Nitrogen isotopes of amino acids in marine sediment: A burgeoning tool to assess organic matter quality and changes in supplied nitrate δ15N
Compound-specific nitrogen (N) isotopic analysis (δ15N) of amino acids (AA), or CSI-AA, is a novel approach to understand N cycling. We expand upon initial observations at a productive, hypoxic margin to provide insight into source and transformation of sedimentary organic nitrogen from varied depositional conditions in a complex N cycling zone, the eastern tropical Pacific. δ15NAA patterns are generally well preserved, matching original mixed plankton inputs with some evidence for microbial degradation. There is appreciable δ15N offset between total N (bulk) and total AA N, where AA N is enriched. Source AA, previously understood to record changing baseline δ15N, are fundamentally different in marine sediments. We use modeling to rigorously test whether the total AA δ15N instead tracks changing baseline δ15N. Lastly we employ this new understanding of δ15NAA in sediments to compare with published whole sediment (δ15Nbulk) records, demonstrating the utility and wealth of information provided by CSI-AA.