The Marine carbon cycle. Carbonate chemistry Carbon pumps Sea surface pCO 2 and air-sea flux The sink for anthropogenic CO 2 Seawater Carbonate chemistry Inorganic carbon exists as several forms in sea water: Hydrated dissolved CO 2 gas.
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CO2(g) + H2O H2CO3
H2CO3 H+ + HCO3-
HCO3-H+ + CO32-
Typically, 90% of the carbon exists as bicarbonate,
9% as carbonate,
1% as dissolved CO2 and undissociated H2CO3 (usually lumped together).
Seawater Partial pressure of CO2
[CO2(g)] = x pCO2sw = where
*pCO2air is determined by the atmospheric mixing ratio, i.e. if the mixing ratio is 370ppm and atmospheric pressure is 1 atm, pCO2air is 370 atm.
Global mean air-sea flux, calculated from pCO2 measurements
The flux is set by patterns of sea-surface pCO2sw, forced by:
The surface wind-driven circulation
The overturning thermohaline circulation
Water cools and sinks
Water warms and upwells?
Global ocean biological production
In high productivity regions, CO2 is taken out of the surface water by plankton growth and sinks in a particle "rain" to depth.
CO2 + H2O CH2O +O2.
Ca++ + 2HCO3- H2O +CO2 + CaCO3
Coccolithophores -- calcite precipitating plankton
Bomb radiocarbon x 1020 atoms
Tropospheric bomb radiocarbon
The atmospheric bomb tests of the 50s and 60s injected a “spike” of radiocarbon into the atmosphere which was subsequently tracked into the ocean. This signal provides a good proxy for anthropogenic CO2 over decadal time scales.
Log10 number of deaths per conflict
3-D model outputs for surface pCO2
How well is the global ocean sink known?
Estimates of the global ocean sink 1990-1999
ReferenceSink (Pg C yr-1)
IPCC (2001) 1.7+/- 0.5
OCMIP-2 Model 2.5+/- 0.4
(ten ocean carbon
Not very well!
Will ocean uptake change in the future?
Source: Manabe and Stouffer, Nature 364, 1993
Iron fertilisation -- deliberate or
pH change mediates against calcite-
Reduction in THC offset by increased
efficiency of nutrient utilisation
Other unforeseen ecosystem changes
ProcessEffect on CO2 uptake