Lecture 6 - The Carbon Cycle

Question 1

What are four allotropes of pure carbon?

Answer 1

Diamond

Graphite

Carbon nanotubes

Buckminsterfullerene

Question 2

What is the bonding in diamond?

Answer 2

Each carbon atom is bonded to four others in a tetrahedral structure which makes diamond very hard.

Question 3

What is the bonding in graphite?

Answer 3

Carbon atoms are arranged in hexagonal layers which makes graphite soft.

Question 4

What is the bonding in carbon?

Answer 4

Carbon has four valence electrons which means that it can form four covalent bonds.

Question 5

What are five important molecules for life that have a carbon backbone?

Answer 5

DNA

Amino Acids

Proteins

Carbohydrates

Lipids

Question 6

What is the difference between inorganic carbon and organic carbon?

Answer 6

Inorganic carbon is extracted from ores and minerals.

Organic carbon is found in nature and plants.

Question 7

What are six forms of inorganic carbon?

Answer 7

Carbon monoxide.

Carbon dioxide.

Carbonic acid.

Bicarbonate ion.

Carbonate ion.

Calcium carbonate.

Question 8

What are sources and sinks?

Answer 8

Sources add carbon to the atmosphere.

Sinks remove carbon from the atmosphere.

Question 9

When can a system be said to be in equilibrium?

Answer 9

When the sources are equal to the sinks.

Question 10

What are the four main reservoirs of carbon on Earth?

Answer 10

Lithosphere - Carbonate rocks are the dominant carbon forms.

Hydrosphere - Bicarbonate and carbonate are the main dissolved ions.

Atmosphere - Carbon monoxide, carbon dioxide and methane.

Biosphere - Organic carbon.

Question 11

How do ions enter the ocean from rivers?

Answer 11

Erosion and transport of material.

For example:

CaCO₃ + CO₂ + H₂O —> Ca²⁺ + 2HCO₃⁻

Question 12

What is the main input of carbon into the ocean?

Answer 12

HCO₃⁻ in river water.

4 x 10¹⁴ g year⁻¹ of HCO₃⁻ are delivered this way.

Question 13

What are the reactions for the dissolution of CO₂?

Answer 13

CO₂ ₍g₎ <—> CO₂ ₍ₐq₎

CO₂ ₍ₐq₎ + H₂O <—> H₂CO₃

Question 14

What pH does carbonic acid buffer seawater to?

Answer 14

8.1-8.2

Question 15

What reactions occur to buffer carbonic acid?

Answer 15

H₂CO₃ <—> HCO₃⁻ + H⁺

HCO₃⁻ <—> CO₃²⁻ + H⁺

Question 16

How does the form of dissolved carbon change as a function of pH?

Answer 16

Low pH = CO₂

Mid pH = HCO₃⁻

High pH = CO₃²⁻

Question 17

In seawater, what is the main form of dissolved carbon?

Answer 17

HCO₃⁻

Question 18

What is the order in terms of magnitude of dissolved inorganic carbon, dissolved organic carbon and particulate organic carbon?

Answer 18

Dissolved inorganic carbon > dissolved organic carbon > particulate organic carbon.

Question 19

Where is dissolved organic carbon derived from?

Answer 19

Decomposing organic matter and includes amino acids, phenols, fatty acids, sterols and carbohydrate molecules.

It has an average age of 3,400 years.

Question 20

What is the main reservoir of carbon?

Answer 20

The lithosphere.

Question 21

What are the dominant forms of carbon in the lithosphere?

Answer 21

Carbonate rocks and sediments/deposits derived from organic matter including coal and oil.

Carbonate rocks are usually calcium carbonate deposits, crystallised in two forms: calcite and aragonite.

Question 22

Why is most of the Earth’s carbon in rocks?

Answer 22

They have formed throughout the Earth’s history since the oceans first existed. They have accumulated slowly and eroded even slower which means they have been locked away.

Question 23

How does the ocean balance calcium concentrations?

Answer 23

The incoming calcium from rivers must be balanced by the formation of carbonate sediments.

These are mainly in the form of calcium carbonate exoskeletons.

Question 24

How are carbonate sediments formed?

Answer 24

When organisms die and sink through the water column, it may fall to the seafloor and form sediment or redissolve into calcium and bicarbonate ions.

Calcium carbonate dissolves easier in cold water and so warm shallow seas have carbonate sediments blanketing the floor.

Question 25

How are organic carbon deposits formed?

Answer 25

When organic matter is buried which requires the production of organic carbon by photosynthesis to be greater than the rate of decomposition.

There are no substantial forms of organic carbon forming today.

Question 26

What two cycles does carbon move around in?

Answer 26

The geochemical cycle which results in slow exchange of carbon over millions of years.

The biogeochemical cycle which results in faster exchanges.

Question 27

What happens during the long term carbon cycle?

Answer 27

Carbon dioxide in the atmosphere weathers rocks on the Earth’s surface which releases ions that travel to the oceans.

These ions are used by organisms and eventually form carbonate sediments on the ocean floor.

These carbonate sediments persist for millions of years before they are subducted into the Earth’s interior where they are broken down and released as carbon dioxide in hydrothermal vents and volcanic emissions.

Question 28

Why is the geochemical carbon cycle a very important process?

Answer 28

It has kept carbon dioxide concentrations below 1% for the past 100 million years.

Question 29

Why do Mars and Venus have differing atmospheric carbon dioxide levels to Earth?

Answer 29

Mars doesn’t have any active plate tectonics and no surface liquid water. This means that carbon locked away in rocks cannot be released by weathering or subduction into the atmosphere which has resulted in very low carbon dioxide concentrations.

Venus is very tectonically active and so carbon dioxide is continually released into its atmosphere. The absence of liquid water has prevented the removal of carbon dioxide from the atmosphere and resulted in very high concentrations.

Question 30

What dominates the biogeochemical cycle?

Answer 30

The short-term cycling of carbon as a consequence of photosynthesis. It is a balance between carbon dioxide consumption in photosynthesis and carbon dioxide production through respiration and decomposition.

Question 31

What is gross primary productivity?

Answer 31

The total rate of photosynthesis per unit area.

Question 32

What is net primary productivity?

Answer 32

The total rate of photosynthesis per unit area in excess of respiration.

Question 33

How much carbon is stored in terrestrial biomass?

Answer 33

480-1080 x 10¹⁵ g C.

Question 34

What is litter and what’s its average turnover time?

Answer 34

All plant material not attached to a living plant including decomposing roots.

The average turnover time is 1.5 years.

Question 35

How does the biogeochemical cycle influence the geochemical cycle?

Answer 35

Marine organisms generate the calcium carbonate that form the ocean sediments.

Land plants and soil microorganisms elevate carbon dioxide concentrations in soil pore spaces encouraging the weathering of minerals.

Land plants and soil microbes excrete organic compounds which accelerates the weathering of minerals.

Question 36

What is a flux?

Answer 36

The movement of material from one place to another.

They are usually expressed as a rate. eg. g cm⁻² s⁻¹.

Question 37

What are important fluxes in the carbon cycle?

Answer 37

Photosynthesis

Plant respiration

Ocean and atmosphere exchange

Soil respiration

Fossil fuel usage and land-use change.

Question 38

How much carbon is removed per year by photosynthesis and how much is stored in plants in any one time?

Answer 38

120 Pg year⁻¹.

610 Pg is stored at any one time.

Question 39

What is autotrophic respiration?

Answer 39

Plant respiration.

Question 40

What is heterotrophic respiration?

Answer 40

Respiration from microorganisms living in the soil.

Question 41

How much carbon a year is released by heterotrophic respiration?

Answer 41

120 Pg year⁻¹.

Question 42

What happens during ocean-atmosphere exchange?

Answer 42

Inorganic carbon is released at the interface of the ocean’s surface and surrounding air through the process of diffusion.

Question 43

What factor makes the oceans able to store more carbon?

Answer 43

The formation of carbonate ions.

Question 44

Why is very little carbon stored biologically in the ocean?

Answer 44

Aquatic plants do not have large, woody trunks, they decompose very quickly.

Question 45

What does the modern day carbon cycle include?

Answer 45

Anthropogenic fluxes such as land use change and burning fossil fuels.

Question 46

How much carbon is released into the atmosphere per year by combustion of fossil fuels?

Answer 46

6-8 Pg year⁻¹.

Question 47

How has land use change caused a carbon flux to the atmosphere?

Answer 47

Forests and other ecosystems contain more carbon than grasslands and crops, 1.5 Pg year⁻¹ are released into the atmosphere.