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. –This rapidly reacts with H 2 O to form undissociated carbonic acid: CO 2(g) + H 2 O  H 2 CO 3 –Which can dissociate by loss of H + to form bicarbonate ion: H 2 CO 3  H + + HCO 3 - –which can dissociate by further loss of H + to form carbonate ion: HCO 3 -  H + + CO 3 2- Typically, 90% of the carbon exists as bicarbonate, 9% as carbonate, 1% as dissolved CO 2 and undissociated H 2 CO 3 (usually lumped together).

Seawater Partial pressure of CO 2 The partial pressure of CO 2 of the sea water (pCO 2sw ) determines whether there is flux from air to sea or sea to air: –Air-to-sea Flux is proportional to (pCO 2air * - pCO 2sw ) pCO 2sw is proportional to dissolved CO 2(g) : [CO 2(g) ] =  x pCO 2sw = where  is the solubility of CO 2. The solubility decreases with increasing temperature. *pCO 2air is determined by the atmospheric mixing ratio, i.e. if the mixing ratio is 370ppm and atmospheric pressure is 1 atm, pCO 2air is 370  atm.

Global mean air-sea flux, calculated from pCO 2 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 pCO 2sw, 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 CO 2 –pCO 2 of air is rising and this tends to favour a flux from atmosphere into the ocean.

Poleward-going currents are warm water They are associated with cooling water Tend to be regions of uptake of CO 2 from the atmosphere. Equator-going currents – vice-versa The surface wind-driven 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 CO 2 and may subsequently sink. The water upwells in other parts of the world ocean, particularly the equatorial Pacific.  Upwelling regions are usually sources of CO 2 to the atmosphere – deep water has high CO 2 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 CO 2 concentration. The overturning thermohaline circulation

In high productivity regions, CO 2 is taken out of the surface water by plankton growth and sinks in a particle "rain" to depth. Global ocean biological production

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 (  CO 2 ) 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.

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. Soft tissue Carbonate

Ocean Carbon: The Biological (soft tissue) Pump This mechanism acts continually to reduce the partial pressure of CO 2 (pCO 2 ) 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 CO 2 in the water. Most of this goes to form organic matter via the reaction: CO 2 + H 2 O  CH 2 O +O 2. Because the buffer factor  ~10, this has a large effect on surface pCO 2, decreasing it by 2-3 times. The reverse reaction occurs by (mostly bacterial) respiration at depth, and increases CO 2 concentration there. Depth

Surface pCO 2, nutrient and surface temperature in the North Atlantic pCO 2 (  atm ) Nitrate (  M ) SST (°C) MarAprMayJunJulAugSepOct Nitrate SST pCO 2

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 CaCO 3 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 pCO 2 and therefore atmospheric CO 2. The net reaction is: Ca HCO 3 -  H 2 O +CO 2  + CaCO 3 

Coccolithophores -- calcite precipitating plankton

The Solubility Pump This mechanism also tends to increase deep sea carbon at the expense of surface ocean and atmospheric carbon. The solubility of CO 2 increases as temperature decreases. So cold water, which is what forms deep water, tends to dissolve CO 2 from the atmosphere before it sinks. Deep water would therefore have a higher CO 2 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  CO 2, hence pCO 2. 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 pCO 2 there. 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 CO 2 The oceans are close to steady-state with respect to atmospheric CO 2. Prior to the industrial revolution, the oceans were a net source of order 0.5 Pg C yr -1 CO 2 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 CO 2 (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 14 carbon. 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.)

Log 10 number of deaths per conflict Bomb radiocarbon x atoms 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 CO 2 over decadal time scales.

3-D model outputs for surface pCO 2 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 ReferenceSink (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 CO 2 forces more into the oceans. But, the buffering capacity of the ocean becomes less as CO 2 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….

Source: Manabe and Stouffer, Nature 364, 1993

Possible Marine biological effects on Carbon uptake, next 100 years. Iron fertilisation -- deliberate or inadvertent NO 3 fertilisation pH change mediates against calcite- precipitating organisms Reduction in THC offset by increased efficiency of nutrient utilisation Other unforeseen ecosystem changes ProcessEffect on CO 2 uptake ?

Conclusions The ocean CO 2 sink is affected both by circulation and biology. Changes in either would affect how much CO 2 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.