Lecture 16 Oxygen distributions and ocean ventilation. Thermocline Ventilation and Deep Water Formation Oxygen Utilization rates. Aerobic respiration Oxygen is consumed and nutrients are released. (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 138 O 2 Algal Protoplasm bacteria
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Oxygen distributions and ocean ventilation
Thermocline Ventilation and Deep Water Formation
Oxygen Utilization rates
Oxygen is consumed and nutrients are released.
(CH2O)106(NH3)16(H3PO4) + 138 O2
106 CO2 + 16 HNO3 + H3PO4 + 122 H2O + trace elements
The oxidation of the NH3 in organic matter to NO3
is referred to as nitrification
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.
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.
What is it conveying? (at the surface? and at depth?)
Remineralization keeps the biological pump pumping!
Waters will move mostly along surfaces of constant density.
DP = [PO4] - [PO4] = RPO4/O2 x AOU
DN = [NO3] - [NO3] = RNO3/O2 x AOU
on s = 27.0 to 27.2
Takahashi et al, 1985
Remineralization Ratios versus Depth
average for 400m to 4000m
P N C O2
1 : 16±1 : 117±14 : 170±10
Anderson and Sarmiento, 1994)
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.
CH2O + O2 = CO2 + H2O
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
Conservative, non-radioactive tracers (CFC-11, CFC-12, CFC-13, SF6)
t1/2 = 12.5 y
3H 3He + b
as H20 (or HTO)
Time series of northern hemisphere atmospheric concentrations
and tritium in North Atlantic surface waters.
When will CFCs not be a good clock?
Comparison of atmospheric history of tritium and 14C
(as of GEOSECS)
A good tracer for
¼ of surface
Tritium/Helium Age (yr)
Why does Tritium concentration slightly different from Tritium/Helium Age?
see Jenkins (1998) JGR, 103, 15,817
AOU versus age
Example for one
sθ = 26.80
Jenkins (1982), Nature, 300, 246
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
Integrated OUR from
100m to depth
OUR = 5.7 mol O2 m-2 yr-1
Comparison with O2 Flux approach
OUR New Production
Convert the integrated O2 consumption to the POC flux required
Use 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 new
production 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!
How and why do we define the quantity called AOU?
Apparent Oxygen Utilization
AOU = O2sat -O2
ΔO2 = ΔO2sat - ΔAOU
(from Deutsch et al)
Air-sea O2 flux
timeSurface fingerprints: ventilation
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…
Similar AOU anomalies may be caused by increased export flux, with very different signatures of O2/CO2 gas exchange.
Air-sea O2 flux
Tritium3H: t1/2 = 12.5 y
3H 3He + b
as H20 (or HTO)
3H = A conservative, radioactive tracer
In rain in Ireland
Locate and define the outcrop of this isopycnal (constant density) surface
– thermocline penetration
Meridional Section in the Pacific
Surface O2 Saturation