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Lecture 16 Oxygen distributions and ocean ventilation PowerPoint PPT Presentation


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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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Lecture 16 Oxygen distributions and ocean ventilation

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Lecture 16

Oxygen distributions and ocean ventilation

Thermocline Ventilation and Deep Water Formation

Oxygen 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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Surface density, isopycnal outcrops

Waters will move mostly along surfaces of constant density.


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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


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Remineralization Ratios versus Depth

average for 400m to 4000m

P N C O2

1 : 16±1 : 117±14 : 170±10

Anderson and Sarmiento, 1994)


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Compare with Hedges Model

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


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Atmospheric Record of Thermocline Ventilation Tracers

Conservative, non-radioactive tracers (CFC-11, CFC-12, CFC-13, SF6)

down 2%

CFC-11

Propellent

Styrofoam

CFC-12

Air conditioning

Refrigerators

CFC-113

solvent

SF6

Transformers

Nike/Mercedes

down 10%

Tritium3H:

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?


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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 for

thermocline ventilation

¼ of surface


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Example: Thermocline ventilation: Tritium

Tritium/Helium Age (yr)

Why does Tritium concentration slightly different from Tritium/Helium Age?

see Jenkins (1998) JGR, 103, 15,817


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Example:Oxygen

Utilization Rate

calculated from

AOU versus age

Example for one

density surface

sθ = 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

Integrated OUR from

100m to depth

 OUR = 5.7 mol O2 m-2 yr-1


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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 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!


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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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Atm.

Mixed layer

thermocline

ventilation

outcrop

Air-sea O2 flux

subtropics

ΔAOU

time

Surface fingerprints: ventilation

Atm.

Mixed layer

Decrease

ventilation

thermocline

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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outcrop

Surface fingerprints: export

Atm.

Atm.

Mixed layer

Mixed layer

Increase

export flux

thermocline

thermocline

Export flux

Similar AOU anomalies may be caused by increased export flux, with very different signatures of O2/CO2 gas exchange.

subtropics

Air-sea O2 flux

ΔAOU

time


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Tritium3H: t1/2 = 12.5 y

3H  3He + b

as H20 (or HTO)

3H = A conservative, radioactive tracer

In rain in Ireland


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Example: Thermocline ventilation: CFCs

Locate and define the outcrop of this isopycnal (constant density) surface


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Tritium is a conservative tracer for water (as HTO)

– thermocline penetration

Eq

Meridional Section in the Pacific


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Deep Ocean Respiration

AOU

OUR =

age

Oxygen

Utilization

Rate:


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Surface O2 Saturation


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Oxygen Distribution


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