macro-nutrient transport pathways and interactions with the iron cycle. export and remineralization...
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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:
Mick Follows, Stephanie Dutkiewicz, Payal Parekh: MITTaka Ito: University of Washington
Ric Williams: University of Liverpool
Return of macro-nutrients to euphotic zone requires diapycnal transfer
Atlantic basin
Return of macro-nutrients to euphotic zone requires diapycnal transfer
Pacific basin
Atlantic basin
Upwelling in Southern Ocean major return pathway...
Mode and intermediate waters formed equatorwards of ACC - pathway for macro-nutrients to northern basins
(Sarmiento et al., 2003)
surface nitrate surface silica
Mode waters Hanawa and Talley (2001)
Residual mean flow transports nutrients northwards to sites of mode and intermediate water formation
Marshall (1997)
Ψres
= ΨEkman
+ Ψeddy
Residual mean flow transports nutrients northwards to sites of mode and intermediate water formation
Marshall (1997)
Ψres
= ΨEkman
+ Ψeddy
NO3 (uM)
upwelling+NO
3
-Fe
dust
iron limited macro-nutrient limited
subduction
● balance between northward residual mean advection and export sets subducted nutrient concentration
● export sensitive to iron availability
Macro-nutrient supply to the subtropical gyres.
WOCE A20 North Atlantic
Convergence of horizontal Ekman nitrate flux in N. Atlantic (10-3 mol N m-2 yr-1)
Lateral Ekman transfer into N. Atlantic subtropical gyres
Williams and Follows (1998)
Role of eddies? Residual mean flow
North Atlantic subtropical gyre: schematic nutrient budget
Ekman transfer significant source to subtropical bowl
(~ mol N m-2 yr-1) organic export (Jenkins, 1988)
Connecting southern and northern hemispheres:“nutrient stream” centred at σ
θ = 27.0 supplies nutrients
to northern gyres (Pelegri and Csanady, 1991)
σθ = 27.0
PO4* (“conservative”) (μM) NO
3 (μM)
Nutrient stream outcrops close to intergyre boundary in winter
March NO3 and σ
θ (10m)
World Ocean Atlas
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
Aeolian Iron Source (mmol Fe m-2 yr-1)
Luo et al (2003)
Sensitivity studies with uniform “high” and “low” aeolian iron flux...
difference in primary producitivity(high – low) aeolian iron supply (g C m-2 y-1)
Sensitivity of primary production to aeolian iron source
More dust, higher productivity
More dust, lower productivity
Pacific basin reflects regulation of intergyre exchange
Atlantic productivity reflects southern ocean surface macro-nutrient utilization
Atlantic
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.
Two regimes:
- iron limited upwelling regions - macro-nutrient limited subtropical gyres + Atlantic
Export and remineralization of sinking, organic particles moves nutrients to denser isopyncals
Atlantic basin
Pelegri and Csanady (1991): “nutrient stream”
core of stream at σθ ~ 27.0
v NO3 at 36N
Numerical model
Eulerian mean(Ekman) upwelling(200m)
Residual meanupwelling (200m)
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