Decadal diagenetic effects on δ13C and δ15N studied in varved lake sediment

To assess the long-term (27 yr) effects of sediment aging on stable carbon and nitrogen isotope values (delta C-13 and delta N-15), we used a collection of eight freeze cores of annually laminated (varved) lake sediment collected from 1979 to 2007 in Nylandssjon (northern Sweden). Previous research has shown that 20-23% of carbon and 35% of nitrogen is lost in 27 yr. Material from specific years was compared in the cores, e. g., delta C-13 and delta N-15 of the surface varve of the 1979 core was followed in cores retrieved in 1980, 1989, 1993, 2002, 2004, and 2006. delta C-13 increased by 0.4-1.5% during the first 5 yr. After this initial increase, only minor fluctuations were recorded. There is a good correlation between the magnitude in delta C-13 changes and the initial carbon and nitrogen concentrations, indicating that the initial sediment composition is important for the C-13 fractionation. delta N-15 gradually decreased by 0.3-0.7% over the entire 27-yr period. The lack of correlation with the initial sediment composition and the gradual decrease in delta N-15 indicates a microbial control on delta N-15 change. The diagenetic changes in the stable isotope values that occur in Nylandssjon are small, but of the same magnitude as the down-core variation in the varves deposited 1950-2006. Diagenetic effects should be considered when delta C-13 and delta N-15 are used to study organic matter sources or paleoproductivity, especially when dealing with recent trends or small changes. Based on our findings, diagenetic effects for delta C-13 are observed during the first 5-10 yr, whereas no delimitation can be recommended for delta N-15.