Nitrogen Cycling in the Canadian Arctic and Sub-Arctic: Constraints from Dual Nitrate Isotope Data

Nadine Lehmann1, Carolyn Buchwald1, Samuel H. Davin2, Julie Granger3, Markus Kienast1, Moritz F Lehmann4, Owen Sherwood5 and Jean-Eric Tremblay6, (1)Dalhousie University, Department of Oceanography, Halifax, NS, Canada, (2)Université du Québec à Montréal, GEOTOP - Centre de recherche en géochimie et géodynamique, Montreal, QC, Canada, (3)University of Connecticut, Department of Marine Sciences, Groton, CT, United States, (4)University of Basel, Department of Environmental Sciences, Basel, Switzerland, (5)Dalhousie University, Department of Earth and Environmental Sciences, Halifax, NS, Canada, (6)Laval University, Biology, Quebec City, QC, Canada
Abstract:
The Canadian Arctic Ocean plays a key role in the marine nitrogen cycle. It provides a connection between the North Pacific, which hosts regions of denitrification, and the North Atlantic, an area of extensive N2 fixation. Here, we present water column natural abundance nitrogen (N) and oxygen (O) isotope ratios of nitrate (NO3-) collected during the Canadian Arctic GEOTRACES expedition in 2015 throughout the Canadian Archipelago, Baffin Bay and the Labrador Sea, as well as samples from the Maria S. Merian 45 and ArcticNet cruises collected during the same year along the Labrador Shelf. These data shed light on both the origin and internal cycling of nitrate in the Canadian Arctic and sub-Arctic.

Nitrate isotope values in the western Archipelago show a pronounced relative enrichment in 15N and a coincident minimum in δ18O in the upper halocline layer, indicative of both benthic denitrification upstream on the Bering and Chukchi shelves and remineralization along the transit. This subsurface peak in nitrate δ15N associated with the cold Pacific-derived halocline shows a progressive decrease along a west-to-east transect, from ~8‰ in the Canada Basin to ~7‰ in southern Lancaster Sound and ~6‰ in western Baffin Bay. The concurrent minimum in nitrate δ18O in the upper halocline seems largely preserved throughout its transit through the Archipelago, with values slightly increasing from ~0‰ in the western Archipelago to ~0.5‰ in southern Lancaster Sound. The Baffin Island Current and the Labrador Current subsequently carry this 15N-enrichment and relatively low-δ18O signature southward along the western Baffin Bay and into the Labrador Sea.

The observed trends in nitrate isotope ratios described here will be discussed in the context of regional circulation patterns and N biogeochemistry in the central Archipelago and further downstream in the sub-Arctic North Atlantic.