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Global Change
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Biogeochemical Processes
Innumerable biological, geological, and chemical processes cycle elements throughout the ecosphere. The few discussed in this section should
give the reader an idea of their variety and complexity. As an example, consider a group of organisms called the prokaryotes: the bacteria and bluegreen algae. The processes that these organisms are
involved with (summarized in Table 1) include:
• methane production and oxidation
• sulfur reduction and oxidation
• nitrogen fixation, nitrification, and
denitrification.
This list is given only as an example; some of
these processes will not be discussed in the text.
These prokaryotic processes may take place
in a variety of ways, such as (1) autotrophy, in
which the organisms convert inorganic carbon in
the environment to organic matter; (2) heterotrophy, in which the products from the breakdown
of organic compounds are used to make new
organic materials; and (3) mixotrophy, in which
both inorganic and organic compounds are used
to make organic matter.
• the capture of carbon dioxide from the
atmosphere and its conversion to organic
matter (fixation of CO2)
• the release of CO2 back to the atmosphere
(through respiration and decay)
• fermentation of sugar
Table 1. Biogeochemical reactions involving prokaryotes
Element
Process
Summary of partial
chemical reactions
Examples of organisms
involved in process
Carbon
CO2 fixation
CO2 + H2 ⇒ (CH2O)n +
A2 (A = O, S)
Photoautotrophs:
cyanobacteria, purple and green
sulfur bacteria
Chemoautotrophs:
sulfur and iron oxidizing bacteria
Methanogenesis
COO- + H2 ⇒ CH4
Methanogenic bacteria
Methanotrophy
CH4 + O2 ⇒ CO2
Methanotrophic bacteria
Fermentation
(CH2O)n + O2 ⇒ CO2
Anaerobic heterotrophic bacteria
Respiration
(CH2O)n + O2 ⇒ CO2
Aerobic heterotrophic bacteria
Sulfur reduction
SO4 + H2 ⇒ H2S
Sulfur-reducing bacteria
Sulfur oxidation
H2S ⇒ S0
Purple and green sulfur phototrophs
S0 + O2 ⇒ SO4
Sulfur oxidizing bacteria
N2 fixation
N2 + H2 ⇒ NH4
Phototrophic bacteria, nitrogen-fixing
heterotrophic bacteria
Nitrification
NH4 + O2 ⇒ NO2, NO3
Nitrifying bacteria
Sulfur
Nitrogen
Denitrifying bacteria
Denitrification
NO2, NO3 ⇒ N2O, N2
__________________________________________________________________
After Stolz et al., 1989
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Understanding Global Change: Earth Science and Human Impacts
Principles of chemical reactions
Atoms and elements
Every object in the universe is composed of matter. Because matter can be converted to energy, it is essentially a form of energy. Matter is composed of atoms, which are the smallest particles of an element that can
exist either alone or in combination. An atom is also the smallest particle that can enter into a chemical reaction. Most atoms never change; they only combine with other atoms to make different substances. Radioactive
atoms, however, do change and eventually decay into stable, nonradioactive atoms.
Elements consist of atoms of the same kind and, when pure, cannot be decomposed by a chemical change.
There are 106 known elements; 103 are listed in the periodic table (Figure 2). The elements most used commercially by people, in order of use, are carbon (C), in the form of coal, oil, and gas; sodium (Na), in table salt
and other products; iron (Fe), used in the steel industry; and nitrogen (N), sulfur (S), potassium (P), and calcium (Ca), all used in fertilizers or as soil conditioners for our food supply.
Compounds
When two or more atoms are bonded together in a definite proportion, a compound is formed. Examples of
compounds discussed in this text are water (H2O), carbon dioxide (CO2), salt (NaCl), and sugar (e.g., glucose,
C6H12O6 ). (All of the compounds named in the text are listed in Table 2.) The numbers in these chemical formulas are the number of atoms of each substance in the compound. If only one atom of a substance is in the
compound, no number is given. The universe is composed of millions of these compounds, all created from the
elements given in the periodic table. The smallest particle of a compound that can exist and exhibit the properties of that compound is called a molecule.
A compound is a pure substance that can be decomposed by a chemical change. The atoms in the chemical
compound may rearrange themselves, or they may separate from the compound to form different compounds.
These changes and interactions among compounds are called chemical reactions.
Chemical equations
A chemical equation expresses a chemical reaction involving compounds or elements. The chemicals that
react together, called reactants, generally are shown on the left-hand side of the equation and the products on
the right-hand side. Consider the decay of plant material (represented by the chemical compound CH2O, a carbohydrate), which requires the oxygen gas (the chemical compound O2) in the earth’s atmosphere. The simplest chemical equation representing this process is
CH2O + O2 ⇒ CO2 + H2O (1)
The arrow pointing right indicates that this process is irreversible; the plant material will be completely oxidized to CO2 and H2O in the presence of atmospheric oxygen. Other processes are highly reversible, and these
are usually represented by a double arrow. For example, the equilibrium between calcium carbonate and its
dissolved calcium and carbonate ions (atoms or molecules that have lost or gained electrons, with the number
lost or gained shown as a positive or negative superscript) is represented as
CaCO3 ⇔ Ca2+ + CO32-
(2)
In chemical processes, matter cannot be created or destroyed. Thus, when a chemical equation is written,
the total number of atoms of any particular element on the left-hand side of a chemical equation must be
made to equal the total number of atoms of that element on the right-hand side of the equation. This is the
process of balancing a chemical equation. Balancing the equation expresses the fact that molecules usually
react in such a way as to bear simple, integral, numerical relationships to one another.
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Global Biogeochemical Cycles and the Physical Climate System
If these relationships are known, it is possible to calculate the masses of reactants and products by using
known atomic and molecular weights. In chemical terms, the amount of a substance is expressed in moles. One
mole of a substance is the amount that contains as many elementary entities as there are atoms in 12 grams of
carbon. This number is termed Avogadro’s constant, and its value is equal to 6.022 x 1023. In the chemical
equation given above for the equilibrium of CaCO3 and its dissolved chemical species, one mole of CaCO3 will
dissolve in water to make one mole of Ca2+ and one mole of CO32-. In terms of mass, 100 grams of CaCO3 will
react to give 40 grams of Ca2+ and 60 grams of CO32-. If only 10 grams of CaCO3 were to dissolve, then the
same proportions of Ca2+ and CO32- would be present at the equilibrium: 4 and 6 grams, respectively.
Figure 2. Periodic table of the elements. Each box includes an element’s atomic number,
chemical symbol, and atomic weight.
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Understanding Global Change: Earth Science and Human Impacts
Table 2.
Chemical formulas and names used in this module
Al2Si2O5(OH)4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .kaolinite
Ca2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .calcium ion
CaCO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .calcium carbonate
CaSiO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .calcium silicate
Ca5(PO4)3(OH,F) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbonate fluoroapatite
CH2O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbohydrate
(CH2O)106(NH3)16H3PO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .organic matter in marine phytoplankton
CH4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .methane
CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbon dioxide
CO32- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbonate ion
CS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbon disulfide
C6H12O6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .sugar (glucose)
DIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .dissolved inorganic carbon
DMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .dimethyl sulfide, (CH3)2S
HCO3- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .bicarbonate ion
HNO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .nitric acid
H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .molecular hydrogen
H2O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .water
H2S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .hydrogen sulfide
H2SO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .sulfuric acid
H3PO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .phosphoric acid
H4SiO40 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .monomeric silicic acid
KAlSi3O8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .orthoclase feldspar
MSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .methane-sulfonic acid
NaAlSi3O8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .albite
NaCl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .sodium chloride, common table salt
NH3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ammonia
NH4+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ammonium ion
NH4NO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ammonium nitrate
(NH4)2SO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ammonium sulfate
NMHC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .nonmethane hydrocarbon
NO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .nitric oxide
NO3- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .nitrate ion
NOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .oxides of nitrogen
N2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .diatomic nitrogen
N2O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .nitrous oxide
OCS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .carbonyl sulfide
OH* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .hydroxyl radical
OH- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .hydroxyl ion
O2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .diatomic oxygen (pure oxygen molecules)
O3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ozone
PAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .peroxylacetyl nitrate
PH3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .phosphine or swamp gas
PO43+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .phosphate ion
SO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .sulfur dioxide
SO42- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .sulfate ion
SOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .oxides of sulfur
SiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .silica
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Global Biogeochemical Cycles and the Physical Climate System
Incidentally, the carbon dioxide and donor
molecule used for photosynthesis are not the
only requirements for plant growth. Plants also
need nitrogen, phosphorus, sulfur, potassium,
and a dozen or so trace elements, like zinc and
iron. As we shall see below, human activities are
changing the atmospheric concentrations of these
nutrients as well as that of carbon, with various
possible effects on plants.
The photosynthetic reactions that produce
organic matter on land differ from those in the
ocean because the proportions of carbon, nitrogen, sulfur, and phosphorus in land vegetation
differ from those in marine plankton. The ratio of
C:N:S:P in marine plankton is 106:16:1.7:1.
Known as the Redfield ratio, this proportion is
fairly constant for the surface-dwelling, microscopic plants (phytoplankton) of the world’s
oceans. The C:N:S:P ratio for land plants is more
variable but averages 882:9:0.6:1. The amount of
carbon is so much greater in land vegetation
because it is stored as cellulose in the structural
tissues of trees and grasses.
From this summary, it can be seen that
photosynthesis (among other things) links the
biogeochemical processes and cycles of the individual organic elements of carbon, nitrogen,
phosphorus, and sulfur. These elements, plus
hydrogen and oxygen, are the major constituents
of organic matter. Those six elements and about a
dozen or so minor elements are necessary for the
maintenance of organic structures and the physiological functions of living organisms.
Photosynthesis
We begin with perhaps the most important
biogeochemical process of all, photosynthesis. It is a
photoautotrophic process, that is, an autotrophic
reaction in the presence of light. Nutrients such as
phosphate (PO43-) and nitrate (NO3-) are also necessary for this reaction to occur.
In the early stages of our planet’s formation,
the atmosphere was very different from that of
today. There was no free molecular oxygen (O2),
which most of today’s life forms require. In fact,
oxygen was a very powerful poison for the simple organisms that lived in this early, oxygendeficient (anaerobic) world. Both the organisms
and the earth had to evolve to a stage where the
organisms produced oxygen and emitted it to
their environment before more advanced life
forms could evolve. Photosynthesis, the process
of constructing complex organic molecules from
simple inorganic ones in the presence of light,
was a critical step in the evolution of life and
allowed the mass of living organisms to grow to
the level of today. In our world, the mass of living
organisms on earth is equivalent to about 600 billion tons of carbon. More than 99% of this carbon
is in land plants; the remainder is stored in
marine plants and in animals.
Photosynthesis is basically a chemical reaction
or process in which carbon-, hydrogen-, and oxygen-bearing chemical compounds (carbohydrates)
are synthesized from atmospheric CO2 and H2O or
another chemical compound that can act as a
hydrogen donor. The generalized reaction is
Respiration and Decay of
Organic Matter
energy + nCO2 + 2nH2A ⇒ (CH2O)n +
(3)
nH2O + 2nA
In the life cycle, photosynthesis in plants is
balanced by the complementary processes of respiration and decay in plants and animals. In
plants, respiration is the breakdown of the complex organic molecules that were formed during
photosynthesis. The chemical reactions for respiration and decay are the reverse of those shown
above for the production of organic material. The
generalized reaction is
C6H12O6 + 6H2O + 6O2 ⇒ 6CO2 +
12H2O + energy
(5)
where H2A is a hydrogen donor molecule,
(CH2O) is a carbohydrate, and n stands for any
number. In higher plants, the donor molecule is
water, and n = 6. Thus for these plants the specific
reaction is
energy + 6CO2 + 12H2O ⇒ C6H12O6 +
6H2O +6O2
(4)
For photosynthetic sulfur bacteria the donor molecule is hydrogen sulfide (H2S), and for nonsulfur
purple bacteria it is organic compounds.
DRAFT
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Understanding Global Change: Earth Science and Human Impacts
Compare this with reaction 4. The amount of
energy released is about 686 kilocalories (kcal) for
each mole of C6H12O6 (glucose, the most common
form of sugar in living things) that is broken down.
In animals, the respiratory oxidation of foods
—that is, the loss of electrons from the carbon in
carbohydrates, occurring during digestion—
provides energy for a variety of uses, including
maintenance of body temperature, muscular
movement, and synthesis of complex organic
compounds.
During the oxidation of organic matter, CO2,
nitrogen- and phosphorus-bearing nutrients, and
bioessential trace elements (e.g., iron) are returned
to the environment to be used again in the production of more organic matter. When O2 is available, it is the oxidizing agent (oxidant); however,
in oxygen-depleted (anoxic) waters, sediments,
and soils, other oxidants are used. These include
nitrate, sulfate, and iron and manganese oxides.
The chemical equations for respiration and
decay, either in an oxygenated or in an anoxic
environment, are more complex than the generalized reaction for photosynthesis given above. For
example, the chemical composition of average
marine phytoplankton—a relatively simple form
of life—is (CH2O)106(NH3)16H3PO4: 106 molecules of carbohydrate, 16 of ammonia, and 1 of
phosphoric acid. When dead phytoplankton react
with O2 in an oxygenated environment, the products are carbon dioxide, nitric acid, phosphoric
acid, and water:
Weathering of Rocks
Another very important set of biogeochemical processes is that involved with the breakdown of rocks exposed to rain, wind, and ice.
Weathering prepares rock for erosion and transportation. Its products are dissolved chemical
species and solids derived from changes in the
primary minerals of the rock being weathered.
The solid products are predominantly clay minerals; there are also dissolved products, predominantly calcium, carbon, and silicon. Ultimately,
the products of weathering are either carried by
water, blown as dust, or carried by glaciers to the
ocean. Of the approximately 20 billion tons of
solids and dissolved materials reaching the ocean
annually from the land, more than 80% is delivered by rivers. However, high-temperature chemical reactions in the presence of seawater along
the great submarine midocean ridges are significant sources of dissolved calcium, silica, and iron
for the oceans.
An example of a chemical weathering reaction is the weathering of the mineral albite (the
inorganic chemical compound NaAlSi3O8), found
in igneous rocks like basalt, to the clay mineral
kaolinite [Al2Si2O5(OH)4]. The reaction takes
place principally in the presence of soil water
and groundwater that contain significant amounts
of dissolved CO2. Although the ultimate source
of the CO2 is the atmosphere, much of it does not
come directly from the air but is produced in
soils by the respiration of plants and the decay of
dead plants and animals. Because of these
processes, the concentration of CO2 in soils may
be one or more orders of magnitude greater than
that of the atmosphere. The elevated CO2 levels
give rise to acidic soil solutions, and these corrosive, low-pH soil solutions are responsible for the
weathering of rock minerals like albite:
(CH2O)106(NH3)16H3PO4 + 138O2 ⇒
106CO2 + 16HNO3 + H3PO4 + 122H2O
(6)
For an example of respiration and decay in an
anoxic environment, let us consider the reduction
of sulfur in sulfate (SO42-) in the pore waters of
anoxic sediments. Bacteria use the oxygen originally bound in the sulfate to oxidize organic matter. Again using phytoplankton as the organic
matter, the equation for this chemical reaction is
2NaAlSi3O8 + 2CO2 + 11H2O ⇒
Al2Si2O5(OH)4 + 2Na+ + 2HCO3- + 4H4SiO40 (8)
(CH2O)106(NH3)16H3PO4 + 53SO42- ⇒
106CO2 + 16NH3 + H3PO4 + 53S2- + 106H2O (7)
The products of this reaction, besides the
kaolinite, are sodium ion, bicarbonate ion, and
monomeric silicic acid.
In regions where human activities such as
coal burning release considerable amounts of sulfur and nitrogen oxide gases to the atmosphere,
This time, in addition to carbon dioxide, phosphoric acid, and water as in reaction 6, the products include ammonia (NH3) and sulfide (S2-).
8
Global Biogeochemical Cycles and the Physical Climate System
inorganic carbon brought to the oceans annually
by rivers. The other half of the riverborne carbon
is released to the ocean and atmosphere when
skeletal carbonate minerals are formed.
Dissolved silica is also removed from the
oceans in the skeletons of marine organisms.
Certain of these organisms—planktonic diatoms
(algae), radiolarians (protozoans), dinoflaggelates
(protozoans), and benthic sponges—use dissolved silica to form their shells of opaline silica.
After these organisms die, most of the opal dissolves, because the oceans throughout their
extent are undersaturated with respect to this
chemical compound. Only about 40% of the total
annual production of skeletal silica sinks below
the parts of the ocean that daylight reaches (the
euphotic zone). Most of this siliceous material dissolves en route to the seafloor; only 5% of that
produced in the euphotic zone accumulates in
marine sediments. This amount is about equivalent to the annual input of dissolved silica to the
oceans by rivers.
such as the midwestern and eastern United States
and southern China, the pH of rainwater and
consequently soil water may be lower (more
acid) than natural values. This happens because
the gases oxidize and react with water in the
atmosphere and then rain out as sulfuric and
nitric acids, respectively. This phenomenon is the
environmental problem of acid deposition (often
called acid rain), which in extreme forms can be
responsible for increased fish mortalities in lakes
and decreased agricultural production.
Deposition in the Oceans
When the solid and dissolved products of
weathering reach the ocean, the solids settle out
because of their weight and are deposited on the
seafloor as gravel, sand, silt, and mud. How long
the dissolved products remain in the ocean
depends on how long it takes them to enter into a
chemical or biochemical reaction. As an example
of the periods involved, dissolved sodium in the
ocean has a long residence time, about 55 million
years. At the other end of the time scale, the residence time of dissolved silica is only 20,000 years.
Sodium and magnesium
In contrast to carbon and silica, which are
removed from the ocean primarily by biological
processes, riverborne dissolved sodium and magnesium are removed to a significant extent by
inorganic chemical reactions. Both of these elements are involved in hydrothermal reactions
between seawater circulating through midocean
ridges and the basalt rock making up the ridges.
In the hydrothermal reaction process, sodium
and magnesium are removed from the seawater.
Sodium is also removed from the ocean by the
precipitation of halite (common table salt, sodium
chloride) from seawater. This process is very
important as a removal mechanism for sodium
and chlorine, but only occurs when the right set
of climatic and tectonic conditions are achieved.
Only seawater in relatively isolated arms of the
sea can be sufficiently evaporated to reach halite
saturation. Thus, because such environments are
scarce today, it is likely that sodium and chlorine
brought to the oceans by rivers are currently
accumulating in seawater.
Some magnesium is also removed from seawater by chemical processes in the pore waters of
Calcium and silica
Many of the processes by which dissolved
constituents are removed from the ocean involve
marine organisms. In today’s oceans, dissolved
calcium and bicarbonate are precipitated as carbonate minerals in the skeletons of several kinds
of marine organisms: planktonic foraminifera (protozoans), pteropods (mollusks), and Coccolithophoridae (algae), and bottom-dwelling (benthic)
corals, echinoids, mollusks, and coralline algae. Of
the total production of skeletal carbonate in the
oceans, equivalent to about 1 billion tons of carbon
per year, 80% is redissolved in the ocean as skeletal debris sinks to the seafloor. This efficient recycling is due to the fact that although the surface
ocean is oversaturated with respect to calcium carbonate, the deeper sea is undersaturated with
respect to this mineral. The remaining 20% of the
ocean’s carbonate production accumulates in shallow-water and deep-sea sediments.
The amount of carbon in these sediments
only accounts for about one-half of the dissolved
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Understanding Global Change: Earth Science and Human Impacts
sediments. These processes taking place during
the burial of sediments are collectively referred to
as diagenesis. The relative importance of diagenetic and hydrothermal reactions for the removal of
magnesium from seawater is a topic of current
scientific research and debate.
We can conclude from the above discussion
that the circulation of material through the ecosphere is complex and involves myriad chemical,
biological, and geological processes. The system
is truly biogeochemical in nature. On all time and
space scales, if the composition of the ecosphere
is regulated, the regulation is controlled by a
complex of interwoven inorganic and organic
processes. The maintenance of the equable environment, including climate, that is required for
life to exist on earth is a product of this interacting and interwoven web of biogeochemical
processes and cycles.
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