NOAA/WDS Paleoclimatology - Santa Barbara Basin Bulk and Compound Specific Nitrogen Isotope Data

The nitrogen (N) isotopic composition (d15N) of bulk sedimentary N (d15Nbulk) is a common tool for studying past biogeochemical cycling in the paleoceanographic record. Empirical evidence suggests that natural fluctuations in the d15N of surface nutrient N are reflected in the d15N of exported planktonic biomass and in sedimentary d15Nbulk. However, d15Nbulk 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 (d15NAA) are novel measurements with the potential to decouple d15N changes in nutrient N from trophic effects, two main processes that can influence d15Nbulk records. As a proof of concept study to examine how d15NAA can be applied in marine sedimentary systems, we compare the d15NAA 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 d15NAA signatures of both planktonic biomass and sinking POM are well preserved in such surface sediments. However, we also observed an unexpected inverse correlation between d15N value of phenylalanine (d15NPhe; 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, d15NPhe values should in fact be directly dependent on shifts in ecosystem trophic position. While this result may appear incongruent with current applications of d15NAA 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 d15NAA parameters are ecosystem trophic position, which determines relative partitioning of 15N into source AA versus trophic AA pools, and the integrated d15NAA of all common protein AA (d15NTHAA), which serves as a proxy for the d15N of nutrient N. Together, we suggest that these can provide a coupled picture of regime shifts in planktonic ecosystem structure, d15N at the base of food webs, and possibly additional information about nutrient dynamics.