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

Abstract

To assess the long‐term (27 yr) effects of sediment aging on stable carbon and nitrogen isotope values (δ 13 C and δ 15 N), 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., δ 13 C and δ 15 N of the surface varve of the 1979 core was followed in cores retrieved in 1980, 1989, 1993, 2002, 2004, and 2006. δ 13 C 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 δ 13 C changes and the initial carbon and nitrogen concentrations, indicating that the initial sediment composition is important for the 13 C fractionation. δ 15 N 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 δ 15 N indicates a microbial control on δ 15 N 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 δ 13 C and δ 15 N 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 δ 13 C are observed during the first 5‐10 yr, whereas no delimitation can be recommended for δ 15 N.