Carbon Cycling - Aquatic and Terrestrial

Question 1

Tell me about the chemistry of carbon

Answer 1

Carbon is in group 4 of the periodic table and can therefore gain or lose 4 electrons when it forms chemical bonds

Question 2

What are some of the isotopes of pure carbon?

Answer 2

Diamond, graphite and buckminster fullerene

Question 3

Tell me about the structure of graphite

Answer 3

Graphite is a very soft form of carbon consisting of carbon atoms arranged in layers comprising hexagonal arrangements of carbon atoms

Weak intermolecular forces between layers means that they can slide over each other. For this reason graphite is used as a lubricant

Graphite can also exist amorphously ( without clearly defined shape) such as BMF (which are football shapes structures formed from single layers of carbon that have wrapped around to form a ball

Diamond too where each carbon atom bonds to four other carbon atoms in a tetrahedral arrangement. Giant ionic lattice

Question 4

What are some important inorganic molecules made up of carbon?

Answer 4

Carbon monoxide
Carbon dioxide
Carbonic acid H2CO3
Bicarbonate ion HCO3-
Carbonate ion
Calcium carbonate 

Carbonic acid buffer system

Question 5

Carbon is distributed between 4 main reservoirs what are they?

Answer 5

Atmosphere - carbon dioxide, carbon monoxide and methane
Hydrosphere (oceans) - bicarbonate an carbonate are the main dissolved forms of carbon in the hydrosphere
Lithosphere ( crust) -carbonate rocks
Terrestrial biosphere (e (the collective term for all the world’s land vegetation, soil, etc) - organic carbon

Question 6

What is the largest carbon reservoir on earth?

Answer 6

The Lithosphere

The majority of the carbon here is locked into carbonate rocks and the remainder is present as kerogens (oils)

Question 7

Is the amount of carbon stored in a reservoir important?

Answer 7

The amount of carbon stored in a reservoir is less important than the annual movement of carbon between reservoirs

Earth contains 10^23 g of carbon

Most buried in sedimentary rocks as organic compounds and carbonates

Question 8

What are the two main cycles?

Answer 8

1. The geochemical cycle

Slow exchange between reservoirs

2. Bio geochemical cycle - rapid exchange between reservoirs

Question 9

Talk about carbon in the hydrosphere

Answer 9

Oceans contains the largest pool of C near the earths surface but most of it is not involved in exchange with the atmosphere

When exchange occurs carbon dioxide in the atmosphere will dissolve in the oceans rapidly forming carbonic acid ( H2CO3)

CO2 (g) CO2 (aq)

and rapidly forms carbonic acid

CO2 + H2O H2CO3

Question 10

Talk about how carbonic acid maintains a constant ph in oceans

Answer 10

Carbonic acid buffers seawater to a constant pH of 8.1

Carbonic acid has 2 hydrogen ions which can be released to buffer the oceans to a constant pH of 8.1-8.2

H2CO3 HCO3- + H+

And. HCO3- CO3^2- + H+

Question 11

Talk about the distribution of carbon species in water as a function of pH

Answer 11

At low pH (acidic conditions) carbon is in the form of carbonic acid. As pH increases carbonic acid will lose a hydrogen ion ( proton) to form the bicarbonate ion . At high pH both hydrogen ions are lost forming the carbonate ion. In seawater at pH 8.2 the majority of dissolved carbon will be in the bicarbonate form with a small proportion as carbonate and carbonic acid

Question 12

The pH or seawater remains relatively constant ( pH of 8.2) because of the buffering action of carbon dioxide in the water

Answer 12

Write the equation which represents this

Question 13

Note: DIC= dissolved inorganic carbon

Answer 13

The average DIC concentration in the ocean is 2.35 mmol kg-1

Total ocean volume = 1370 x 10^6 Km3

Question 14

What is the main mechanism by which ions enter ocean water from rivers

Answer 14

Erosion and subsequent transport of materials are the main mechanisms by which ions enter ocean water from rivers. This equation is an example of that process. Limestone rock ( CaCO3) interacts with carbon dioxide in the atmosphere to generate calcium and bicarbonate ions. Carbon dioxide is bought into close contact with the limestone surface by first dissolving in rainfall

CaCO3 (s) + CO2 (g) + H2O -> Ca2+ + 2HCO3- (aq)

CaCO3 = lithosphere ( weathering)

And CO2 from the atmosphere

Question 15

What is the main input of carbon into oceans

Answer 15

The main input of carbon into the oceans is HCO3- in river water

0.4 x10^5 g yr-1 of C as HCO3-

500 x 10^12 g yr-1 of Ca^2+

Question 16

What’s the relationship between dissolved inorganic carbon and dissolved organic carbon in oceans

Answer 16

The amount of dissolved inorganic carbon in oceans is much greater than the amounts of dissolved organic carbon. Organic carbon is largely derived from decomposed organic matter

Question 17

What is the main reserve of carbon and which forms is it present in?

Answer 17

The lithosphere is the main reservoir of carbon on Earth containing more than 99 % of global carbon. In the lithosphere the dominant forms of carbon are carbonate rocks and sediments/deposits derived from organic matter including coal and oil

The carbonate rocks are usually calcium carbonate deposits. Calcium carbonate can crystallise into 2 forms ( polymorphs);calcite and aragonite. Organic carbon can occur as large deposits e.g coal or can be more widely dispersed. e.g where oil had migrated and coated the individual grains within a rock

Question 18

Talk about carbonate rock formation and locked up carbon

Answer 18

Carbonate rocks have been formed throughout Earth’s history since oceans first existed. This slow accumulation and even slower erosion and weathering has resulted the majority of Earth’s carbon being locked away in these rocks

Question 19

How are calcium concentrations maintained?

Answer 19

Seawater has a relativity constant calcium concentration. To maintain this steady state carbon coming into the oceans from rivers must be balanced by formation of carbonate sediments

For seawater to maintain a fairly constant Ca2+ composition an equivalent amount of Ca must be deposited as CaCO3 in ocean sediments

Large areas of ocean floors are covered in carbonate deposits

Calcium carbonate skeletons of organisms sink to the ocean floors after the animals die

Under some conditions the calcium carbonate dissolves again ( where the pressure is high and the temperature is low)

Question 20

Sedimentation - carbonated

Answer 20

Carbonate sediments are formed from marine organisms. Organisms like foraminifera and coccoliths use calcium and bicarbonate ions from seawater to form their exoskeletons

When these organisms die and sink down through the water column the calcium carbonate skeletons may survive to form carbonate sediment or may dissolve re-releasing calcium and bicarbonate ions

Eventually sediments are subducted into the earths interior

The massive quantities of carbon in the earths crust are the result of a slow accumulation of these materials over long periods of earths history

Although most substances dissolve more in warm water compared to cold water. Calcium carbonate dissolves more easily in cold water than warm water

This means that in shallow ( not a lot of pressure ) warm tropical seas carbonate sediments will blanket the ocean floor whereas in deep, cold parts of the oceans carbonate sediments will dissolve ( DIC?)

Question 21

How long will a carbon atom that forms marine carbonate spend locked away in the sediment before being released

Answer 21

400 million years

Question 22

Talk about sedimentation in the form of organic deposits

Answer 22

Coal and oil are formed by the burial of organic matter

Organic deposits including coal and oil are formed when organic matter is buried at a rate faster than it can be decomposed. Whilst this was common in the past, there are no substantial deposits of organic carbon currently forming on Earth. Burning of fossil fuels is returning organic carbon to the atmosphere at a greater rate than carbon is being removed

Earliest organic carbon is present in 3.8 billion year old rocks

Question 23

What are the 2 cycles the carbon cycle can be split into?

A long term geochemically driven cycle and a short term bio geochemical cycle. In the long term cycle carbon dioxide in the atmosphere weathers rock on the earths surface releasing ions that travel to the oceans

In the oceans these ions are used by organisms and end up forming carbonate sediments on the ocean floor

These carbonate sediments may persist for millions of years before they are eventually subducted back into the Earths interior where they are broken down releasing carbon dioxide

Answer 23

A long term geochemically driven cycle and a short term bio geochemical cycle. In the long term cycle carbon dioxide in the atmosphere weathers rock on the earths surface releasing ions that travel to the oceans

In the oceans these ions are used by organisms and end up forming carbonate sediments on the ocean floor

These carbonate sediments may persist for millions of years before they are eventually subducted back into the Earths interior where they are broken down releasing carbon dioxide

Question 24

Why have atmospheric carbon dioxide been below 1 percent for the past 100 million years.? Compare this to our neighbouring planets why is this mechanism important for controlling carbon dioxide?

Answer 24

The Geochemical cycling of carbon on Earth where carbon is locked away in carbon sediments, has maintained atmospheric carbon dioxide concentrations below 1% for the past 100 million years

Mars does not have any active plate tectonics or water at its surface in liquid form, therefore carbon that might have been locked away in carbonate rocks in the past can no longer be released by weathering or subduction back into the atmosphere. This had resulted in very low carbon dioxide concentrations in the Martian atmosphere

In contrast, Venus is very tectonically active with carbon dioxide being continually released into its atmosphere. This high concentration of carbon dioxide and Venus’s position closer to the sun than Earth have warmed the Venetian atmosphere to greater than 100 degrees Celsius. The absence of liquid water at the surface of Venus prevents the removal of carbon dioxide from the atmosphere

Question 25

What is the bio geochemical cycle dominated by? What is litter? What is the average turnover time for carbon litter in 1.5 years?

Answer 25

Occurs in addition to the main, underlying geochemical carbon cycle

The biogeochemical carbon cycle is dominated by short term cycling of carbon as a consequence of photosynthesis

1) CO2 used in photosynthesis to form organic matter
2) Organic matter decomposes to regenerate CO2

Litter: all plant material not attached to a living plant ( includes decomposing roots)

Average turnover time for carbon in litter is 1.5 years

CO2 + H2O = glucose + O2

This cycle occurs in addition to the slower geochemical cycle and is a balance between carbon dioxide consumption in photosynthesis and carbon dioxide production through respiration and decomposition

Question 26

What is gross primary productivity?

Answer 26

Total rate of photosynthesis per unit of area

Question 27

What is net primary productivity?

Answer 27

Total rate of photosynthesis per unit area in excess of respiration

Question 28

What is the amount of carbon in terrestrial biomass?

Answer 28

480-1080 x 10^15 g C

Question 29

In what ways does the bio geochemical cycle influence the underlying geochemical cycle?

Answer 29

Marine organisms generate the calcium carbonate that form the ocean sediments

Land plants and soil microorganisms elevate carbon dioxide in soils encouraging weathering of minerals