Lecture 17
Oxygen distributions and ocean ventilation
Thermocline Ventilation and Deep Water Formation
Oxygen Utilization rates
The Ocean Conveyor
What is it conveying? (at the surface? and at depth?)
Summary of vertical circulation
What is happening above the thermocline boundary? Fill in appropriate arrows.
Sinking and Remineralization
Remineralization keeps the biological pump pumping!
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%
Time series of northern hemisphere atmospheric concentrations
and tritium in North Atlantic surface waters. When will CFCs not be
a good clock.
Changing atmospheric gases
Comparison of atmospheric history of tritium and 14C
Tritium 3H: τ1/2 = 12.5 y
⇒ 3He + β
as H20 (or HTO)
3H
3H
= A conservative, radioactive tracer
In rain in Ireland
Surface density, isopycnal outcrops
Waters will move mostly along surfaces of constant density.
Example: Thermocline ventilation: CFCs
Locate and define the outcrop of this isopycnal (constant density) surface
Tritium is a conservative tracer for water (as HTO)
– thermocline penetration
Eq
Meridional Section in the Pacific
Tritium Contours
(as of GEOSECS)
¼ of surface
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
Example: Deep horizontal flow: 14C
Oxygen is a tracer of both physical and
biological changes
Apparent
Oxygen
Utilization
AOU = O2sat -O2
∆O2 = ∆O2sat - ∆AOU
Surface O2 Saturation
Composite cruise track
Oxygen Distribution
Apparent Oxygen Utilization
How and why do we define the quantity called AOU?
Surface fingerprints: ventilation
Atm.
Atm.
Mixed
layer
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…
thermocline
ventilation
Air-sea
O2 flux
outcrop
subtropics
∆AOU
time
Surface fingerprints: export
Atm.
Atm.
Mixed
layer
Mixed
layer
Increase
export flux
thermocline
Similar AOU anomalies
may be caused by
increased export flux,
with very different
signatures of O2/CO2
gas exchange.
thermocline
Export flux
subtropics
Air-sea
O2 flux
∆AOU
time
outcrop
Example:Oxygen
Utilization Rate
calculated from
AOU versus age
Example for one
density surface
σθ = 26.80
Jenkins (1982), Nature, 300, 246
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
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 was
= 3.8 mol C m-2 y-1
Two independent estimates – remarkably close agreement!
Deep Ocean Respiration
Oxygen
Utilization
Rate:
AOU
OUR =
age