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

Abstract

<p>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 (δ<sup>15</sup>N), 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 δ<sup>15</sup>N<sub>(nitrate)</sub> collected off Greenland in the eastern Labrador Sea show low δ<sup>15</sup>N<sub>(nitrate)</sub>, which mixes with more <sup>15</sup>N-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 δ<sup>15</sup>N 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.</p><p>Two downcore sedimentary δ<sup>15</sup>N records from the NE and NW Labrador Sea coast both show high δ<sup>15</sup>N 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 δ<sup>15</sup>N of ca. 4.5‰ at the core top, in contrast to a much more subtle decrease in the NW Labrador Sea (surface sediment δ<sup>15</sup>N of ca. 6.5‰). The decreasing δ<sup>15</sup>N 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 δ<sup>15</sup>N decrease in the NW Labrador Sea.</p>

Stable isotope ratios in seawater nitrate reflect the influence of Pacific water along the northwest Atlantic margin

“…With respect to sediments, δ 15 N is confounded by site-tosite differences in sedimentation rates and diagenetic effects (Robinson et al, 2012). Nevertheless, known spatial patterns track the expected distribution of fPW, with lower values of 4 ‰-6 ‰ in the central Labrador Sea and Southwest Greenland margin and higher values of 6 ‰-9 ‰ on the Labrador Shelf and in northern Baffin Bay (Muzuka and Hillaire-Marcel, 2000;Cormier et al, 2016;Kienast et al, 2020;Limoges et al, 2020). Thus, by extension, downcore trends in δ 15 N should reflect advection-related temporal changes in fPW.…”

Stable isotope ratios in seawater nitrate reflect the influence of Pacific water along the Northwest Atlantic margin

“…With respect to sediments, δ 15 N is confounded by site-to-site differences in sedimentation rates and diagenetic effects (Robinson et al, 2012). Nevertheless, known spatial patterns track the expected distribution of fPW, with lower values of 4 -6 ‰ in the central Labrador Sea and Southwest Greenland margin, and higher values 6 -9 ‰ on the Labrador shelf and northern Baffin Bay (Muzuka & Hillaire-Marcel, 2000;Cormier et al, 2016;Kienast et al, 2020;Limoges et al, 2020). Thus, by extension, downcore trends in δ 15 N should reflect advection-related temporal changes in fPW.…”

Biomarker Records of Environmental Shifts on the Labrador Shelf During the Holocene

“…(2022a, 2022b) with license Creative Commons Attribution 4.0 International. Bayesian age model code is available at Zenodo (Kolling et al., 2023) with license Creative Commons Attribution 4.0 International.…”