Ageing of organic matter in incubated freshwater sediments; inferences from C and H isotope ratios of methane

Abstract

The freshwater sediments were incubated under anaerobic conditions for 570 and 879 days to investigate the potential variations in methanogenic pathways due to increasing sediment age and recalcitrance of organic matter. The methanogenic pathways did not shift from acetate fermentation toward CO2 reduction, as indicated by the observed variations of the isotopic composition of methane in natural conditions. It appeared, however, that the observed decrease of methane concentration (from 86 to 39%) and continuous increase in d13 C(CH4) (from -69.7 to -59.0) and dD(CH4) values (from -381 to -320) resulted mainly from exhaustion of at least one methanogenic substrate in the incubated sediments. To better understand processes controlling the variations of d13 C(CH4) and dD(CH4) values relative to ageing of organic matter, the method of principal component analysis (PCA) was used. This method offers good comparison of the relationships between variables when a larger number of parameters control a given process in the same time period. In this study, the PCA indicated three distinctive factors that controlled decomposition of organic matter during the incubation. Factor 1 explained 33% of observed variations among the variables and had positive (0.93-0.92) loadings for electric conductivity and DIC concentration and negative loading for d13 C(CH4) values (-0.72). Factor 2 accounted for 28% and had high positive loading for dD(CH4) value (0.86) and high negative loading for methane concentration (-0.81). Factor 3 accounted for 19% and exhibited high positive loadings for temperature (0.90) and d13 C(DIC) value (0.69). Factors 1 and 2 were directly linked to the methanogenesis and indicated that bigger accumulation of bio-products in sediments is likely important for variations of d13 C and dD of methane. This study shows that method of principal component analysis might be a useful tool while studying biogeochemical carbon cycle during early digenesis of freshwater sediments.