Compound specific amino acid δ15N in marine sediments: A new approach for studies of the marine nitrogen cycle

The nitrogen (N) isotopic composition (delta N-15) of bulk sedimentary N (delta N-15(bulk)) is a common tool for studying past biogeochemical cycling in the paleoceanographic record. Empirical evidence suggests that natural fluctuations in the.515N of surface nutrient N are reflected in the delta N-15 of exported planktonic biomass and in sedimentary delta N-15(bulk) However, delta N-15(bulk) is an analysis of total combustible sedimentary N, and therefore also includes mixtures of N sources and/or selective removal or preservation of N-containing compounds. Compound-specific nitrogen isotope analyses of individual amino acids (delta N-15(AA)) are novel measurements with the potential to decouple delta N-15 changes in nutrient N from trophic effects, two main processes that can influence delta N-15(bulk) records. As a proof of concept study to examine how delta N-15(AA) can be applied in marine sedimentary systems, we compare the delta N-15(AA) signatures of surface and sinking POM sources with shallow surface sediments from the Santa Barbara Basin, a sub-oxic depositional environmental that exhibits excellent preservation of sedimentary organic matter. Our results demonstrate that delta N-15(AA) signatures of both planktonic biomass and sinking POM are well preserved in such surface sediments. However, we also observed an unexpected inverse correlation between.515N value of phenylalanine (delta N-15(AA)); the best AA proxy for N isotopic value at the base of the food web) and calculated trophic position. We used a simple N isotope mass balance model to confirm that over long time scales, delta N-15(AA) values should in fact be directly dependent on shifts in ecosystem trophic position. While this result may appear incongruent with current applications of delta N-15(AA) in food webs, it is consistent with expectations that paleoarchives will integrate N dynamics over much longer timescales. We therefore propose that for paleoceanographic applications, key delta N-15(AA) parameters are ecosystem trophic position, which determines relative partitioning of N-15 into source AA versus trophic AA pools, and the integrated delta N-15(AA) of all common protein AA (delta N-15(AA)), which serves as a proxy for the delta N-15 of nutrient N. Together, we suggest that these can provide a coupled picture of regime shifts in planktonic ecosystem structure, delta N-15 at the base of food webs, and possibly additional information about nutrient dynamics. (C) 2014 Elsevier Ltd. All rights reserved.