Nitrogen cycling and circulation in Baffin Bay investigated with isotopic measurements of N2, N2O, and NO3-

Cara C Manning1, Annie Bourbonnais2, Julie Granger3, Roberta Claire Hamme4, Laurence Yeung5, David Armando Valerio6, Edward D Young7, Zhiyin Zheng8, Nadine Lehmann9, Jean-Eric Tremblay10 and Philippe Tortell1, (1)University of British Columbia, Earth, Ocean and Atmospheric Sciences, Vancouver, BC, Canada, (2)University of South Carolina, School of Earth, Ocean, and Environment, Columbia, United States, (3)University of Connecticut, Marine Sciences, Groton, United States, (4)University of Victoria, School of Earth and Ocean Sciences, Victoria, BC, Canada, (5)Rice University, Houston, United States, (6)Rice University, Houston, TX, United States, (7)University of California Los Angeles, Earth, Planetary, and Space Sciences, Los Angeles, United States, (8)University of British Columbia, Vancouver, BC, Canada, (9)Dalhousie University, Department of Oceanography, Halifax, NS, Canada, (10)Laval University, Biology, Quebec City, QC, Canada
Abstract:
Baffin Bay is a marginal sea connecting Pacific waters traveling through the Arctic to the North Atlantic via the adjacent Labrador Sea. Pacific source waters entering Baffin Bay via the Canadian Arctic Archipelago have a fixed nitrogen (N) deficit as a result of denitrification, while the North Atlantic is a site of significant nitrogen fixation. Here we use isotopic and concentration measurements of N2, N2O, and NO3- in Baffin Bay and surrounding waters to determine water mass origins and quantify fixed N sources and sinks. The long residence time of Baffin Bay Bottom Water allows distinctive geochemical signatures to accumulate. Excess N2O and N2 in basin deep waters, as well as a fixed N deficit (negative N*), indicate that sedimentary denitrification of organic matter is a significant fixed nitrogen sink within Baffin Bay. Isotopomeric measurements of N2O, which show strong enrichments in δ18O with depth, indicate that N2O derived from incomplete denitrification has diffused out of the sediments and into the water column. We quantify the excess N2 from denitrification using clumped isotope measurements (for the first time in an oceanic setting) and compare the results with N2/Ar-based estimates of excess N2 from denitrification. Isotopic measurements of NO3- suggest that the sedimentary denitrification is fueled by remineralization of organic matter produced in northern Baffin Bay, thereby regulating the nutrient inventories that are exchanged with adjacent oceanic regions. This study demonstrates that coupled isotopic measurements of multiple nitrogen species provide new insights into biogeochemical cycling and water mass circulation in a rapidly changing Arctic system.
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