Transcript No Slide Title

9
Carbon Cycle
•
•
•
Carbon cycle is critically important to climate
because it regulates the amount of CO2 and CH4
in the atmosphere.
Carbon, like water, continuously cycles between
various reservoirs.
Fig (Harvey, fig 2.6) illustrates the major carbon
reservoirs as a series of interconnected boxes.
•
•
Numbers in the boxes is estimated pre-industrial amount of
carbon in gigatonnes (Gt, or billions of tonnes).
Numbers on the arrows give the estimated pre-industrial flux
(Gt per year) between the boxes (in the direction of the
arrow).
1
9
Carbon Cycle
Different studies agree on the following points:
1. The amount of carbon in the land biota is roughly
comparable to the amount of carbon in the atmosphere.
2. The amount of carbon in the soil and in the detritus is
about twice the amount in either the atmosphere or
above-ground land biota.
3. The amount of carbon in the ocean mixed layer, which
interacts directly with the atmosphere, is comparable to
the amount of carbon in the atmosphere itself.
4. The overwhelming majority of the total carbon in the
biosphere + atmosphere + ocean system is in the deep
ocean.
2
Carbon Cycle
Respiration
Photosynthesis
Atmosphere
(consumers)
Decomposition
(soil & detritus)
(primary producers)
oxidized biospheric carbon
(from Riverine input)
Flux in = (120 – R)Land + (100 + aR)Ocean = 220 – (1 – a) R [Gt (C)/yr]
Flux out = (120)Land + (100)Ocean = 220 [Gt (C)/yr]
From these numbers, we conclude that there was a net CO2 flux out [(1 – a)R]
3
of the pre-industrial atmosphere.
Atmosphere – Ocean Exchange Processes
The Ocean Surface Layer – Fig 2.6 & 2.7
When CO2 enters sea water, the following chemical reactions take place:
CO2 dissolves in water
Carbonic acid
dissociates into ions
Bicarbonate dissociates
Net Reaction
gas
CO2
liquid

H 2O
carbonicacid

H 2CO3( aq)
hydrogenion
H

H 2CO3
carbonate
CO32
carbonicacid

hydrogenion

H
bicarbonate
HCO3
bicarbonate

HCO3
H 2O  CO2( gas)  CO32  2HCO3
Carbon thus occurs as several species in seawater:
CO2 gas, H2CO3, HCO3-, and CO32-
4
Carbon Cycle
Things to note:
• The transfer of CO2 between Atmosphere and
Ocean driven by the difference in CO2 partial
pressures (pCO2).
• The concentration of CO2 as gas (pCO2) in
seawater is very small.
– ~ 90% of the inorganic carbon is in the form of
bicarbonate HCO3– ~ 10% is in the form of carbonate CO32– Less than 1% is in the form of CO2
• The oceans hold so little CO2 because it is
converted into other forms of carbon.
5
Biological processes in surface layer:
Photosynthesis uses CO2
for soft tissue construction
CO2  H 2O  CH 2O  O2
Skeleton construction
creates calcium carbonate
Ca 2   2 HCO3  CaCO3  H 2O  CO2
Biological Pump
Some of the soft tissue and skeletal material
(CaCO3) ends up in the deep ocean through
sinking dead micro-organisms (flux B ~ 10 Gt (C)
per year).
6
Harvey, Figs 2.7 & 2.8
Dissolved Inorganic Carbon (DIC)
• DIC
Dissolved Inorganic Carbon; Collective name for carbon
in the form of (dissolved) CO2, HCO3- and CO32– Note that an older (but still used) name for DIC is
Total CO2 = CO2
• Concentration Profiles (DIC and TPO4)
– Photosynthesis (and hence biological growth) depletes surface
waters of nutrients (DIC, phosphate, nitrate,…).
• Maximum nutrient concentrations around depth of 1 km, max DIC at
somewhat greater depth.
– The profiles take the form they do mainly because
• Nutrients removed from solution in surface waters by photosynthesis,
• Nutrients are returned to solution in deep water as organic matter is
decomposed, and
• Atmospheric CO2 dissolves more readily in cold waters at high
latitudes, which sink to the deep sea-floor on account of their low
7
temperature and increased density.
Processes that balance downward transfer of DIC
• DIC transferred upward by
– Turbulent Diffusion
• Because DIC increases with depth, turbulent diffusion tends to
transfer DIC upwards.
– Convective (Thermohaline overturning)
• Causes upward flow of DIC because the water that sinks tends to
have a lower DIC concentration than the water that rises (which has
absorbed DIC at the bottom transported there by the biological
pump).
• However, as warm surface waters flow poleward, they cool and
absorb more CO2 from the atmosphere. As a result, the DIC
concentration in sinking water increases to that in the water that
upwells at lower latitude.
• Consequently, net vertical transfer of DIC due to advective
overturning is rather small.
8
Carbon Pumps
Three factors account for the excess DIC in the deep ocean:
(1) The water in the deep ocean is colder and thus can hold more CO2 at
equilibrium with the atmosphere,
(2) the deep ocean contains remineralized CO2 from organic particles that
sink from the surface ocean, and
(3) the deep ocean contains CO2 derived from the dissolution of CaCO3
in particles that sink from the surface ocean.
Oceanographers refer to these factors as the three carbon pumps,
1. the solubility pump,
2. the biological pump (also known as organic or soft tissue pump), and
3. the carbonate pump
9
Why Atmospheric CO2 Concentration Is So Low
Can now see that there are two reasons why the
atmospheric CO2 concentration is low and why
most of the carbon is in the oceans:
1. Because of the chemistry of DIC, and the fact that
most of oceanic carbon does not occur as dissolved
CO2.
2. Because DIC concentration is
•
•
Low in the surface mixed layer, but high in the deep ocean.
The atmosphere is in direct contact only with the mixed
layer, and is largely isolated from the deep ocean.
10
Summing up
•
The biological pump maintains low DIC concentration in the surface
layer.
–
This is possible
even though DIC is in much greater concentrations in the deep ocean,
and the mixing processes within mixed layer are intense,
–
because mixing between mixed layer and deeper water is much slower.
•
Low DIC in surface water plus the chemistry of DIC results in low
pCO2 of surface water.
•
Low pCO2 of surface water results in low atmospheric pCO2,
–
since atmospheric pCO2 tends to adjust to the pCO2 of the water with
which it is in contact.
11
Check out this paper
Distribution of Anthropogenic CO2 in the
Oceans
http://www.pmel.noaa.gov/pubs/outstand/feel2331/anthropogenic.shtml
12