Lecture 16 Oxygen distributions and ocean ventilation Thermocline Ventilation and Deep Water...
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Transcript of Lecture 16 Oxygen distributions and ocean ventilation Thermocline Ventilation and Deep Water...
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Lecture 16 Oxygen distributions and ocean ventilation
Thermocline Ventilation and Deep Water FormationOxygen Utilization rates
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Aerobic respiration
Oxygen is consumed and nutrients are released.
(CH2O)106(NH3)16(H3PO4) + 138 O2
Algal Protoplasm
bacteria
106 CO2 + 16 HNO3 + H3PO4 + 122 H2O + trace elements
The oxidation of the NH3 in organic matter to NO3 is referred to as nitrification
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Apparent Oxygen Utilization (AOU)
Apparent Oxygen Utilization or AOU.
AOU is defined as:
AOU = O2' - O2
where: O2' = value of O2 the water would have if it was in equilibrium with the atmosphere at the temperature and salinity of the water.
This is called saturation. This implies that all waters are in equilibrium with the atmosphere (100% saturated) when they sink to become the deep ocean water.
O2 is the dissolved oxygen actually measured in the same water sample.
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Nutrients versus AOU
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Oxidative and Preformed Nutrients versus Depth
1 mol O2 = 106/138 mol CO2 + 16/138 mol HNO3 + 1/138 mol H3PO4
consumed = 0.77 CO2 + 0.12 HNO3 + 0.0072 H3PO4
But vertical profiles are not the best way to study this problem.
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The Ocean Conveyor
What is it conveying? (at the surface? and at depth?)
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Meridional Overturning Sinking and Remineralization
Remineralization keeps the biological pump pumping!
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Winter Outcrops of Isopycnal Surfaces
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Waters will move mostly along surfaces of constant density.
Surface density, isopycnal outcrops
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P = [PO4] - [PO4] = RPO4/O2 x AOU N = [NO3] - [NO3] = RNO3/O2 x AOU
on = 27.0 to 27.2 Takahashi et al, 1985
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Remineralization Ratios versus Depth
Anderson and Sarmiento, 1994)average for 400m to 4000mP N C O2
1 : 16±1 : 117±14 : 170±10
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It is clear that more O2 (~170 moles) is actually required to respire sinking organic matter than was originally calculated from the RKR equation (138 moles).
The RKR type organic matter has an oxidation state as for carbohydrate (CH2O). Real plankton have 65% protein, 19% lipid and 16% carbohydrate (from NMR studies)
The higher O2 demand suggests that sinking organic matter has more of a lipid-like nature.
Instead of:CH2O + O2 = CO2 + H2O
More like:CH2 + 3/2 O2 = CO2 + H2O
Real plankton biomass is more like
C106H177O37N17S0.4 instead of C106H260O106N16
Complete oxidation requires 154 moles of O2 instead of 138
Compare with Hedges Model
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Time series of northern hemisphere atmospheric concentrationsand tritium in North Atlantic surface waters. When will CFCs not be a good clock?
Atmospheric Record of Thermocline Ventilation TracersConservative, non-radioactive tracers (CFC-11, CFC-12, CFC-13, SF6)
CFC-11PropellentStyrofoamCFC-12Air conditioningRefrigeratorsCFC-113solventSF6TransformersNike/Mercedes
down 10%
down 2%
Tritium 3H: t1/2 = 12.5 y3H 3He + as H20 (or HTO)
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Changing atmospheric gases
Comparison of atmospheric history of tritium and 14C
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Example: Deep horizontal flow: 14C
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Tritium Contours(as of GEOSECS)
A good tracer forthermocline ventilation
¼ of surface
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Why does Tritium concentration slightly different from Tritium/Helium Age?
Example: Thermocline ventilation: Tritium
Tritium/Helium Age (yr)
see Jenkins (1998) JGR, 103, 15,817
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Example:Oxygen Utilization Rate calculated from AOU versus age
Example for onedensity surfaceθ = 26.80
Jenkins (1982), Nature, 300, 246
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Winter Outcrops of Isopycnal Surfaces
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OUR versus Depth
OUR decreases exponentially with depth below the euphotic zone (Z in m) according to:
ln OUR = -(0.68+0.17) - (0.00295+0.00027) Z
OUR = 5.7 mol O2 m-2 yr-1
Integrated OUR from100m to depth
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Comparison with O2 Flux approach
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OUR New Production
Convert the integrated O2 consumption to the POC flux requiredUse Takahashi et al (1985) stoichiometric ratio to convert C to O2
Integrated OUR x conversion = Integrated C oxidized
5.7 mol O2 m-2 y-1 x 106C/172O2 = 3.51 mol C m-2 y-1
For comparison in the last lecture we calculated the annual newproduction of C from the O2 mass balance in the euphotic zone.From that approach the new production is (using 106C/172O2)
= 3.1 mol C m-2 y-1
Two independent estimates – remarkably close agreement!
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Composite cruise track
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Apparent Oxygen Utilization
How and why do we define the quantity called AOU?
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Oxygen is a tracer of both physical and biological changes
Apparent Oxygen Utilization
AOU = O2sat -O2
ΔO2 = ΔO2sat - ΔAOU
(from Deutsch et al)
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Surface fingerprints: ventilation
thermocline
Mixed layer
Atm.
thermocline
Mixed layer
Atm.
Decrease
ventilation
ventilation
Air-sea O2 flux
ΔAOU
subtropics
time
outcrop
An increase in AOU due to decreased ventilation will cause changes in air-sea fluxes of both O2 and CO2 coincident with the ventilation change…
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Surface fingerprints: export
thermocline
Mixed layer
Atm.
thermocline
Mixed layer
Atm.
Increase
export flux
Export flux
Air-sea O2 flux
ΔAOU
outcrop
time
subtropics
Similar AOU anomalies may be caused by increased export flux, with very different signatures of O2/CO2 gas exchange.
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Tritium 3H: 1/2 = 12.5 y3H 3He + as H20 (or HTO)
3H = A conservative, radioactive tracer
In rain in Ireland
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Locate and define the outcrop of this isopycnal (constant density) surface
Example: Thermocline ventilation: CFCs
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Tritium is a conservative tracer for water (as HTO) – thermocline penetration
Meridional Section in the Pacific
Eq
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OUR = AOU age
Oxygen Utilization Rate:
Deep Ocean Respiration
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Surface O2 Saturation
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Oxygen Distribution