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.  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.