Examining compound-specific nitrogen isotopic composition of amino acids (δ15NAA) as a new proxy for sedimentary organic N

The stable nitrogen (N) isotopic (δ¹⁵N) signature of marine sedimentary N (δ¹⁵N_bulk) is commonly applied as a proxy for the δ¹⁵N of sinking particulate organic matter (δ¹⁵N_POMsink), and by extension the δ¹⁵N of marine primary production. Although a general correspondence between the δ¹⁵N_bulk and δ¹⁵N_POMsink exists in shallow water, diagenesis or mixtures of N sources can affect this relationship. For instance, diagenesis δ¹⁵N_bulk enrichment as a function of water depth (Robinson et al., 2012), and terrestrial N sources can constitute a large portion of total sedimentary N near continental margins (Schubert & Calvert, 2001). Compound-specific amino acid analysis (δ¹⁵N_AA) represents a new approach to address these issues. Proteins and peptides comprise the majority of N-containing molecules in living organisms; hence δ¹⁵N_AA may be a direct proxy for organic N-δ¹⁵N (δ¹⁵N_ON). However, the relationship between δ¹⁵N_AA data and major sedimentary N fractions has not been evaluated.

We analyzed δ¹⁵N_AA and the δ¹⁵N composition of major operationally defined N fractions and their relative contribution to total N from marine POM and shallow sediments collected in Santa Barbara Basin (SBB). Fractions analyzed include bulk, acid-soluble (AS), acid-insoluble (AI), and total hydrolysable amino acid (THAA). Average sedimentary δ¹⁵N_THAA and δ¹⁵N_AS are enriched relative to δ¹⁵N_bulk by 2.9‰ and 1.0‰, respectively and δ¹⁵N_AI is depleted relative to δ¹⁵N_bulk by ~1.5‰. The δ¹⁵N_AS and δ¹⁵N_THAA are closest to subsurface nitrate δ¹⁵N (~8‰) in SBB, consistent with a primary N source, while depleted δ¹⁵N_AI values are consistent with a dominant terrestrial N source. Together, these findings help to characterize the mixture of ON compounds, including hydrolysable AA, found in fresh biomass and suggest that δ¹⁵N_THAA represents a valuable new molecular level proxy for sedimentary proteinaceous material, but requires calibration to reconstruct δ¹⁵N of source N.