1. Carbon Cycle
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• 4th most common element on Earth H, He, O, C
• The building block of life, making up about 50% of the dry
weight of living things.
• Second, the cycling of carbon is tied to the flows of energy in
biosphere
• Carbon, in the form of carbon dioxide (CO2) and methane (CH4),
forms two of the most important greenhouse gases. (without the
greenhouse effect, the Earth’s average temperature would be -33
degree C).
• Elemental form: amorphous C, graphite and diamond.
• oxidation state from +4 to í, and occurs mostly in the +4 state as carbon dioxide
(CO2) and in carbonate (CO3
2í) form.
• The CO2 is a trace constituent in the atmosphere 0.04%.
• Carbonate is present in the lithosphere as calcite (CaCO3), dolomite
(CaMg(CO3)2), and siderite (FeCO3).
• In aqueous form, carbonate exists as H2CO3, HCOí, and CO3
2í.
• Carbon monoxide (CO) is present in the atmosphere as oxidation state +2.
• The PRVWUHGXFHGIRUPRI í LVPHWKDQH +4).
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The Fast Carbon Cycle
• The fast carbon cycle is largely the movement of carbon through life forms on
Earth, or the biosphere.
• Between 1-100 Gt of carbon move through the fast carbon cycle every year.
•
• Plants and phytoplankton are the main components of the fast carbon
cycle.
• Photosynthesis: The chemical reaction looks like this:
• 6CO2 + 6H2O + energy = C6H12O6 + 6O2
• Glucose , carbohydrates, fats amino acids cellulose
•
• carbon from a plant and return it to the atmosphere, but all involve
• Reverts back by same reaction respiration/ Decomposition and fire
• CH2O + O2 = CO2 + H2O + energy
The Slow Carbon Cycle
• Through a series of chemical reactions and tectonic activity, carbon takes between
100-200 million years to move between rocks, soil, ocean, and atmosphere in the
slow carbon cycle.
• On average, .01-1 Gt of carbon move through the slow carbon cycle every year.
• The movement of carbon from the atmosphere to the lithosphere (rocks) begins with
rain. Atmospheric carbon combines with water to form a weak acid—carbonic acid—
that falls to the surface in rain. The acid dissolves rocks—a process called chemical
weathering—and releases calcium, magnesium, potassium, or sodium ions. Rivers
carry the ions to the ocean.
• The slow cycle returns carbon to the atmosphere through volcanoes. Earth’s land
and ocean surfaces sit on several moving crustal plates. When the plates collide, one
sinks beneath the other, and the rock it carries melts under the extreme heat and
pressure. The heated rock recombines into silicate minerals, releasing carbon dioxide.
2. 5
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 CO3
2-
• Less than 1% is in the form of CO2
• The oceans hold so little CO2 because it is converted into
other forms of carbon.
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Photosynthesis uses CO2
for soft tissue construction
Skeleton construction
creates calcium carbonate
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).
Biological processes in surface layer:
2
2
2
2 O
O
CH
O
H
CO
l
2
2
3
3
2
2 CO
O
H
CaCO
HCO
Ca
l
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Dissolved Inorganic Carbon (DIC)
• DIC
Dissolved Inorganic Carbon; Collective name for carbon in the form of (dissolved)
CO2, HCO3-
and CO32-
• Concentration Profiles (DIC)
– 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 temperature and increased density.
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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.
3. 9
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
Solubility pump
• CO2 is more soluble in cold waters.
• In the ocean, CO2 is about two times more soluble in the cold mid-
depth and deep waters than it is in the warm surface waters near the
equator.
• Because these mid-depth and deep waters are formed by the sinking
of cold surface waters in Arctic and Antarctic regions, the formation
of these waters with high CO2 keeps the CO2 concentration of the
atmosphere lower than the average concentration of surface waters.
• Downwelling currents occur in areas where cold, denser
water sinks. These downwelling currents bring dissolved
CO2 down to the deep ocean. Once there, the CO2 moves into
slow-moving deep ocean currents staying there for hundreds
of years.
• Eventually, these deep ocean currents return to the surface in
a process called upwelling. Many upwelling currents occur
along coastlines. When upwelling currents bring deep, cold
ocean water to the surface, the water warms and some of the
dissolved CO2 is released back to the atmosphere.
Downwelling and upwelling currents are important
components of the deep ocean conveyor belt and are
important in physically transporting carbon compounds to
different parts of the oceans.
Surface Ocean – Temp. is variable
-2 to 30°C
Deep ocean – Temp. is stable
-1 to 4°C
Average
Temp. of
the world
ocean is
3.8°C
200 m
4. Biological pump
• Transfers surface organic carbon to the intermediate and deep ocean.
• Not all of the organic matter produced by phytoplankton is respired
in the surface waters where it is produced; some sinks out of the
photic zone to deeper water.
• Eventually, this organic matter is decomposed at depth and reaches
the surface again through ocean circulation.The net effect of the
sinking of organic matter is to enrich the deeper waters relative to
surface waters and thus to reduce the CO2 concentration of the
atmosphere.
Carbonate pump
• The ocean carbonate pump is linked to the biological pump and
plays a very big role in transporting carbon down to deep ocean
sediments where it is stored for very long time scales of millions of
years.
• Some forms of phytoplankton have CaCO3 shells that, in sinking, transfer carbon
from the surface to deeper water, just as the biological pump transfer organic
carbon to depth.
• When shell-builders die and sink, the carbon in their shells is transported down
to the deep ocean where the carbon can become part of deep ocean currents
and seafloor sediments. Many shells dissolve before reaching the seafloor
sediments, a process that releases CO2into deep ocean currents. Shells that do
not dissolve build up slowly on the sea floor forming calcium carbonate (CaCO3)
sediments. Eventually, tectonic processes of high heat and pressure transform
these sediments into limestone.This process locks massive amounts of carbon
away for millions of years.
Carbonate Pump
Biological Pump
Solubility Pump
5. • Marine photosynthesis and the sinking of organic matter out of the
surface water are estimated to keep the concentration of CO2 in air
about 30% of what it would be in their absence.
• Together, the two pumps; solubility pump and biological
pump keep the DIC concentration of the surface waters
about 10% lower than at depth.
• 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.
• The concentration of CO2 in the atmosphere (280 ppmv
preindustrially) would have been 720 ppmv if both pumps were
turned off.
