Nitrogen cycling in the warming Arctic Ocean
Embargo end date25/03/2023
This PhD thesis is investigating fixed nitrogen cycling at a Pan-Arctic scale, in order to better understand the sensitivity of Arctic biogeochemistry to ongoing warming and to identify future implications for long-term carbon fixation, nitrogen mass balance and oxygenation of the Arctic Ocean. Using stable isotopes of nitrate (δ¹⁵N-NO3 and δ¹⁸O-NO3), alongside an extensive suite of biogeochemical and hydrographic data, this project provides spatially and temporally integrated measurements of N supply, uptake and recycling in the Central Arctic Ocean (Nansen, Amundsen and Makarov basins) and the Atlantic inflow regions (Barents Sea, Fram Strait), outlining the processes that control nutrient budgets and fluxes on a Pan-Arctic scale. This study proves the paramount role of bordering Arctic shelves in the reprocessing and redistribution of nutrients throughout the entire Arctic Ocean. Stable isotope data demonstrates that the entire nutrient pool in the Central Arctic halocline has been regenerated and laterally advected from the surrounding shelves. However, nutrient supply and uptake across the Central Arctic basins exhibits an east-west gradient dictated by location of the Transpolar Drift (TPD). Regions situated on the TPD path (i.e. Amundsen and Makarov basins) exhibit surface NO3-depletion and a halocline isotopic signal (δ15N-NO3 ~6.5±0.1‰, δ18O-NO3~-1±0.2‰) consistent with lateral advection of partially nitrified and denitrified shelf bottom waters, traced back to the Siberian shelves. The strong salinity stratification in Amundsen and Makarov basins impedes surface nutrient recharge from the halocline, restricting biological uptake to the NO₃-depleted meltwater layer (top 20-30m). Using an isotopically constrained mass balance model, this study shows that the TPD is not an additional source of (riverine) NO3 to Amundsen/Makarov basins and cannot support a future increase in Central Arctic productivity because any riverine nitrogen inputs concentrated in the TPD are lost to denitrification on the shallow, productive Siberian shelves. Regions outside the TPD influence (i.e. Nansen basin) exhibit Si depletion and a well-mixed Atlantic halocline formed by lateral advection of shelf waters from the Barents Sea. Based on the coupled isotopic signature of preformed nitrate in Barents Sea (δ15N-NO3 ~5±0.1‰, δ18O-NO3~2.8±0.2‰) the origin of nutrients within the Atlantic inflow is traced to the north Atlantic subpolar gyre. Biological uptake peaks in the weakly stratified Nansen basin, extending throughout the entire halocline (100m), eventually limited by Si availability and transient Fe availability. In the eventuality that Si limitation is resolved with ongoing warming, this study estimates that new productivity in Nansen Basin can double from the current 15g C/m2 to ~33g C/m2. However, the probability that Nansen basin productivity will increase with warming is low given the Atlantification of Atlantic inflow regions, which generates further Si-limitation and unstable light conditions through deep mixing, creating an environment restrictive to biological uptake. Thus, although the systematics behind nutrient limitation are very different between Nansen versus Amundsen and Makarov basins, leading to the formation of an east-west biogeochemical gradient which will only strengthen with ongoing warming, this study shows that there is limited potential for primary productivity to increase anywhere in the Central Arctic Ocean. Through complementary use of respiration stoichiometry and δ¹⁸O-NO3 signatures, this study also assesses the role of in situ export production versus lateral advection of shelf organic matter in shaping water column remineralisation and O2 consumption in the Central Arctic. Lighter δ¹⁸O-NO3 (1.4-1.5‰) in Central Arctic deep waters compared to their Atlantic source (~2‰) indicate that ~50% of the deep nutrient pool in the Central Arctic has been recycled, exhibiting a strong correlation with deep O2 concentrations and ventilation age. It is revealed that ~90% of the carbon remineralisation and O2 consumption measured in Central Arctic deep waters are associated with the lateral supply of shelf organic matter. These findings prove that the Arctic shelf carbon pump drives long-term carbon sequestration in the deep Central Arctic, turning the study area into a carbon sink, with potential mitigating effects on global warming trends. The heavier δ¹⁵N-NO₃ signature (5.2‰-5.5‰) in Central Arctic deep waters compared to their Atlantic source (~4.9‰) reflects regional differences in nitrogen supply, reinforcing the strong sensitivity of deep Arctic biogeochemistry to far-field shelf nutrient cycling. There is a westward increase in deep δ¹⁵N-NO₃, reaching 5.5‰ in Makarov basin explained by remineralisation of isotopically heavy organic matter laterally advected from the Siberian shelves. This study further identified heavier δ¹⁸O-NO3 signatures and lower N* in bottom waters of Makarov basin indicative of benthic denitrification impacting the bottom water nitrate pool. It is argued that this denitrification signal is a recent feature of Makarov basin developing in response to warming-induced increases in Siberian shelf productivity and changes in the control mechanisms of shelf exports. Benthic denitrification is expected to amplify and expand across the Central Arctic bottom waters with ongoing warming, as the concentration-based diffusion of O2 into the deep sediment layer decreases. Under current climate change, the Arctic Ocean is becoming a greater sink for fixed nitrogen. As a result, the stoichiometry of Arctic exports to the North Atlantic is changing. Excess Si and PO4 exports through Fram Strait fuel 10% of nitrogen fixation in North Atlantic. These fluxes are expected to amplify with ongoing warming which can lead to autotrophic shifts and biogeochemical changes that may propagate at a global scale through the meridional overturning circulation.