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Macro-Nutrient Transport Pathways and Interactions with the Iron Cycle.

Export and remineralization of sinking, organic particles moves nutrients to denser isopyncals:. Macro-Nutrient Transport Pathways and Interactions with the Iron Cycle. Mick Follows , Stephanie Dutkiewicz , Payal Parekh : MIT Taka Ito : University of Washington

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Macro-Nutrient Transport Pathways and Interactions with the Iron Cycle.

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  1. Export and remineralization of sinking, organic particles moves nutrients to denser isopyncals: Macro-Nutrient Transport Pathways and Interactions with the Iron Cycle. Mick Follows, Stephanie Dutkiewicz, Payal Parekh: MIT Taka Ito: University of Washington Ric Williams: University of Liverpool

  2. Return of macro-nutrients to euphotic zone requires diapycnal transfer Atlantic basin

  3. Return of macro-nutrients to euphotic zone requires diapycnal transfer Pacific basin

  4. Upwelling in Southern Ocean major return pathway... Atlantic basin

  5. surface nitrate surface silica Mode and intermediate waters formed equatorwards of ACC - pathway for macro-nutrients to northern basins (Sarmiento et al., 2003)

  6. Mode waters Hanawa and Talley (2001)

  7. Marshall (1997) Residual mean flowtransports nutrients northwards to sites of mode and intermediate water formation Ψres = ΨEkman + Ψeddy

  8. Marshall (1997) Residual mean flowtransports nutrients northwards to sites of mode and intermediate water formation Ψres = ΨEkman + Ψeddy

  9. iron limited macro-nutrient limited • balance between northward residual mean advection and export sets subducted nutrient concentration • export sensitive to iron availability dust subduction upwelling +NO3 -Fe NO3 (uM)

  10. Macro-nutrient supply to the subtropical gyres. WOCE A20 North Atlantic

  11. Role of eddies? Residual mean flow Lateral Ekman transfer into N. Atlantic subtropical gyres Williams and Follows (1998) Convergence of horizontal Ekman nitrate flux in N. Atlantic (10-3 mol N m-2 yr-1)

  12. North Atlantic subtropical gyre: schematic nutrient budget Ekman transfer significant source to subtropical bowl (~ mol N m-2 yr-1) organic export (Jenkins, 1988)

  13. Connecting southern and northern hemispheres: “nutrient stream” centred at σθ = 27.0supplies nutrients to northern gyres (Pelegri and Csanady, 1991) σθ = 27.0 PO4* (“conservative”) (μM) NO3 (μM)

  14. Nutrient stream outcrops close to intergyre boundary in winter March NO3 and σθ (10m) World Ocean Atlas

  15. Illustration in a global biogeochemical model: regulation of macro-nutrient pathways by aeolian iron source (Dutkiewicz et al., 2005) Modeled Surface Chl (mg m-3) • Explicit, coupled phosphorus, silica and iron cycles • Two phytoplankton classes: Diatoms and "other" phytoplankton • Single grazer • Prescribed aeolian iron source

  16. Aeolian Iron Source (mmol Fe m-2 yr-1) Luo et al (2003) Sensitivity studies with uniform “high” and “low” aeolian iron flux...

  17. Sensitivity of primary production to aeolian iron source difference in primary producitivity (high – low) aeolian iron supply (g C m-2 y-1) More dust, higher productivity More dust, lower productivity

  18. Pacific basin reflects regulation of intergyre exchange

  19. Atlantic productivity reflects southern ocean surface macro- nutrient utilization Atlantic

  20. Summary • Lateral transfer in surface ocean is significant route for diapycnal return of macro-nutrients to light isopycnals. (Southern Ocean, inter-gyre boundaries) • Advection by residual mean flow (Ekman + eddy) is key physical process • Balance between residual mean advection and iron stress regulates the lateral fluxes of macro-nutrients.

  21. Two regimes: - iron limited upwelling regions - macro-nutrient limited subtropical gyres + Atlantic

  22. Export and remineralization of sinking, organic particles moves nutrients to denser isopyncals Atlantic basin

  23. Pelegri and Csanady (1991): “nutrient stream” core of stream at σθ ~ 27.0 v NO3 at 36N

  24. Numerical model Eulerian mean (Ekman) upwelling (200m) Residual mean upwelling (200m)

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