Carbon Cycling - Aquatic and Terrestrial Flashcards
(29 cards)
Tell me about the chemistry of carbon
Carbon is in group 4 of the periodic table and can therefore gain or lose 4 electrons when it forms chemical bonds
What are some of the isotopes of pure carbon?
Diamond, graphite and buckminster fullerene
Tell me about the structure of graphite
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
What are some important inorganic molecules made up of carbon?
Carbon monoxide Carbon dioxide Carbonic acid H2CO3 Bicarbonate ion HCO3- Carbonate ion Calcium carbonate
Carbonic acid buffer system
Carbon is distributed between 4 main reservoirs what are they?
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
What is the largest carbon reservoir on earth?
The Lithosphere
The majority of the carbon here is locked into carbonate rocks and the remainder is present as kerogens (oils)
Is the amount of carbon stored in a reservoir important?
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
What are the two main cycles?
- The geochemical cycle
Slow exchange between reservoirs
- Bio geochemical cycle - rapid exchange between reservoirs
Talk about carbon in the hydrosphere
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
Talk about how carbonic acid maintains a constant ph in oceans
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+
Talk about the distribution of carbon species in water as a function of pH
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
The pH or seawater remains relatively constant ( pH of 8.2) because of the buffering action of carbon dioxide in the water
Write the equation which represents this
Note: DIC= dissolved inorganic carbon
The average DIC concentration in the ocean is 2.35 mmol kg-1
Total ocean volume = 1370 x 10^6 Km3
What is the main mechanism by which ions enter ocean water from rivers
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
What is the main input of carbon into oceans
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+
What’s the relationship between dissolved inorganic carbon and dissolved organic carbon in oceans
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
What is the main reserve of carbon and which forms is it present in?
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
Talk about carbonate rock formation and locked up carbon
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
How are calcium concentrations maintained?
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)
Sedimentation - carbonated
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?)
How long will a carbon atom that forms marine carbonate spend locked away in the sediment before being released
400 million years
Talk about sedimentation in the form of organic deposits
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
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
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
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?
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
