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 CYCLEObservationsProcessesClimate change impactsFuture 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

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.

Oxygen and marine ecosystem

Figure 1. Keeling et al., (2010)

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 ZoneFigure 3a

HH L

Ocean vertical motions

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

TemperatureHeat content of the ocean

Integrated effect of respirationBiological productivityAging of water masses

Declining oxygen in warming climateWarming and circulation change

Temperature increase

Slower vertical exchange of waters

Future projection• Model simulations for the 21st century

future climate

• What are the uncertainties?

Expanding OMZ?!

Deutsch et al. (2011)

California Current Region

Stramma et al. (2008)

Tropical Pacific Region

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

Computercode

Initial cond (time=t) T, S, nutrient, carbon, alkalinity

Boundary cond:Heating, wind, dust, atmos pCO2, dust…

time = t+1T, S, nutrient, carbon, alkalinity, biological productivity, …

A hindcast simulationClimatology

Model climatology

O2 on sq = 26.8• Expansion of OMZ during late 20th

century

• Minimum extent of OMZ around mid

1970s Eastern tropical Pacific O2

ENSO cycle and O2 Compensations

+O2 inventoryOMZ

contraction

-O2

inventoryOMZ

expansionMajor El-Nino events

O2- AOU

O2sat

The Mechanism: Upwelling and AOU

La-Nina El-Nino

Deutsch et al. (2011)

• Colder and increased O2sat

• Stronger lateral O2 supply

• Increased biological O2

consumption

OMZ expansion

• Warmer and decreased O2sat

• Weaker lateral O2 supply

• Decreased biological O2

consumption

OMZ contraction

Land vs ocean CO2 uptake

Land vs ocean CO2 uptake

+ Ocean outgassing

Decline of oceanic O2

O2 outgassing

Slope = a

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 circulation

Oxygen, ocean biogeochemistry and climate

Heat, water, momentum exchange

Light, SST, dust

Greenhouse effect

Changes in the carbon cycle and ocean CO2 uptake.

Biological Productivity

Atmospheric Climate

Nutrient Supply

Changes in ocean circulation and biological productivity

Oceanic carbon, O2

cycle

Transport + mixing

Photosynthesis + respiration

O2 changes indicate: