The Marine carbon cycle.Carbonate chemistryCarbon pumpsSea surface pCO2 and air-sea fluxThe sink for anthropogenic CO2
Seawater Carbonate chemistry • Inorganic carbon exists as several forms in sea water: • Hydrated dissolved CO2 gas. • This rapidly reacts with H2O to form undissociated carbonic acid: CO2(g) + H2O H2CO3 • Which can dissociate by loss of H+ to form bicarbonate ion: H2CO3 H+ + HCO3- • which can dissociate by further loss of H+ to form carbonate ion: 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 • The partial pressure of CO2 of the sea water (pCO2sw) determines whether there is flux from air to sea or sea to air: • Air-to-sea Flux is proportional to (pCO2air* - pCO2sw) • pCO2sw is proportional to dissolved CO2(g): [CO2(g)] = x pCO2sw = where • is the solubility of CO2. The solubility decreases with increasing temperature. *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 • Air-sea flux is variable. • In some regions the net flux is from sea to air, in others from air to sea. • Averaged over the whole ocean, the net flux is into the ocean, about 2 Pg C yr-1
What sets the net air-sea flux? The flux is set by patterns of sea-surface pCO2sw, forced by: • Ocean circulation; • Is surface water is cooling or heating? • Is water being mixed up from depth? • Ocean biology; • Is biological activity strong or weak? • Is calcium carbonate being precipitated? • The rising concentration of atmospheric CO2 • pCO2 of air is rising and this tends to favour a flux from atmosphere into the ocean.
The surface wind-driven circulation • Poleward-going currents are warm water • They are associated with cooling water • Tend to be regions of uptake of CO2 from the atmosphere. • Equator-going currents – vice-versa
The overturning thermohaline circulation Water cools and sinks Water warms and upwells? • The Northern North Atlantic is a region of strong cooling, associated with the North Atlantic drift. • Cooling water takes up CO2 and may subsequently sink. • The water upwells in other parts of the world ocean, particularly the equatorial Pacific. • Upwelling regions are usually sources of CO2 to the atmosphere – deep water has high CO2 and the water is being warmed. • This circulation controls how rapidly old ocean water is brought to the surface, and therefore how quickly the ocean equilibrates to changes in atmospheric CO2 concentration.
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.
Ocean carbon “pumps” • Deep water has higher (10-20%) total carbon content and nutrient concentrations than surface water. There are several processes contributing to this: • The "Solubility pump" tends to keep the deep sea higher in total inorganic carbon (CO2) compared to the warm surface ocean. • The “Biological pump(s)" – the flux of biological detritus from the surface to deep, enriches deep water concentrations. There are two distinct phases of the carbon in this material: • The "soft tissue" pump enriches the deep sea in inorganic carbon and nutrients by transport of organic carbon compounds. • The calcium carbonate pump enriches the deep sea in inorganic carbon and calcium.
Carbonate Soft tissue Ocean biological pumps • Falling dead organisms, faecal pellets and detritus are "remineralised" at depth. Remineralization occurs • By bacterial activity. • By inorganic dissolution of carbonate below the lysocline. • The different phases have different depth profiles for remineralisation.
Ocean Carbon: The Biological (soft tissue) Pump • This mechanism acts continually to reduce the partial pressure of CO2 (pCO2) in the surface ocean, and increase it at depth. • Over most of the ocean, upwelling water is depleted of inorganic carbon and nutrients (nitrate and phosphate) by plankton. • In the process they remove about 10% of the inorganic CO2 in the water. Most of this goes to form organic matter via the reaction: CO2 + H2O CH2O +O2. • Because the buffer factor ~10, this has a large effect on surface pCO2, decreasing it by 2-3 times. • The reverse reaction occurs by (mostly bacterial) respiration at depth, and increases CO2 concentration there. Depth
Surface pCO2, nutrient and surface temperature in the North Atlantic 360 Mar Apr May Jun Jul Aug Sep Oct 340 18 SST SST (°C) 320 8 16 ) atm m ( 300 6 2 pCO 2 pCO 14 Nitrate ( m 4 280 M ) 12 2 260 Nitrate
The biological (calcium carbonate) pump. • This mechanism also transfers carbon from the surface ocean to the deep sea. • Some of the carbon taken up by the biota in surface waters goes to form calcium carbonate. • The CaCO3 sinks to the deep sea, where some of it may re-dissolve and some become sedimented. The redissolution can only occur below the lysocline, which is shallower in the Pacific than the Atlantic. • In contrast to the soft tissue pump, this mechanism tends to increase surface ocean pCO2 and therefore atmospheric CO2 . The net reaction is: Ca++ + 2HCO3- H2O +CO2 + CaCO3
The Solubility Pump • This mechanism also tends to increase deep sea carbon at the expense of surface ocean and atmospheric carbon. • The solubility of CO2 increases as temperature decreases. So cold water, which is what forms deep water, tends to dissolve CO2 from the atmosphere before it sinks. • Deep water would therefore have a higher CO2 content than most surface water, even without any biological activity.
Biological influence on air-sea flux. • Blooms of plankton fix carbon dioxide from the water and lower CO2, hence pCO2. • Particularly marked in the North Atlantic which has the most intense bloom of any major ocean region. • In the equatorial Pacific, plankton blooms are suppressed by lack of iron – part of the explanation for high pCO2there. • In the equatorial Atlantic, upwelling is less intense and there is more iron from atmospheric dust.
Circulation influence on air-sea flux • Warm currents, where water is cooling, are normally sink regions (NW Atlantic, Pacific). • Source regions for subsurface water, where water is cooled sufficiently to sink are strong sinks (N. N. Atlantic, temperate Southern ocean).. • Tropical upwelling zones, where subsurface water comes to the surface and is strongly heated, are strong sources (equatorial Pacific).
The ocean sink for anthropogenic CO2 • The oceans are close to steady-state with respect to atmospheric CO2. • Prior to the industrial revolution, the oceans were a net source of order 0.5 Pg C yr-1 CO2 to the atmosphere. Today they are net sink of order 2 Pg C yr-1. • The main factor controlling ocean uptake is the slow overturning circulation, which limits the rate at which the ocean mixes vertically. • Two methods are being used to calculate the size of the ocean sink. • Measurements of atmospheric oxygen and CO2 (last lecture). • Models of ocean circulation. These are of two types: • Relatively simple box-diffusion models “calibrated” so that they reproduce the uptake of tracers such as bomb-produced 14carbon. • Ocean GCMs which attempt to diagnose the uptake from the circulation. (However, the overturning circulation is difficult to model correctly. In practice these models are also tested against ocean tracers.)
300 Bomb radiocarbon x 1020 atoms 200 100 1950 1960 1970 1980 1990 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 • Capture the basic elements of the sources and sinks distribution. • Considerable discrepancies with one another and with the data (Southern Ocean, North Atlantic).
How well is the global ocean sink known? Estimates of the global ocean sink 1990-1999 Reference Sink (Pg C yr-1) IPCC (2001) 1.7+/- 0.5 Estimate (Keeling oxygen technique) OCMIP-2 Model 2.5+/- 0.4 Intercomparison (ten ocean carbon models). Not very well!
Will ocean uptake change in the future? • Yes: the models forecast that the sink will increase in the short term as increasing atmospheric CO2 forces more into the oceans. • But, the buffering capacity of the ocean becomes less as CO2 increases, tending to decrease uptake. • Also, if ocean overturning slows down, this would tend to decrease the uptake. • Changes in ocean biology may also have an impact….
Possible Marine biological effects on Carbon uptake, next 100 years. Iron fertilisation -- deliberate or inadvertent NO3 fertilisation pH change mediates against calcite- precipitating organisms Reduction in THC offset by increased efficiency of nutrient utilisation Other unforeseen ecosystem changes Process Effect on CO2 uptake ?
Conclusions • The ocean CO2 sink is affected both by circulation and biology. Changes in either would affect how much CO2 is taken up by the ocean. Climate change may cause both. • Because different methods agree roughly on the size of the global ocean sink, it has generally been assumed that we know it reasonably well. • However, there is an increasing discrepancy between the most accurate methods. Our present understanding allows us to specify the sink only to ~35%. • We cannot at present specify how it changes from year to year or decade-to-decade. • Acccurate knowledge of the ocean sink would enable us (via atmospheric inverse modelling) to be much more specific about the terrestrial sinks – useful for verification of Kyoto-type agreements.