxidation: (CH2O)106(NH3)13.6(H3PO4)0.84. [ABSTRACT FROM AUTHOR]