Earth System Science and Gaia
Peter Westbroek
The Earth is unique
No object in the solar system is known in more detail than the Earth. Yet, of all the heavenly bodies around,
there is not one as enigmatic as our own familiar planet. The astronomical perspective makes us aware that
our surroundings are not commonplace, as we usually think, but highly aberrant and unique. We are
reminded of astronaut James Lovell who, on his way to the moon, made the first pictures of the Earth from
outer space. ‘It was a moment of great excitement’, he recalls. ‘When I saw that colorful globe, as big as my
thumb nail, in the immense black sky with brilliant stars, I knew that this was home, more so than the street
where I live, or even my own country America’.
Major distinctive features of the Earth include the following. In the first place, of course, our planet has a
prominent biosphere. The biotic habitat not only includes the troposphere, the oceans and the continental
surface, but penetrates several kilometers into the solid Earth, where temperatures up to 120°C may
prevail.
Secondly, liquid water covers about two thirds of the planetary surface. Water is thought to have
episodically flooded parts of the Martian surface, but the Earth’s ocean has persisted for more than 3.8
billion years. Furthermore, an ocean is suspected to exist on the Jovian moon Europa; if it exists, it is
covered with a permanent sheet of ice. Only during particularly cold spells, especially in the Late
Proterozoic (some 600 million years ago) glaciers appear to have reached the Earth’s equator (‘snowball
Earth’, Hoffman and Schrag, 2000), but it is highly doubtful whether the ice completely separated the liquid
ocean below it from the atmosphere.
A third distinctive feature of the Earth is the fact that the planet steadily turns itself inside out. Rocks are
involved in a cyclic process of renewal and disintegration, known as the rock cycle. Their melting in the
deep Earth and their subsequent solidification is followed by uplift, exposure, weathering, erosion, transport
and sedimentation until a new phase of burial and melting sets in. Plate tectonics has been the dominating
geodynamic regime over at least 2.7 billion years, while less ordered mechanisms appear to have
prevailed in earlier days. A particularly characteristic effect of the rock cycle is the differentiation of rock
types, ranging broadly from the dense mantle-derived peridotites, into less dense ocean basalts and finally
into the relatively light-weight continental crust of diorite and granite. The continents, which may be
considered as buoyant slugs resulting from this differentiation, owe their relief to isostatic ‘floating’ on the
underlying mantle. They appear to be a very characteristic feature of the Earth. Note that recently
discovered magnetic anomalies on the surface of Mars may be interpreted as signs of plate tectonic activity
in the early stages of that planet’s history. It is thought that crustal motions on Mars have come to a
standstill long ago, owing to smaller volume and hence more rapid cooling of that planet.
Finally, also the Earth’s atmospheric composition is aberrant. Whereas Venus and Mars have atmospheres
that are oxidized and largely consist of CO2, and the large outer planets Jupiter, Saturnus and Uranus
retained their original reduced atmospheres (hydrogen – methane - ammonia), the Earth has a mixture of
oxidized and reduced components (oxygen and nitrogen), while CO 2 only occurs in trace amounts. In
particular the presence of low concentrations of methane in combination with oxygen indicates that, unlike
the atmospheres of the other planets, the air we breathe is far removed from thermodynamic equilibrium.
Deepening of our insight into the unique character of the planet that we inhabit is now perceived as a major
challenge for science and a requirement for achieving a sustainable development for future generations.
Earth System Science and Geobiology
There is a wealth of evidence suggesting that the four major spheres on the outer Earth, the biosphere, the
hydrosphere, the solid Earth and the atmosphere, did not arrive at their aberrant state independently, but by
mutual interaction. The nature of these interactions is the subject of intense research, commonly referred
to as ‘Earth System Science’ (fig. 1). In this approach, the Earth is considered as a coherent system with
the various components interacting. The system is subject to cosmic and internal or endogenic forcing,
whereby material exchange with the internal and external domains is thought to be negligible on a
multimillion years times scale. On the billion years time scale, however, these material exchanges can not
be ignored.
The great scientific uncertainty concerning the nature of our planet’s behavior stems in part from the fact
that several fundamental mechanisms are essentially unknown. For instance, a coherent system theory of
biological organization is missing (see however Kooijman 2000) and we are ignorant about the effects that
the biota may have on endogenic activity (see however Anderson, 1984). Furthermore, a systems approach
to the Earth requires that the multiple interactions be quantified as rigorously as possible, not only in the
present situation, but also, by reconstruction from the geological record, in the geological past.
The major stumbling block for progress in this field may be in the required transdisciplinary approach, as
this asks for a profound cultural change in the practice of research. The impressive progress of science in
the past 50 years caused a steady progression of specialization. Each discipline developed its own jargon,
concepts and assumptions. As a result, a technical discussion between scientists from different fields
became increasingly difficult. It is easily said that specialists in widely different disciplines must collaborate,
but it turns out to be very hard to select transdisciplinary problems that can be adequately addressed. It is
encouraging to see however that in scientific circles there is a growing enthusiasm for a new, integrated
science of the Earth. Thousands of researchers make their specialized knowledge available in many
transdisciplinary projects related to the System Earth.
The understanding and quantification of the processes that link the biota with the rest of the world (fig. 1)
remain the most elusive aspect of Earth System Science. Until a few decades ago, it was customary, in
both geology and biology, to regard life as merely adapting to the interplay of non-biological forces. The
reverse, i.e., the influence of life on the Earth’s dynamics was largely ignored. The main reason was that
this influence is highly non-linear and therefore particularly difficult to quantify. As long as the science was
concerned with reconstructing the broad outline of planetary history, geology and biology could better
advance separately. However, with growing insights into the details, the study of life as a geological force
became unavoidable. At present, the interactions between the living world and the non-living environment
are perceived as a core subject in Earth System Science, known as Geobiology (fig. 1).
In the following, I can merely illustrate a few of the geobiological interactions with some examples. Yet, I
hope that this modest approach will help to communicate the flavor of this burgeoning field. By way of an
introduction, it is useful to first epitomize some major features of the history of the Earth System.
Earth System History in a day
The Solar system (and thus also the Earth) is estimated to be 4.57 billion years old. In order to develop a
first intimation with the time scale relationships of major geological events, it is useful to condense this long
span of time into a single 24-hour day running from midnight to midnight. One hour in the Earth-day
represents 190 Ma (million years), one minute 3 Ma and one second 50 thousand years.
On this contracted time scale the origin of the Moon was at 20 minutes after midnight and the accretion of
the Earth ended at 0 hr 30 minutes. By that time, the cooling of the planetary surface and the formation of
the Earth’s core were completed and the original, dense atmosphere was lost. Until 3:55 AM an intense
bombardment by meteorites destroyed most of the crust, and it is unlikely that life, if it was there, could
have survived for extended periods of time. Despite this highly destructive regime, the oldest known
remains of continental crust survived from 2:55 AM. Recently, minute crystals of the mineral zircon were
discovered in a much younger parent rock, which was dated at 1 AM (Halliday, 2001). This is remarkable,
as zircon is formed during granite formation, and in the presence of water. Thus, only half an hour after the
end of accretion, continents, and probably even oceans, may have been present.
The oldest indications of biological activity, carbon isotope signatures indicative of the biological fixation of
carbon, date from 4 AM, only 5 minutes after the meteorite bombardment came to a halt, and the earliest
fossils, of bacterial origin, were found in rocks as old as 4:45. The oldest recognizable remains of plate
tectonic activity were formed at 10 AM. The Earth’s atmosphere remained reducing, with CO 2 and methane
as major constituents) until about midday, when appreciable amounts of oxygen (to about 1% of present
levels) began to accumulate. This event marks a major bottleneck for biological evolution, as, in principle,
oxygen is a particularly aggressive poison. The hitherto prevalent anaerobic life forms were forced to retreat
into anoxic refugia, such as the deep sea and the sediments, and only those with sufficiently effective
detoxifying mechanisms were capable to survive in direct contact with the open air.
Eukaryotic cells such as ours, with organelles and a nucleus, made their entry at about 5 PM. Thus, for 13
hours life has been exclusively bacterial. It should be stressed that the importance of bacteria has been
much underrated in the past. We now know that virtually all metabolic pathways evolved early in Earth
history within the bacterial world, and even today the biogeochemical cycles are predominantly catalyzed by
bacteria. Eukaryotes, which themselves originated from bacterial by symbiosis, only occupy a limited niche
of the vast repertoire of environments inhabited by the biota at large. Multicellular life arose around 8 PM,
and the major body plans of the extant animals evolved within a surprisingly short period of 3 minutes
around 9:10 PM, shortly after the severe ice age thought to have brought snowball-earth situations.
Vascular plants colonized the continents at 9:50. At 2 minutes to midnight, humans arrived, and the
industrial revolution took place at 3.7 milliseconds before midnight.
On the Earth-day time scale, global cycling makes a memorable impression. At its present rate, the mean
cycling time of the total sedimentary mass is 2 hours, and the ocean floor is renewed every half hour. The
water in the oceans is being renewed every second and stirred at 200 revolutions per minute. In just 18
seconds, the rivers would deliver a mass of dissolved material equivalent to that already in solution in the
oceans. It takes 0.2 and 140 milliseconds, respectively, to process all atmospheric CO 2 and O2 through the
biosphere. Clearly, the uninterrupted presence of life from 4 AM to the present severely limits variations in
the living environment, despite the frantic dynamics of the planetary system. For the continued reproduction
of conditions under which life can survive, not only the lithosphere, but also the other components of the
outer Earth system – hydrosphere, atmosphere, and biosphere – must have been in a recycling mode for a
long time.
Broadly speaking, the geological archive shows that the Earth system did not develop by gradual change,
but was characterized by an alternation of long periods of stasis and sudden ‘events’, critical
reorganizations that gave rise to new mechanisms and life forms. Some of these events are indicated
above. In addition to oscillating regimes (e.g., between warm (‘green-house’) and cold (‘ice-house’) worlds),
long-term trends are becoming apparent, not only in biological evolution, but also in the physics, chemistry
and biology of global dynamics. Major examples of such trends are the conversion of a reduced to an
oxidized atmosphere and the advent of plate tectonics.
Life and the rock cycle
The concept of the rock cycle was first formulated by James Hutton, by the end of the 18th century. In its
present form it is connected with the theory of plate tectonics. The relief at the surface of the solid earth is
generated by ’endogenic’ forces, such as mountain formation, volcanism, sea-floor spreading and
subduction. The endogenic processes are driven by radioactive decay in the deeper Earth. On the other
hand, ‘exogenic’ processes, which are powered by solar radiation, combine to equalize the relief. As soon
as they appear at the Earth’s surface, the rocks are weathered. The resulting debris (pebbles, sand and
clay) and dissolved materials (e.g., ions of calcium, bicarbonate and silica) are washed by rivers, glaciers
and wind into the oceans (erosion and transport) and end up as sediments, ultimately those that cover the
deep sea floor. From there the band wagon of sea-floor spreading transports the sedimentary mass via
subduction zones into the deeper domains of the Earth. They are compressed and heated, and brought
back up to the planetary surface in a subsequent cycle of mountain building and volcanism.
Through their strategic position at the interface of rocks, water and air, and in the light of the Sun, living
systems can play an active role in particular in those processes of the rock cycle taking place on the
outside of the planet: weathering, erosion, transport and sedimentation. The nature of this biological
intervention is best revealed by considering the contrast between the slow motion of plate tectonics and
mountain formation in comparison with the frantic activity of life. Essential nutrients, such as phosphate,
iron, copper or molybdenum are supplied in minute fluxes to the biota from the depths of the Earth. Hence,
biological activity is maintained through the evolution of a huge variety of mechanisms whereby organisms
can exploit to their own advantage the very limited nutrient supplies.
Nutrient supplies are increased in the first place by dramatic speeding up of weathering by the activity of
biological systems. Fungi, bacteria and plant roots penetrate into small cracks in the rocks and create
microenvironments where the minerals can readily disintegrate. From a biological point of view, weathering
may be compared with mining: raw materials are extracted from the rocks and supplied to living systems.
The nutrients are then kept in circulation by extensive biological recycling. The weathered surface layers or
soils play an important role in this repeated utilization. They form an indispensable substrate for vegetation:
here, the debris of dead plants are broken down by organisms and the liberated nutrients are made
available to the plants again. Excess nutrients are washed out or withdrawn from the biologically catalyzed
circulation by storage in the tissues, by precipitation or evaporation, whereas limiting nutrients are usually
recycled very efficiently. The nutrient fluxes tend to be diverted by the living communities in such a way that
their concentration is adapted to local needs.
Although living systems promote the breakdown of rocks in the weathering process, the resulting debris is
kept in place as soils through multiple mechanisms. Overgrowth, roots and slime production are among the
stabilizing factors. Nevertheless, the re-utilization of the nutrients is never exhaustive. The biologically
catalyzed cycles are leaky and over the longer term the nutrients are diluted. Living communities are
undermined and disappear; the soil cover is washed away and fresh rocks are brought to the surface. Now,
the process can start all over. Meanwhile, the washed-out nutrients may stimulate biological activity
downstream. In this way, life in the oceans is maintained by these nutrient fluxes.
Ultimately, the used end-products of the biota are dumped on the deep-sea floor and enclosed in the slowly
accumulating sediments. Sea-floor spreading will bring the refuse to a deep-sea trough where it is
subducted. The sedimentary mass is subjected to high temperatures and pressures and pushed up during
the formation of a mountain ridge. Thus, it is made available as raw material for a new cycle. Not only does
plate tectonics serve as a sewage system for life, it also is responsible for the regeneration of fresh
nutrients from the fluxes and refuse. It is doubtful whether life would persist for long in the absence of plate
tectonics. While Figure 2a shows the classical, physical view of the rock cycle, Figure 2b shows the same
process from a biological point of view.
The behavior of living systems with respect to toxic materials is totally different. Like the nutrients, these
substances may be liberated through the weathering of rocks. In addition, toxic compounds occur in air and
in water, and they may even be generated as by-products of biologically catalyzed reactions. A huge variety
of mechanisms has been described whereby poisonous materials are actively removed from biological
systems. For instance, heavy metals may accumulate in the cell walls of many bacteria, to be kept away
from cellular machinery.
Dead or living organisms, loaded with heavy metals, are often taken up in sediments, so that the overlying
waters are cleansed. In the ocean, poisonous materials are withdrawn from the upper water layers where
planktonic organisms may bloom. Toxic nitrogen and sulfur compounds as well as, for instance, mercury,
may be volatilized and released into the atmosphere. Thus, in contrast to the nutrients, toxic materials tend
to be withdrawn from the biological circulation, or are even removed altogether from the biosphere. Some
can be converted into useful or harmless materials and then be further channeled through the nutrient
cycles. Ultimately, most toxic materials will end up in the deep-sea sediment, together with the final remains
of the biota. After a very long time they may be brought back into circulation by the internal dynamics of the
Earth.
One aspect needs further clarification. The interaction of the biota with the rock cycle is an energy
consuming process. Without solar radiation the process would soon come to an end. The light is captured
by the biota and transformed into chemical energy. This energy is conducted through a fine network through
the biosphere and sets the entire machinery in operation. Finally, it escapes in a low-grade form into space.
The biosphere is a flimsy, but exceedingly complex and highly energized shell around the Earth. The
geochemical fluxes wind together here into elaborate self-organizing and self-perpetuating networks. In the
remote past they have emerged from non-biological geochemical fluxes. This is life: a very special
geochemical process. Biochemistry is a constituent part of geochemistry.
Emiliania – amplification in biological forcing
The example of the marine unicellular alga Emiliania may serve to demonstrate the highly non-linear
character of the biological involvement in Earth dynamics. The cell (fig. 3a), about a hundredth of a
millimeter in diameter, is surrounded with elegant scales of calcium carbonate, or chalk. These scales, or
coccoliths, are produced in a special vesicle inside the cell and when their formation is completed they are
transported to the cell surface. Emiliania occurs in astronomical numbers in the oceans. The cells tend to
divide each night and periodically, in particular during the late spring in the North Atlantic at mid-latitudes,
there seems to be nothing to stop their proliferation. Huge blooms are the result, which may be viewed from
space (fig. 3b). These blooms may last for several weeks, until they are terminated by grazing crustaceans
or viral infection. The cells and the coccoliths sink to the ocean floor. Emiliania is one of the most numerous
organisms on Earth.
Emiliana in one out of several hundred species of planktonic organisms that produce calcium carbonate
and live in the upper, illuminated, layer of the ocean water column. Together, they influence the rock cycle
as well as the world’s climate. The export of calcium carbonate and organic material from the blooms to the
deep ocean removes carbon from the atmosphere. This process effects the concentration of carbon
dioxide, the greenhouse gas. In addition, the blooms emit a sulfur-containing gas to the atmosphere, DMS.
Higher up in the atmosphere, this gas is oxidized to sulfuric acid droplets which facilitate the formation of
white clouds. Because these clouds reflect the solar radiation, they have a cooling effect. In addition, they
cause the precipitation of natural acid rain.
The chalk and organic carbon that sink to the ocean floor form thick layers which accumulated during
millions of years. Such layers are locally exposed in the chalk cliffs of Dover, Cap Blank Nez (Boulonnais)
and Etretat. These sediments form a geological archive, used to reconstruct the long history of this system.
The origin by evolution of coccolith formation occurred some 200 million years ago. It depended on the
‘invention’ of a minute vacuole in the cell, and extended the area of calcium carbonate sedimentation from
the periphery of the oceans (coral reefs) to the main ocean floor. Model studies suggest that this geological
event had major climatic consequences. As the huge masses of calcium carbonate on the ocean floor
entered into the deeper Earth at subduction zones, they gave rise to a substantial increase of CO 2
emissions into the atmosphere through volcanism. Without the tiny vacuoles we might now be locked up in
a very severe ice age.
The Emiliania phenomenon provides a good example of transdisciplinary science. More than a hundred
scientists participate in this research. Among them are geneticists, physiologists, ecologists,
oceanographers, climatologists, geologists and modelers. The underlying idea is that the understanding
and quantification of this phenomenon may serve as a model for the study of geobiological interactions in
general.
Oxygen
A telling example of the interaction between biological and geological processes is the problem of how the
original, reducing atmosphere could change into the present oxidized atmosphere some 2.2 billion years
ago. Figure 4 represents the underlying mechanism. Free oxygen is a by-product of photosynthesis, the
process whereby certain microorganisms, algae and plants convert solar radiation into the chemical energy
on which virtually all further life depends. The relatively inert carbon dioxide and water are converted into a
reactive mixture – organic matter (CH2O) and oxygen. Respiratory processes reverse the reaction, so that
carbon dioxide and water are regenerated. It must have had its beginnings shortly after the origin of life.
The cycle is leaky, however: a small portion (about one thousandth) of the organic carbon which is
produced in the biological cycle is enclosed in the accumulating sediments. It remains there for a very long
time (the mean residence time is in the order of 300 million years), until it is exposed by tectonic forces at
the continental surface. It then reacts with oxygen and is re-converted into CO2 and water by weathering.
Thus, a gigantic reservoir of organic matter could be formed in the Earth’s crust, about 15,000 times the
total living biomass of today. For each molecule of organic carbon withdrawn in this way from the biological
circulation, one molecule of oxygen was liberated. Originally, however, the gas could not accumulate in the
atmosphere. It reacted immediately with reduced iron and sulfur brought up by the rock cycle, and as a
result huge reservoirs of oxidized iron and sulfate (rust and gypsum) were stored in the crust together with
the organic carbon. Thus, a steady geochemical trend, whereby iron and sulfur were transferred from the
reduced to the oxidized state, was characteristic for the early development of the Earth. Finally, a steady
state situation emerged where the oxidation and reduction rates of iron and sulfur equaled out. It was at this
point that oxygen could freely accumulate in the atmosphere.
Thus, in Figure 4, the biological cycle acts as the producer and the geological cycle as the accumulator of
atmospheric oxygen. As a result of these interactions, the Earth has stored a huge amount of fossil energy
over geological time; this planet is a chemical battery with the oxidized pole at the outer and the reduced
pole at the interior of the planet. The fact that the atmospheric composition is far removed from
thermodynamic equilibrium is a corollary of this energized state of the planetary system.
It is also clear that biology or geology alone cannot explain the evolution of the oxygen atmosphere. Present
research, focusing on the finer details of this crucial trend in Earth history, requires an ever closes
collaboration between specialists in these two fields. This research now relates to the origin and evolution
of photosynthesis and respiration, and to the efficiency of these processes, as they depend on nutrient
availability in the oceans. At least as significant are secular changes, both in the accommodation space for
sediment accumulation and in weathering rates. It now appears that the present level of oxygen at 21% of
the total atmosphere was only reached in the Late Paleozoic, less than 400 million years ago. The
colonization of the continents by vascular plants is likely to have played a crucial role in this event.
It should be noted that the scenario of fig. 4 may not fully explain the original oxygenation of the
atmosphere. The figure accounts for recycling of the organic reservoir in the geological cycle, but does not
provide a mechanism by which the size this reservoir could sufficiently increase for free oxygen to
accumulate. Catling et al. 2001, and Kasting 2001 introduce a new hypothesis which they believe to operate
over and above the recycling of organic matter. They provide evidence suggesting that abundant methane,
released by biological systems under the original anoxic atmosphere, would disintegrate in the high
atmosphere upon exposure to solar radiation. Molecular hydrogen released in this process would have
escaped on a massive scale into space, leaving the outer earth in an increasingly oxygenated state. Once
oxygen could accumulate in the atmosphere, the methane was oxidized in the lower atmosphere, so that
the flux of hydrogen into space was quenched. It should be emphasized that this proposal can only
strengthen the argument that the evolution of atmospheric oxygen can only be understood by
transdisciplinary research involving both biology (the production of methane) and the physical Earth
sciences (disintegration of the methane and hydrogen release).
Global regulation and Gaia
The preceding examples may sufficiently illustrate the importance of life as a forcing factor in Earth
dynamics. It should be kept in mind that this influence is rarely if ever exerted by biological activity alone.
The majority of the geological processes affected by biological systems would also proceed in the absence
of life. Only in combination with physical and chemical processes could the immense impact of life
accumulate over geological time. The idea of global regulation brings the argument one step further.
Lovelock (1979, 1988. 2000) drew attention to the fact that life continued without interruption from 3.8 billion
years to the present day, despite a 25% increase in solar radiation and major catastrophic events, such as
meteorite impacts that periodically devastated the external planetary domain in this long period of time.
Lovelock explained this observation by proposing that the Earth is an organized entity. Global regulatory
mechanisms, emerging from the multiple interactions between life and its environment, automatically
maintain favorable conditions for the long-term survival of the biota. There is no need to invoke forethought
or planning. No deus ex machina is required. The system, which he termed ‘Gaia,’ maintains itself like the
flame of a candle. Lovelock compared the Earth with a superorganism, which maintains internal
homeostasis, just as we maintain the blood pressure or temperature in our bodies.
The ‘Gaia hypothesis’ has been refuted on the ground that it cannot be falsified (Kirschner, 1991). Gaia
would be true no matter what the facts are. Furthermore, the analogy with a superorganism is flawed
because the Earth does not reproduce, and cannot evolve through natural selection in the absence of
competitors. In addition, geological history shows that the Earth is not truly homeostatic, as the system has
undergone major changes during its existence.
Despite these criticisms, an impressive influence of the Gaia hypothesis on Earth System Science cannot
be denied. Firstly, the Gaia idea gave a new fuel to the systems approach, which had already been
explored by Vernadsky, and in more recent years by the schools of Robert Garrels, Heinrich Holland and
many others. Secondly it brought to the fore the idea of global regulation. Since the Gaia idea has been put
forward, several globally operating feedback mechanisms, both negative and positive, have been proposed.
Examples are the calcium silicate carbonate cycle, shown in Figure 5, and modeled by Berner (1994) (see
also Lovelock and Kump, 1994, and Westbroek, 2000), and a proposed mechanism for the regulation of
atmospheric oxygen (Watson and Lenton).
Despite these early advances, our understanding of global regulation is very incomplete. Few feedbacks
have been studied in any detail and many are likely to be discovered in the future. It should be borne in
mind that Gaia is about the behavior of the entire earth system as it emerges from all co-evolutionary
interactions and feedbacks together. Clearly, a deep understanding Gaia is still out of reach. Nevertheless, I
personally feel that the term Gaia is to be preferred over the more commonly used ‘Earth System Science’.
The latter term gives little incentive to biologists to collaborate with geologists. Furthermore, ‘Gaia’ reminds
us of our profound ignorance of the Earth’s dynamics and history. And finally, the name of the ancient
Greek goddess attracts the attention of the public at large and helps to bring world-wide respect for the
planet that we inhabit.
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Catling, D.C., Zahnle, K.J., McKay, P., 2001. Biogenic methane, hydrogen escape, and the irreversible
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Figure 1. The subject of Earth System Science
Figure 2. The rock cycle. A. From a geophysical, and B. from a geobiological point of view.
Fig. 3. Emiliania. Left: a single cell (diameter one hundredth of a millimeter) with coccoliths of chalk. Right:
a bloom South of Iceland, as seen from space.
Figure 4. Production and accumulation of atmospheric oxygen through the coupling of the biological and
geological cycles of organic carbon. Note rapid recycling and small global reservoirs in the biological cycle,
and slow recycling and huge reservoirs in the geological cycle.
Fig. 5. The calcium silicate-carbonate cycle as a global thermostat. Weathering of one mole of calcium
silicate (CaSiO3) the continents requires 2 moles of atmospheric CO2 (and water – not shown). Rivers
transport the dissolved weathering products (ions of silicic acid (H 4SiO4), calcium (Ca++) and bicarbonate
(HCO3-)) into the ocean. These substances are removed from the ocean water by precipitation of silica
(SiO2) and limestone or chalk (CaCO3). Upon the precipitation of limestone, one mole of CO 2 is returned to
the atmosphere. Subduction of the deep-sea sediments at deep-sea trenches is followed by re-generation
of calcium silicate and the volcanic release of one mole of CO 2. An increase in subduction rate leads to an
increased atmospheric concentration of CO2 and to climatic warming. At higher temperatures, the
weathering rate increases and CO2 is pumped away from the atmosphere more rapidly, leading to a
lowering of the temperature. This system acts as a global thermostat over times scales larger than 1000
years. Asterisks indicate biologically enhanced processes.