Is selective preservation of nitrogenous organic matter reflected in the δ13C signal of lacustrine sediments?

The δ13C and C/N ratios of organic matter in core from a lake near Kalamazoo, SW Michigan, representing ∼12 k years were measured. The δ13C values of two fractions, one obtained by demineralization using dilute hydrochloric acid (δ13CHCl) and the other a fraction obtained by HF–HCl digestion (δ13CHF–HCl), ranged from −24 to −12‰ with the δ13CHF–HCl being generally more negative. Both fractions showed a similar pattern of variation with depth. The δ13C values are less negative when compared to sediment organic δ13C values from other Midwestern lakes, but are consistent with a dissolved inorganic carbon (DIC) source. The C/N ratios of the HCl fraction ranged from 11 to 12, typical of organic carbon in midwestern lakes. However, the C/N ratios of the HF–HCl residue varied from 13 to 25 suggesting that this fraction had lost components enriched in nitrogen. Because compounds such as proteins and amino acids (hereafter referred to as NC for `nitrogenous compoundsʹ) in sediments are more labile, enriched in δ13C and lower in C/N ratios, compared to more stable lignin and cellulose, it is interpreted that the HF–HCl digestion removed these compounds. A Δ13C parameter defined as: Δ13C=δ13CHCl−δ13CHF–HCl should, in principle, be controlled by fluctuations in the preservation of NC in the water column. Previous studies have linked preservation of nitrogenous compounds in sediments to existing climate. This hypothesis is supported by a good correlation between the Δ13C parameter and the previously demonstrated climate-sensitive organic δD record in this lake core. Moreover, the δ13C correlated strongly with percentage organic carbon content in this core. This behavior is consistent with a process where increased organic carbon production and burial depletes the DIC in 12C, thereby reducing the fractionation between the organic and inorganic reservoirs.