Amino Acid Carbon Isotopes in Core-Top Sediments of the Northern Labrador Sea and Baffin Bay Track the Origins of Export Productivity across Environmental Gradients

Compound-specific stable isotope analysis of amino acids is emerging as a powerful new tool for tracing biosynthetic origins of organic carbon in food webs. Applications of this technique have mostly involved the tissues of living organisms. By contrast, there have been relatively few investigations of amino acid δ¹³C in detrital materials, specifically marine sediments. We examined amino acid-specific δ¹³C in core-top sediments collected from the northern Labrador shelf, slope and basin, the west Greenland shelf, and Baffin Bay. The collection sites span environmental gradients in water depth, proximity to land, and duration of seasonal sea ice coverage. The overall patterns in amino acid δ¹³C were remarkably similar to those of field-collected and laboratory-cultured microalgae representing of a diverse range of ocean regions globally. Analysis of the δ¹³C of essential amino acids showed that (eukaryotic) microalgae represented the dominant source of amino acid carbon in all samples, thus indicating tight benthic-pelagic coupling across all the sites investigated. The between-site variability in δ¹³C of total hydrolysable amino acids was only about 1‰, compared to up to 6‰ for individual amino acids. The significance of this variability with respect to proportional contributions of different plankton functional groups will be discussed. Overall, our results highlight the potential for using amino acid δ¹³C as a new tool to investigate the origins of export productivity in marine environments.