Lecture 16 Oxygen distributions and ocean ventilation

Thermocline Ventilation and Deep Water FormationOxygen Utilization rates

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

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.

Nutrients versus AOU

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.

The Ocean Conveyor

What is it conveying? (at the surface? and at depth?)

Meridional Overturning Sinking and Remineralization

Remineralization keeps the biological pump pumping!

Winter Outcrops of Isopycnal Surfaces

Waters will move mostly along surfaces of constant density.

Surface density, isopycnal outcrops

P = [PO4] - [PO4] = RPO4/O2 x AOU N = [NO3] - [NO3] = RNO3/O2 x AOU

on = 27.0 to 27.2 Takahashi et al, 1985

Remineralization Ratios versus Depth

Anderson and Sarmiento, 1994)average for 400m to 4000mP N C O2

1 : 16±1 : 117±14 : 170±10

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

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)

Changing atmospheric gases

Comparison of atmospheric history of tritium and 14C

Example: Deep horizontal flow: 14C

Tritium Contours(as of GEOSECS)

A good tracer forthermocline ventilation

¼ of surface

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

Example:Oxygen Utilization Rate calculated from AOU versus age

Example for onedensity surfaceθ = 26.80

Jenkins (1982), Nature, 300, 246

Winter Outcrops of Isopycnal Surfaces

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

Comparison with O2 Flux approach

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!

Composite cruise track

Apparent Oxygen Utilization

How and why do we define the quantity called AOU?

Oxygen is a tracer of both physical and biological changes

Apparent Oxygen Utilization

AOU = O2sat -O2

ΔO2 = ΔO2sat - ΔAOU

(from Deutsch et al)

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…

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.

Tritium 3H: 1/2 = 12.5 y3H 3He + as H20 (or HTO)

3H = A conservative, radioactive tracer

In rain in Ireland

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

Example: Thermocline ventilation: CFCs

Tritium is a conservative tracer for water (as HTO) – thermocline penetration

Meridional Section in the Pacific

Eq

OUR = AOU age

Oxygen Utilization Rate:

Deep Ocean Respiration

Surface O2 Saturation

Oxygen Distribution