Transcript No Slide Title

The Marine carbon cycle.
Carbonate chemistry
Carbon pumps
Sea surface pCO2 and air-sea flux
The 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+ + CO32Typically,
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
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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.
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.
Carbonate
Soft
tissue
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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
Ocean Carbon: The Biological (soft tissue) Pump
Nitrate
1 (or phosphate)
2
3
Concentration
Total
CO2
Surface pCO2, nutrient and surface
temperature in the North Atlantic
360
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
16
300
8
6
14
Nitrate (M)
pCO2 ( atm)
320
18
SST (°C)
340
4
280
12
260
2
The biological (calcium carbonate) pump.
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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 redissolve 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
Coccolithophores -- calcite precipitating plankton
The Solubility Pump
Increasing
CO2 concentration
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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.
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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
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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.)
Bomb radiocarbon x 1020 atoms
Tropospheric bomb radiocarbon
300
200
100
1950
1960
1970
1980
1990
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.
3-D model outputs for surface pCO2
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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)
Estimate
(Keeling oxygen
technique)
1.7+/- 0.5
OCMIP-2 Model
Intercomparison
(ten ocean carbon
models).
2.5+/- 0.4
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….
Source: Manabe and Stouffer, Nature 364, 1993
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
?
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.