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

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.