The Marine carbon cycle. Carbonate chemistry Carbon pumps Sea surface pCO2 and air-sea flux The sink for anthropogenic CO2

1
The Marine carbon cycle.Carbonate chemistry
Carbon pumpsSea surface pCO2 and air-sea
fluxThe sink for anthropogenic CO2
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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).
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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.

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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

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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.

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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

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The overturning thermohaline circulation

• 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.

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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.
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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.

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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.

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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
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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
pCO
2
14
Nitrate (
m
4
280
M
)
12
2
260
Nitrate
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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?

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Coccolithophores -- calcite precipitating plankton

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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.

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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.

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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).

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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.)

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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
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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).

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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!
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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.

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Source Manabe and Stouffer, Nature 364, 1993
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Possible Marine biological effects on Carbon
uptake, next 100 years.
Process Effect on CO2 uptake

• 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

?
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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.