Introduction to Biogeochemical Cycles Jean-Marc Mayotte jean_marc.mayotte@geo.uu.se

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

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.

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

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

Hydrosphere The total mass of all liquid water on Earth

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

Biosphere Global sum of all ecosystems on Earth

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

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)…

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.

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

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

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)

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?

Water cycle (in box diagrams) Reservoir Pool size [10³km³], pool size [%], flux [km³/y] Atmosphere 13 (0.0009%) (9d) Salt lakes 104 (0.008%) (10-1000y) precipitation 111 000/y 40 000/y Freshwater lakes 125 (0.009%) (1-100y) 71 000/y Polar ice, glaciers 29 000 (2.08%) (16 000y) Rivers 1.2 (0.00009%) (12-20d) 425 000/y precipitation Evaporation Evapotranspiration Evaporation 385 000/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

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?

The Carbon cycle

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.

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)

Photosynthesis and respiration

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

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

The Carbon Cycle Sinks Fossil Fuels

The Carbon cycle Human Influence Removal rate >> replacement rate

The Methane cycle

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

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?

The Oxygen cycle

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.

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)

The Oxygen cycle Fluxes

The Oxygen cycle Sinks

Human influence

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?

The Nitrogen cycle

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.

Nitrogen cycle in the biosphere

The Nitrogen cycle Fluxes

The Nitrogen cycle Sinks

The Nitrogen cycle Human Influence

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?

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.

Eutrophication

The Sulfur cycle

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.

The Sulfur cycle (preindustrial)

The Sulfur cycle (mid-1980s)

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?

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.