Unit 4.2 Cycles*

Question 0

Explain what is meant by the term ‘biogenic element’.

Answer 0

An element which is essential to the biosphere, e.g. oxygen, carbon, hydrogen and macronutrients.

Question 1

List the forms in which oxygen can occur in the atmosphere, geosphere, hydrosphere and biosphere.

Answer 1

• constituent of rocks, e.g. silicates (SiOx) or carbonates
• water molecule
• gas dissolved in water (H2O)
• part of dissolved compounds
• oxygen (O2) and ozone (O3)
• constituent of other gases (e.g. CO2)
• essential element of living organisms, e.g. cellulose or blood

Question 2

Explain what is meant by the term ‘biogeochemical cycle’.

Answer 2

Cycles of biogenic elements. The biosphere is very important in these cycles.

Elements are continually transformed from one chemical compound to another as they pass through the biosphere and geosphere and, for this reason, the process is termed biogeochemical cycling.

Question 3

Describe the formation and destruction of rocks in the rock cycle.

Answer 3

Destruction through uplift and weathering breaks rocks into smaller pieces which may then be deposited as sediments, which are lithified and form sedimentary rocks.

Destruction through melting, forming the magma (a mixture of metal cations and silicon ions) which will form igneous rocks when cooled.

Metamorphism does not destroy rocks; although their chemical composition remains unchanged, the minerals are transformed and arranged into bands.

Question 4

Explain what drives the rock cycle.

Answer 4

Tectonic forces drive the rock cycle, and most activity is concentrated at plate boundaries:

• igneous: melting mantle beneath divergent plate boundaries and above the subducting plate at convergent plate boundaries
• metamorphic: increases in pressure and temperature occur in continental collision zones and within subduction zones as crust is dragged downwards
• sedimentary: increased weathering in mountain ranges caused by continental collision

Question 5

Apply systems concepts of matter and energy flow across system boundaries to various types of systems on the Earth.

Answer 5

E.g. Tectonic cycle open because both energy and matter transfer to and from the asthenosphere.

E.g. Earth in general is closed: energy enters, but no significant matter enters from outside.

Question 6

Distinguish open and closed systems.

Answer 6

Closed system: energy can enter and leave the system, but matter can not. Example: Earth receives solar radiation and emits infrared, but (pretty much) no matter is transferred.

Open system: both energy and matter can enter and leave the system. Example: rock cycle, as matter is exchanged between mantle and crust at divergent and convergent plate boundaries.

Question 7

Explain why the Earth is effectively a closed system, and why the rock cycle is an open system.

Answer 7

Earth receives solar radiation and emits infrared, and therefore there is a transfer of energy. However, apart from meteorites and spaceships, there is no transfer of matter; these two exceptions are so small that they don’t count.

Matter transfers between the crust and the mantle as part of the rock cycle. Material from the mantle enters the crust at divergent plate boundaries, where it cools to form igneous rocks. Material from the crust returns to the mantle at subduction zones, where oceanic rocks are forced downwards into the mantle.

Question 8

Explain how the tectonic cycle produces a net transfer of material from the mantle to the rock cycle, atmosphere, hydrosphere and biosphere.

Answer 8

Material is transferred from the mantle at divergent plate boundaries, and returned to the mantle at convergent plate boundaries. However, the amount entering the crust is slightly higher than the amount returning to the mantle.

Volcanoes can introduce water and gases from the interior

Small amounts of subducting oceanic lithosphere may remain in the crust:

• slivers may be ‘scraped off’ as the two plates rub
• small amounts may melt, forming magma which may solidify at depth or rise and erupt through volcanoes

Question 9

Order the four spheres by increasing average residence time of elements.

Answer 9

Geological is longest.

Oceanic.

Atmospheric, although some forms have extremely short lifespan and are rapidly changed to another form (e.g. CO).

Biological is shortest.

Question 10

Undestanding that recycling of elements is essential for life on Earth.

Answer 10

As Earth is a closed system, there is a finite quantity of the elements which are vital for life (or in the case of nitrogen, a finite quantity of the element is accessible).

Question 11

Appreciate that biogeochemical cycling involves the cycling of material through both the biosphere and geosphere.

Answer 11

E.g. The nitrogen, carbon, sulfur and phosphorus cycles are all examples of biogeochemical cycling. All four elements are biogenic, and all have significant geological reservoirs.

Question 12

Explain how the global carbon cycle consists of a number of subcycles and that these subcycles operate on varying time-scales.

Answer 12

The terrestrial subsystem cycles carbon between living material, detritus and soil, and the atmosphere. Transfer is rapid: carbon may be cycled between all three in a single season, in the case of the leaf of a deciduous tree.

The oceanic subsystem (100s - 1000 years) is dominated by the biological pump, in which carbon is absorbed by phytoplankton during photosynthesis, and then falls through the ocean column either as faecal pellets of zooplankton, or carbonate (CaCO3) shells. Dissolved as it falls, approximately 1% of pellets reach the floor, and 0.1% become buried in sediments; carbonate shells are only deposited above the carbonate compensation zone (approx. 4 km depth).

Carbon in the geological subsystem has a residence time of millions of years. It enters the subsystem through oceanic sediments, or the formation of coal and gas.

Question 13

Calculate residence times for the main reservoirs of carbon on Earth.

Answer 13

Residence time = capacity / flux

Question 14

Describe the processes that transfer carbon between the various reservoirs.

Answer 14

Photosynthesis and respiration between biosphere and atmosphere.
Absorption and release of CO2 between ocean and atmosphere.

Decay between biosphere and soil/detritus.
Runoff between soil/detritus and ocean.

Sedimentation between ocean and geological.
Storage between detritus/soil and geological.

Question 15

Recognise the respective roles of biology and geology in the carbon cycle.

Answer 15

Biological cycles are the most active, while geological is the main long-term storage of carbon.

Question 16

Describe the effect of human intervention in the carbon cycle.

Answer 16

Through human intervention, transfer from the long-term geological cycle to the short-term atmosphere is rapidly increased, and is now far in excess of transfer into the geological reservoir. As a result, there is a net increase in carbon in the atmosphere and ocean, which has significant consequences for the climate, and oceanic ecosystems.

Question 17

Name the Earth’s major reservoirs of nitrogen.

Answer 17

Atmosphere: dinitrogen (N2) makes up about 80% of atmosphere. This is the biggest nitrogen reservoir.

Nitrogen is also present in the oceans, and soils.

Question 18

Describe how nitrogen is transferred between its reservoirs.

Answer 18

Nitrogen is transferred from atmosphere to biosphere through fixation by bacteria, and by lightning strikes.

Nitrogen is transferred from biosphere to soil through decay, and back again through assimilation.

Nitrogen is transferred from soil to atmosphere by ammonia volatilisation, and denitrification.

Nitrogen is transferred from soil to watercourse through leaching of nitrates.

Nitrogen is transferred from geological to atmosphere through burning fossil fuels. (or is it?)

Question 19

Outline how nitrogen is fixed in nature.

Answer 19

Lightning strikes provide energy to break triple bond in dinitrogen; the resulting N molecules combine with oxygen to form nitrate.

Bacteria such as Frankia and Rhizobium, have developed the ability to fix nitrogen. Both have symbiotic relationships with certain plant species (Alnus, legumes).

Question 20

Describe how nitrogen is fixed by humans.

Answer 20

Nitrogen can be fixed using the Haber-Bosch process. This involves two stages:

The first stage produces hydrogen by combining methane and water at high temperature and pressure and in the presence of a catalyst.

In the second stage, this hydrogen is combined with dinitrogen, again at high temperature and pressure and in the presence of a catalyst, to produce ammonia (NH3).

Question 21

Summarise the processes of ammonia volatization, nitrification and denitrification.

Answer 21

NH3 exists in equilibrium with NH4, but does not bind well with soils and may exist as a gas. Ammonia volatilisation is the loss of this NH3 to the atmosphere.

Nitrification is the oxidation of NH4 (ammonia) to NO3 (nitrate) by soil bacteria, as an energy pathway in well-oxidised soils.

Denitrification is the reduction of nitrate (NO3) to a variety of gases, including N2, NO and N2O, by soil bacteria as an energy pathway in oxygen-poor soils.

Question 22

Explain the influence of agriculture on the nitrogen cycle.

Answer 22

Artificial fertiliser is often administered as a single large dose, meaning that soils are periodically saturated in nitrogen, as opposed to the gradual transfer of natural systems.

Ploughing increasese mineralisation, and also, by introducing atmospheric oxygen into the soil, making conditions strongly oxidising, increases nitrification. The result is more nitrates, which are extremely vulnerable to leaching (as opposed to NH4).

Use of artificial fertilisers also means that there is more nitrogen held within soils. As a result, the natural processes which transfer nitrogen into the atmosphere (ammonia volatilisation, and denitrification) occur more, and so more nitrogen enters the atmosphere as N2, NO or N2O.

Question 23

Name the Earth’s major reservoirs of sulfur.

Answer 23

99.99% of sulfur is held in the oceanic and geological reservoirs.

Oceanic: sulfate (SO4/2-) is 7.68% of sea salt.

Geological: evaporates and shakes (which contain pyrite)

Question 24

Describe how sulfur is transferred between its reservoirs.

Answer 24

The most active reservoirs are atmospheric and biological. The atmosphere receives dimethyl sulfate (CH3)2S from decaying marine organisms, hydrogen sulfide (H2S) from terrestrial organisms, sulfate (SO2/4-) from natural weathering of rocks and from sea spray, and sulfur dioxide (S2O) from burning of fossil fuels; the latter can be transformed in the atmosphere to sulfuric acid (H2SO4). SO2 and H2S are also released by volcanoes.

Question 25

Name the Earth’s major reservoirs of phosphorous.

Answer 25

Almost all phosphorous is held in rocks. Mineral apatite. Phosphorus is present in trace amounts in all rocks.

Phosphorous is found to a much lesser extent in soils and in the sea, especially at depth.

There is no gaseous reservoir of phosphorous.

Question 26

Outline the main differences between the phosphorous cycle and the carbon, nitrogen and sulfur cycles.

Answer 26

There is no gaseous reservoir of phosphorous in the atmosphere. Phosphorous is almost exclusively transferred as (soluble) phosphate in soil or in water.

Question 27

Describe how phosphorous is transferred between its reservoirs.

Answer 27

Phosphorous is released from rocks through natural weathering, forming (soluble) phosphate: HPO4/2-.

Phosphate can be absorbed by organisms (used in creation of DNA, ATP etc.). This is rapidly recycled, with organisms absorbing the phosphates released through decomposition. The result is an internal cycling, which is only broken by soil erosion washing phosphate-containing soil into water courses.

Phosphate travels down rivers, with some deposited in river sediments, but much reaching the ocean.

Once phosphates reach the ocean, they are used and internally recycled among marine organisms. 90% of the phosphate which reaches the ocean is used in this way. The remaining 10% falls through the water column, and so the deep ocean is much river. Where deep ocean upwells, the phosphate rich water is brought to the surface, and there is high productivity.

Phosphates are returned to the geological reservoir through the formation of oceanic sediments.

Question 28

Give examples of the ways in which human activity affects the sulfur and phosphorous cycles.

Answer 28

Sulfur cycle: burning of fossil fuels releases SO2.

Coal contains 6-8% sulfur, although this may be reduced to 2% through scrubbing. Oil contains 0.7% sulfur. The SO2 can lead to acid rain.

Phosphate: use of phosphates for water softening in detergents, and as fertilisers, leads to high levels in water (eutrophication).

Question 29

Consider the consequences of the transfer of material from one biogeochemical reservoir to another.

Answer 29

Human induced transfer may disrupt the time scales, e.g. with the carbon cycle the transfer from geological to atmospheric is far more rapid. It may also alter the levels in any one place, causing disruption to local ecosystems, e.g. nitrogen or phosphorous causing eutrophication.