Concept test
• We, human beings, along with all animals are causing a
net increase of atmospheric CO2 because our breath
contains CO2 when we exhale.
• (1) True
• (2) False
• (3) Debatable
OXYGEN CYCLE
Observations
Processes
Climate change impacts
Future projections
Measuring oxygen in the seawater
• Oxygen is the third most frequently measured
ocean tracer following T and S
• Winkler titration method (1888)
- Wet chemistry, performed on site
- Most accurate
• Electronic and optical sensors
- Calibration is crucial
- Possible to deploy on floats, gliders, etc
Observed oxygen distribution
Observed variability of oceanic O2
• Global ocean deoxygenation?
Gruber et al. (2007)
NPIW
Stramma et al
2008
Oxygen and marine ecosystem
Figure 1. Keeling et al., (2010)
Image from an ROV off the Oregon coast
after a low oxygen event
Low O2 can reduce the respiratory capacity of marine heterotrophs,
leading to reduced physiological performance or death.
Glacial-interglacial changes?
• The intensity of the North Pacific
Oxygen Minimum Zone has varied
over the past 100ky.
• This has implications for the
ocean’s redox chemistry and the
overall fertility of the ocean.
• Reasons for the O2 changes are
unclear.
Van Geen et al. [2003]
Global oxygen cycle: processes
Vertical profile of
observed O2
Controls on atmospheric oxygen
• More than 90% of oxygen molecule is in the atmosphere
• 20% of atmospheric gas
• Opposite tendency relative to CO2
• Source: photosynthesis
• Sink: respiration
• Geologic timescale
• Burial of organic matter leads
to atmospheric O2 increase
Observations of oceanic oxygen
Oceanic oxygen
• Near saturation at the surface
- O2 is within a few percent of saturation with
atmosphere (high concentrations)
- O2 ≈ O2sat(T,S)
• Consumed by respiration
• Depleted at depth
- Hypoxic: below 60 mmolO2/kg
- Suboxic: below 5 mmolO2/kg
Oxygen Minimum Zone
Figure 3a
Ocean vertical motions
H
L
H
Distribution of oxygen-depleted waters
• Indo-Pacific is more depleted in oxygen relative to
the Atlantic
• Oxygen gets depleted in some coastal regions
Downwelling supplies O2 to the ocean
Upwelling and biological productivity
Oxygen utilization
Apparent Oxygen Utilization
O2 = O2 sat (T, S) - AOU
Solubility effect
Biological
effect
Temperature
Heat content of the ocean
Integrated effect of respiration
Biological productivity
Aging of water masses
Declining oxygen in warming climate
Warming and circulation change
O2 = O2 sat (T, S) - AOU
Solubility effect
Biological
effect
Temperature increase
Slower vertical exchange of
waters
Future projection
• Model simulations for the 21st century
future climate
• What are the uncertainties?
Expanding OMZ?!
Tropical Pacific Region
Stramma et al. (2008)
California Current Region
Deutsch et al. (2011)
Decadal-scale fluctuations or climate trend?
Ocean biogeochemistry model
• Divide up the oceans into grid cells
• Input: solar heating, rain rate, surface wind, …
• Given the state at time=t, predicts for t+1
Initial cond
(time=t)
T, S, nutrient,
carbon, alkalinity
Boundary cond:
Heating, wind,
dust, atmos
pCO2, dust…
Computer
code
time = t+1
T, S, nutrient,
carbon, alkalinity,
biological
productivity, …
A hindcast simulation
O2 on sq = 26.8
Climatology
• Expansion of OMZ during late 20th
century
• Minimum extent of OMZ around mid
1970s Eastern tropical Pacific
O2
Model climatology
ENSO cycle and O2 Compensations
+O2 inventory
OMZ
contraction
- AOU
-O2
inventory
OMZ
expansion
O2
O2sat
Major El-Nino events
The Mechanism: Upwelling and AOU
Deutsch et al. (2011)
El-Nino
La-Nina
• Colder and increased O2sat
• Warmer and decreased O2sat
• Stronger lateral O2 supply
• Weaker lateral O2 supply
• Increased biological O2
• Decreased biological O2
consumption
 OMZ expansion
consumption
 OMZ contraction
Land vs ocean CO2 uptake
Land vs ocean CO2 uptake
O2
outgassing
Slope = a
Decline of
oceanic O2
Slope = b
Accounting for ocean O2
outgassing implies that CO2
uptake by land must have been
smaller, and ocean CO2 uptake
larger than previously thought.
+ Ocean
outgassing
Oxygen, ocean biogeochemistry
and climate
Atmospheric
Climate
Greenhouse
effect
Light,
SST,
dust
Heat, water,
momentum
exchange
Ocean
circulation
Nutrient
Supply
O2 changes
indicate:
Biological
Productivity
Transport
+ mixing
Photosynthesis
+ respiration
Oceanic
carbon, O2
cycle
Changes in ocean
circulation and
biological productivity
Changes in the carbon
cycle and ocean CO2
uptake.