EGU2020-12134
https://doi.org/10.5194/egusphere-egu2020-12134
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Isotopic evidence for changes in the origin and cycling of nitrogen in the Labrador Sea during the last 8,000 years

Markus Kienast1, Sam Davin1,2, Kristin Doering1, Dierk Hebbeln3, Stephanie Kienast1, Nadine Lehmann1, Ralph Schneider4, Owen Sherwood5, and Jens Weiser3
Markus Kienast et al.
  • 1Dept. Oceanography, Dalhousie University, Halifax, Canada (markus.kienast@dal.ca)
  • 2GEOTOP Centre de recherche en géochimie et géodynamique, Université du Québec à Montréal, Montreal, Canada
  • 3MARUM – Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany
  • 4Institute of Geoscience, Christian Albrecht’s University Kiel, Kiel, Germany
  • 5Dept. Earth and Environmental Sciences, Dalhousie University, Halifax, Canada

Subsurface nitrate in the Labrador Sea (NW Atlantic) and Baffin Bay is provided by North Pacific water flowing through Bering Strait and the Canadian Arctic as well as by advection from the North Atlantic. Both these nitrate sources are distinct in their isotopic signature (δ15N), owing to benthic denitrification on the Bering, Chukchi and east Siberian shelves and nitrogen fixation in the North Atlantic, respectively. Accordingly, water column profiles of δ15N(nitrate) collected off Greenland in the eastern Labrador Sea show low δ15N(nitrate), which mixes with more 15N-enriched nitrate flowing through Baffin Bay into the northern Labrador Sea. The Labrador Current carries this mixture southward along the western Labrador Sea, toward Newfoundland. The δ15N of surface sediments in the Labrador Sea closely mirrors these water column signals, suggesting that sediments can be used to trace changes in both the source signature of Atlantic versus Pacific-derived nitrate as well as in the admixture of the two source waters.

Two downcore sedimentary δ15N records from the NE and NW Labrador Sea coast both show high δ15N values of ca. 7‰ during the early Holocene (9-7 kyrs BP). In the NE Labrador Sea, this is followed by a long-term decrease toward δ15N of ca. 4.5‰ at the core top, in contrast to a much more subtle decrease in the NW Labrador Sea (surface sediment δ15N of ca. 6.5‰). The decreasing δ15N values along the eastern Labrador Sea are consistent with a Holocene increase in nitrogen fixation in the North Atlantic or an increasing advection of isotopically light nitrate. In turn, an increasing admixture of North-Pacific-derived nitrate, or intensified denitrification on the Bering Shelf would be required to explain the much subdued Holocene δ15N decrease in the NW Labrador Sea.

How to cite: Kienast, M., Davin, S., Doering, K., Hebbeln, D., Kienast, S., Lehmann, N., Schneider, R., Sherwood, O., and Weiser, J.: Isotopic evidence for changes in the origin and cycling of nitrogen in the Labrador Sea during the last 8,000 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12134, https://doi.org/10.5194/egusphere-egu2020-12134, 2020