Is selective preservation of nitrogenous organic matter reflected in the delta C-13 signal of lacustrine sediments?

The delta(13)C and C/N ratios of organic matter in core from a lake near Ka lamazoo, SW Michigan, representing similar to 12 k years were measured. The delta(13)C values of two fractions, one obtained by demineralization using dilute hydrochloric acid (delta(13)C(HCl)) and the other a fraction obtain ed by HF-HCl digestion (delta(13)C(HF-HCl)), ranged from -24 to -12 parts p er thousand with the delta(13)C(HF-HCl) being generally more negative. Both fractions showed a similar pattern of variation with depth. The delta(13)C values are less negative when compared to sediment organic delta(13)C valu es from other Midwestern lakes, but are consistent with a dissolved inorgan ic carbon (DIC) source. The C/N ratios of the HCl fraction ranged from 11 t o 12, typical of organic carbon in midwestern lakes. However, the C/N ratio s of the HF-HCl residue varied from 13 to 25 suggesting that this fraction had lost components enriched in nitrogen. Because compounds such as protein s and amino acids thereafter referred fraction had lost components enriched in nitrogen. Because compounds such as proteins and amino acids (hereafter referred to as NC for 'nitrogenous compounds') in sediments are more labil e, enriched in delta(13)C and lower in C/N ratios, compared to more stable lignin and cellulose, it is interpreted that the HF-HCl digestion removed t hese compounds. A Delta(13)C parameter defined as: Delta(13)C = delta(13)C( HCl) - delta(13)C(HF-HCl) should, in principle, be controlled by fluctuatio ns in the preservation of NC in the water column. Previous studies have lin ked preservation of nitrogenous compounds in sediments to existing climate. This hypothesis is supported by a good correlation between the Delta(13)C parameter and the previously demonstrated climate-sensitive organic delta D record in this lake core. Moreover, the delta(13)C correlated strongly wit h percentage organic carbon content in this core. This behavior is consiste nt with a process where increased organic carbon production and burial depl etes the DIC in C-12, thereby reducing the fractionation between the organi c and inorganic reservoirs. (C) 1999 Elsevier Science B.V. All rights reser ved.