Concept test We, human beings, along with all animals are causing a net increase of atmospheric CO 2 because our breath contains CO 2 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 O 2 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 O 2 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 O 2 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 CO 2 Source: photosynthesis Sink: respiration Geologic timescale Burial of organic matter leads to atmospheric O 2 increase

Observations of oceanic oxygen

Oceanic oxygen Near saturation at the surface -O 2 is within a few percent of saturation with atmosphere (high concentrations) -O 2 ≈ O 2sat (T,S) Consumed by respiration Depleted at depth -Hypoxic: below  molO 2 /kg -Suboxic: below  molO 2 /kg

Oxygen Minimum Zone Figure 3a

H H 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 O 2 to the ocean

Upwelling and biological productivity

Oxygen utilization Apparent Oxygen Utilization 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 Temperature increase Slower vertical exchange of waters

Future projection Model simulations for the 21 st 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 Computer code Initial cond (time=t) T, S, nutrient, carbon, alkalinity Boundary cond: Heating, wind, dust, atmos pCO2, dust… time = t+1 T, S, nutrient, carbon, alkalinity, biological productivity, …

A hindcast simulation Climatology Model climatology O 2 on   = 26.8 Expansion of OMZ during late 20 th century Minimum extent of OMZ around mid 1970s Eastern tropical Pacific O 2

ENSO cycle and O 2 Compensations +O 2 inventory OMZ contraction -O 2 inventory OMZ expansion Major El-Nino events O2O2 - AOU O 2sat

The Mechanism: Upwelling and AOU La-Nina El-Nino Deutsch et al. (2011) Colder and increased O 2sat Stronger lateral O 2 supply Increased biological O 2 consumption  OMZ expansion Warmer and decreased O 2sat Weaker lateral O 2 supply Decreased biological O 2 consumption  OMZ contraction

Land vs ocean CO 2 uptake

+ Ocean outgassing Decline of oceanic O2 O 2 outgassing Slope =  Slope =  Accounting for ocean O 2 outgassing implies that CO 2 uptake by land must have been smaller, and ocean CO 2 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 CO 2 uptake. Biological Productivity Atmospheric Climate Nutrient Supply Changes in ocean circulation and biological productivity Oceanic carbon, O2 cycle Transport + mixing Photosynthesis + respiration O 2 changes indicate: