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

The nitrogen (N) isotopic composition (δ 15 N) of bulk sedimentary N (δ 15 N bulk ) is a common tool for studying past biogeochemical cycling in the paleoceanographic record. Empirical evidence suggests that natural fluctuations in the δ 15 N of surface nutrient N are reflected in the δ 15 N of exported planktonic biomass and in sedimentary δ 15 N bulk . However, δ 15 N 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 (δ 15 N AA ) are novel measurements with the potential to decouple δ 15 N changes in nutrient N from trophic effects, two main processes that can influence δ 15 N bulk records. As a proof of concept study to examine how δ 15 N AA can be applied in marine sedimentary systems, we compare the δ 15 N 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 δ 15 N 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 δ 15 N value of phenylalanine (δ 15 N Phe ; 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, δ 15 N Phe values should in fact be directly dependent on shifts in ecosystem trophic position. While this result may appear incongruent with current applications of δ 15 N 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 δ 15 N AA parameters are ecosystem trophic position, which determines relative partitioning of 15 N into source AA versus trophic AA pools, and the integrated δ 15 N AA of all common protein AA (δ 15 N THAA ), which serves as a proxy for the δ 15 N of nutrient N. Together, we suggest that these can provide a coupled picture of regime shifts in planktonic ecosystem structure, δ 15 N at the base of food webs, and possibly additional information about nutrient dynamics. [ABSTRACT FROM AUTHOR]