BIOGEOCHEMICAL CYCLESIntroduction

Credit: U.S. Department of Energy Genomic Science Program.

THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS

Earth composition was determined by chemical composition of the solar nebula from which the solar system was formed and by the nature of physical processes that concentrated the material from which planets were formed (condensation , accretion)

Most abundant elements: oxygen (in solid earth!), iron (core), silicon (mantle), hydrogen (oceans), nitrogen, carbon, sulfur…

The elemental composition of the Earth has remained essentially unchanged over its 4.5 Gyr history. Extraterrestrial inputs (e.g., from meteorites, cometary material) have been relatively unimportant. Escape to space has been restricted by gravity.

Venus Earth Mars

Radius (km) 6100 6400 3400

Surface pressure (atm) 91 1 0.007

CO2 (mol/mol) 0.96 3x10-4 0.95

N2 (mol/mol) 3.4x10-2 0.78 2.7x10-2

O2 (mol/mol) 6.9x10-5 0.21 1.3x10-3

H2O (atm, mol/mol) 3x10-3 1x10-2 3x10-4

What makes Earth special :

• O2, N2 and H2O are in ratio which do not satisfy a global thermodynamic equilibrium ( ultimately HNO3 should be formed and dissolve in the oceans).

• O2 coexists with combustible biomass (ultimately O2 should be used to oxidize biomass , leading to CO2)

• Acidic material in the atmosphere co-exist with alkaline material in rocks.

Important role of the ‘Biosphere’ : ultimately what ties the different compartments of the Earth system together.

e.g. if you look at earth present atmosphere in comparison to ‘sister’ planets

EVOLUTION OF O2 AND O3 IN EARTH’S ATMOSPHERE

A composite of different ‘domains’ having very different physical and chemical properties

COUPLING and CYCLING between different GEOSPHERES

Physical exchange, chemistry biochemistry are involved

Solar radiation

Radioactive decay

Evidence of coupling between different variable of the system ?

The Gaia hypothesis

The view of a coupled nature of the earth system has not always dominated the development of the key components of Earth sciences, which have indeed evolved into highly refined disciplines (e.g meterology, oceanography etc). For example, the biosphere was considered to be forced by constraints imposed by other component.

The GAIA hypothesis was put forward by Lovelock and Margulis (1974) to provide a basis for integrating all component of the Earth systems.

The Earth’s biota as well as the planet itself are part of a quasi-living entity that has a capacity for self regulation …. ‘ by and for the biosphere’

As the GAIA hypothesis evolved the interdependence of biotal evolution and geophysical/geochemical systems is described in non-teleological terms.

Gas Mole fraction 1 ppm= 1x10-6

Nitrogen (N2) 0.78

Oxygen (O2) 0.21

Water (H2O) 0.04 to < 5x10-3; 4x10-6 -strat

Argon (Ar) 0.0093

Carbon Dioxide (CO2) 370x10-6 (date: 2000)

Neon (Ne) 18.2x10-6

Ozone (O3) ¶ 0.02x10-6 to 10x10 –6

Helium (He) 5.2x10-6

Methane (CH4) 1.7x10-6

Krypton (Kr) 1.1x10-6

Hydrogen (H2) 0.55x10-6

Nitrous Oxide (N2O) 0.32x10-6

Carbon Monoxide (CO) 0.03x10-6 to 0.3x10-6

Chlorofluorocarbons 3.0x10-9

Carbonyl Sulfide (COS) 0.1x10-9

Atmospheric Composition

(average)

red = increased by human activity

¶ Ozone has increased in the troposphere, but decreased in the stratosphere.

The anthroposphere as a ‘new’ component of biogeochemical cycles …

NOAA Greenhouse gas records

Biogeochemical Cycles of elements Beside physical exchange through geospheres (e.g atmosphere-ocean, ocean-continents), there is a complementary set of chemical cycles that we can describe for each of the most important elements (carbon, nitrogen, oxygen, sulfur ..)

Some important definitions:

Reservoir (box, compartment) : An amount of material defined by certain physical, chemical or biological characteristics (e.g. carbon in the atmosphere, carbon in the ocean, carbon in terrestrial biosphere …). Characterize by a content M (mass, mole) or a concentration when normalized per unit of volume.

Source : Characterize a process that increase the content of a reservoir over time (unit ?).

Sink : Characterize a process that decrease the content of a reservoir. The loss is often assumed to be proportional to the content of the reservoir.

Flux : The amount of material transferred from one reservoir to another per unit of time (flux density if normalized per unit of surface) e.g. the rate of evaporation of water to the atmosphere (unit ?).

Budgets : A balance between all source and sinks of a reservoir. If source and sink balance exactly, M is constant and the reservoir is in steady-state.

Turn-over time (or residence time) : Strictly applies to a reservoir in steady state: time required to turn-over the reservoir. Calculated as the ratio of the content to the sum of the sinks (or sources). In atmospheric chemistry, residence time is sometime called lifetime of a species. One can define specific residence times relative to specific sinks. The inverse of residence time can be viewed as a ‘transfer constant’, or a ‘kinetic constant’ if we are talking about chemical reactions (this will be detailed later) .

Additional remarks : one can also work with volume normalized system. The variable characterizing the reservoir are then concentrations, sources and sinks are defined per unit of volume

The bathtub example ….

Could you characterize the evolution of water content m (kg of water) ) in these cases ?

1) Bathtub initially empty with tap on (S in kg.s-1) and drain closed.

2) Bathtub initially at m0 (in kg) with drain (k in s-1) open.

3) Bathtub initially at mo with tap on and drain open

Q : Difference between steady state and equilibrium for a chemical system ?

Steady state solution (dm/dt = 0)

Initial condition m(0)

Characteristic time t = 1/k for• reaching steady state• decay of initial condition

If S, k are constant over t >> t, then dm/dt g 0 and mg S/k: "steady state"

( ) (0) (1 )kt ktdm SS km m t m e e

dt k

• Determine how fluxes of a given element or (group of element) depends on reservoir nature/contents.

• Determine how fluxes are affected by environmental (physical and chemical) factors through different processes .

Building biogeochemical models

Determine how changes (e.g. anthropogenic perturbation) affects fluxes (cycling) and content of reservoirs.

Trans-disciplinary approach !

Biogeochemical Cycle of elements : overall objectives