1. Introduction to Biogeochemical Cycles
Jean-Marc Mayotte

2. Matter cannot be created or destroyed... ...but it can be moved around

3. What is a biogeochemical cycle?
The movement of matter through a closed system (Earth) such that there is no beginning or end of the movement; the matter simply moves between reservoirs.

4. Earth is a closed system (almost
Earth is a closed system (almost... there is some very small gain/loss from space)
We can divide the system into four parts: Atmosphere Hydroshpere Lithosphere Biosphere
The processes in the biosphere can have very large impacts the ”balance” of the cycles and are very important to sustaining life on Earth... hence the emphasis on bio in biogeochemical cycles

5. Atmosphere
The layer of gasses held close to Earth by the force of gravity

6. Hydrosphere
The total mass of all liquid water on Earth

7. Lithosphere
The outermost ”shell” of the Earth
This one!

8. Biosphere
Global sum of all ecosystems on Earth

9. General cycle
Movement to/from influences the fluxes to/from other reservoirs
Movement through the atmosphere is generally rapid
Movement through soils and rocks is generally slow

10. What is the ”matter”?... (sorry)
There are about 30 chemical elements and compounds important to life...
Carbon (C)
Methane (CH₄)
Oxygen (O)
Silica (Si)
Hydrogen (H)
Nitrogen (N)
Potassium (K)
Sulfur (S)
Calcium (Ca)
Phosphorus (P)
Iron (Fe)
Water (H₂O)
Magnesium (Mg)…

11. Why is this important?
Living organisms depend on chemical inputs to sustain their energy needs and well-being. The speed, location and efficiency with which these chemicals are moved, changed and recycled is directly correlated with the diversity and scale of life on Earth. The cycles can also help explain the accumulation of greenhouse gasses and illustrate the impacts of anthropogenic interactions with the environment.

12. Where is the matter moving? Reservoirs
Atmosphere
Plants
Coastal ocean
Soil
Ocean surface
Ocean
Terrestrial
Deep ocean
Ocean floor
Deep sediments/rocks

13. How is the matter moving? Fluxes
Dissolution out of and dissolving into water
Plant life and bacteria are major drivers of chemical cycling
Geologic events (volcanos, subduction, abduction)
Precipitation, errosion, freeze/thaw, evaporation
Sedimentation and burying

14. Sources and sinks
A source is a reservoir with a large outward flux of the chemical in question
A sink is a reservoir with a relatively low outward flux of the chemical in question. A deposited chemical will remain in the sink for a very long period of time (almost permanent on a human timescale)

15. Reading box diagrams Pay attention to:
Reservoir size & turnover time – this will give you an idea of how “active” the process is and how it might respond to large changes (which are “sources” and which are “sinks”?)
Biologic relevance of the reservoir – what does a change in a particular reservoir mean for life living around or within that reservoir?

16. Water cycle (in box diagrams)
Reservoir
Pool size [10³km³], pool size [%], flux [km³/y]
Atmosphere
13 (0.0009%) (9d)
Salt lakes
104 (0.008%) ( y)
precipitation /y
40 000/y
Freshwater lakes
125 (0.009%) (1-100y)
71 000/y
Polar ice, glaciers
(2.08%) (16 000y)
Rivers
1.2 ( %) (12-20d) /y
precipitation
Evaporation
Evapotranspiration
Evaporation /y
River runoff
40 000/y
Soil moisture
67 (0.005%) (280d)
Ocean
1.37x10⁴ (97.61%) (37000y)
Groundwater (active)
4000 (0.29%) (300y)
Schlesinger, 1993; Murray, 1992

17. Discussion:
Assuming that the current trend of warming continues and sea and land ice continues to melt, what would this mean for the hydrologic cycle? How would fresh water deposits be affected?

18. The Carbon cycle

19. The carbon cycle
The carbon cycle encompases a series of processes that are necessary to sustain life as we know it on Earth. Carbon is the primary “building block” for most biological compounds. It is also widely accepted as being responsible for the greenhouse effect as atmospheric CO₂ and CH₄.

20. Important concepts (carbon cycle)
Net Primary Production (NPP) : NPP = Gross photosynthesis – respiration
Photosynthesis: Convert Carbon dioxide and water into sugar and oxygen using the light of the sun to ”fuel” the reaction
Respiration: Process of reactions breaking large molecules into smaller ones, the energy released during the process is used as ”fuel” by the cell
Primarily concerned with aerobic respiration (oxygen dependant), where oxygen and sugars are broken down into Carbon dioxide, water and heat through combustion
Captures Carbon (CO₂)
Releases Carbon (CO₂)

21. Photosynthesis and respiration

22. Important concepts (carbon cycle)
Organic Carbon (natural): usually complex compounds containing carbon produced during the life cycles of plants and animals (such as photosynthesis, respiration and decaying)
Dissolved Organic Carbon (DOC): organic carbon that is dissolved in water
Particulate Organic Carbon (POC): organic carbon incapable of dissolving in water

23. Fluxes The Carbon Cycle Responsible for atmospheric build-up of Oxygen
Volcanos = 0.1 y⁻¹
Fluxes
microbial respiration = 50 y⁻¹
Soil / microbial respiration = 50 y⁻¹
Responsible for atmospheric build-up of Oxygen

24. The Carbon Cycle
Sinks
Fossil Fuels

25. The Carbon cycle Human Influence
Removal rate >> replacement rate

26. The Methane cycle

27. The Methane (CH₄) cycle
Human Influence
Captures 100 times more heat than CO₂!

28. Discussion:
Assuming that human-influenced discharge of carbon dioxide and methane stopped immediately, what do you suppose would happen to the carbon and methane cycle? How would the Earth’s increased temperature affect this? When would the Earth return to its pre-human state?

29. The Oxygen cycle

30. The ”Great Oxidation” and the Oxygen cycle
The ”great oxidation” event, estimated to have occurred 2.4B years ago, is what is responsible for almost all of the free oxygen in the atmosphere. Photosynthesis was producing oxygen but it was being bound to organic matter and dissolved iron and immediately removed from the atmosphere. It wasn’t until these substances were saturated that oxygen begin to accumulate in the atmosphere and the oxygen cycle as we know it today began.

31. The Oxygen cycle
The primary driver of the oxygen cycle is photosynthesis (oceanic and terrestrial)
Almost exactly balanced by respiration
The atmosphere is the largest reservoir of free oxygen and has the longest turnover time (3.7M years)

32. The Oxygen cycle
Fluxes

33. The Oxygen cycle
Sinks

34. Human influence

35. Discussion:
What human influences on the oxygen cycle are not explicitly mentioned in the previous box diagrams? What effects would they have on the Oxygen cycle?

36. The Nitrogen cycle

37. Nitrogen cycle in the biosphere
Nitrogen is a very important part of many biological processes but it cannot be used in its elemental (N) form by ”higher order” plants and animals as it does not react with other chemicals easily. Processes which convert free nitrogen into more accesable forms are fixation and nitrification. Mineralization and denitrification then move it back molecular nitrogen. All of these processes are an integral part of the biosphere.

38. Nitrogen cycle in the biosphere

39. The Nitrogen cycle
Fluxes

40. The Nitrogen cycle
Sinks

41. The Nitrogen cycle
Human Influence

42. Discussion:
Artificial nitrogen fixation (ex: man-made fertilizers) has introduced a vast ammount of fixed nitrogen into the nitrogen cycle. What consequences does this have on the surrounding ecosystems, most notably the aquatic ecosystems?

43. Eutrophication
The addition of artificial nutrients (fixed nitrogen) to natural waters can cause a dramatic increase in population of nutrient demanding organisms (ex: cyanobacteria (algae) and phytoplankton) which can cause a dramatic decline in oxygen levels in the water (hypoxia) thus making the water inhabitable by many marine animals.

44. Eutrophication

45. The Sulfur cycle

46. The Sulfur cycle
The sulfur cycle has been dramatically altered by human interaction. Sulfur (S) is present in fossil fuels and when they are burnt they are released into the atmosphere where it reacts with water and can form what is known as ”acid rain”. Without fossil fuel burning, sulfur would remain in rocks and sediments for very, very long periods of time. There are two sulfur cycles presented: one pre-industrial and one of the current state.

47. The Sulfur cycle (preindustrial)

48. The Sulfur cycle (mid-1980s)

49. Discussion:
Acid rain is caused, in part, by industrial processes injecting sulfur into the atmosphere which, when it reacts with water, can precipitate out of the atmosphere and damage natural and anthropogenic landscapes. Considering that atmospheric processes are very complex and hard to estimate and that there are natural causes of acid rain (ex: volcanos) do you think industries discharging large amounts of sulfur can be held accountable for acid rain?

50. Conclusions
The cycling of chemicals on Earth has been greatly altered by human interaction.
Knowledge of how cycles have been altered is important for many numeric models of the environment and must be accounted for (ex: the global climate models have sub-models of these cycles imbedded in them)
Understanding these cycles is essential to understanding global climate change
Many of the cycles are coupled (one chemical affects another); the cycles are way, way more complex than shown above.
The figures presented here are not complete or absolutely accurate and should only be used to give an idea of the general behavior of the cycling.