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EAS 107, How the Earth Works
Class 7, Text Page 1 of 3
THE HISTORY OF SEA WATER
THE EARTH AS A HEAT ENGINE — A heat engine is a mechanism that transfers heat energy from
a heat source to a heat sink by way of a working fluid and in the process extracts work. The
working fluid cycles back and forth between the source and the sink, like the water in a car’s
radiator (the heat sink for the car’s engine) or the coolant in a refrigerator.
The Earth is actually two heat engines in one: the exogenic (“externally powered”) heat
engine of the atmosphere, hydrosphere, and crust; the endogenic (“internally powered”) heat
engine of the core, mantle, and lithosphere. The exogenic heat engine’s energy source is the sun;
its heat sink is outer space; its working substances are the atmosphere and hydrosphere and, most
important, the water they contain. The endogenic heat engine’s main energy source is the
mantle’s radioactivity; its heat sink is the exogenic heat engine; and its working fluids are the
outer core’s molten iron and the sublithospheric mantle’s fluid-like rock. The engines’ work is
the rearrangement of Earth materials— the crust’s continents and oceans, the lithosphere’s
tectonic plates, the mantle’s convection cells, the ocean’s water masses, the atmosphere’s air
masses.
Why Is Water Important? — Among other reasons, because it helps determine the Earth’s
overall surface temperature. Because it buffers earth-surface temperatures, requiring more
energy per kilogram to warm 1 K or to vaporize than any common substance. Because it is the
single most important heat-transporting component in the exogenic heat engine’s working fluids,
carrying most of the heat conveyed from the tropics to the poles, about half being carried as
sensible heat in warm, wind-driven, polarward-flowing ocean currents, and about half as latent
heat in atmospheric water vapor (i.e. the energy required to evaporate the water). Because it
comes closest of any compound to being the universal solvent in the earth-surface environment
and in biological systems. Because it is the principal medium of transport for sediment and
dissolved weathering products in the rock cycle.
Material Cycles — The exchange of material associated with the cycling of the exogenic heat
engine’s working fluid is referred to as the exogenic cycle; that associated with exogenic heat
engine’s working fluid, as the endogenic cycle. The pathways within and between these two
material cycles are exceedingly complex and involve many subcycles. The rock cycle is
concerned with the interchange of material between the two, with the exogenic engine’s
recycling of the endogenic engine’s igneous and metamorphic rocks into new generations of
sediment and sedimentary rock, and the endogenic engine’s recycling of the exogenic engine’s
output into new generations of igneous and metamorphic rock.
Earth scientists characterize geochemical cycling in terms of bulk material (e.g. the rock
cycle), mineral grains (e.g. one-cycle vs. multicycle sediments), compounds (e.g. the water
cycle), and elements (e.g. the carbon cycle) and their isotopes. Material cycling is described in
terms of reservoirs, reservoir sizes, and fluxes between reservoirs, as illustrated in the diagrams
of the water cycle and rock cycle. Different materials are cycled at very different rate and on
very different time scales. One useful index of the time scale over which significant change
takes place is the residence time with respect to one or another flux into or out of a reservoir, that
is, the time that the reservoir would take to fill, empty, or turn over its inventory:
Residence Time ≡
Reservoir Size
Flux
(7.1)
Where a reservoir has multiple inbound (influx) or outbound (efflux) pathways, residence times
with respect to more than one influx or efflux may be of interest. The residence time concept is
really meaningful only when the reservoir is at or near a steady state on the time scale of interest,
that is, when the reservoir’s size, influxes, and effluxes are essentially constant.
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EAS 107, How the Earth Works
Class 7, Text Page 2 of 3
The Water Cycle — Water is cycled relatively rapidly, as the accompanying diagram shows.
With respect to the global evaporation rate, its residence time in the atmosphere is about 11 days,
and in surface waters on continents, about 25 days. The oceanic reservoir is subdivided into
smaller reservoirs, individual water masses, in which the typical residence time is on the order of
1 ka with respect to total influx or efflux1. For comparison, the residence time of the sodium ion
(Na+) in sea water is about 200 Ma with respect to river influx.
The exogenic cycle and the rock cycle’s exogenic portion are more or less piggy-backed
on the water cycle. A few figures flesh out this impression: Each year, about 23% of incoming
solar radiation goes to evaporate (and re-evaporate) about 0.04% of the water on the Earth’s
surface. Of this 0.04%, the 0.008% that falls as rain or snow on continents carries about 96% of
the particulate and dissolved materials that reach the ocean each year.
The Rock Cycle — As accompanying diagrams show, water is the principal medium of
transport and exchange for others materials in the exogenic half of the rock cycle. As discussed
in earlier classes, water plays an important role in endogenic cycle as well, being instrumental in
the partial melting and fractionation by which the makings of continental crust are extracted from
the mantle at subduction zones.
SEA WATER IN THE ROCK CYCLE
Why Sea Water is Important — Because sea water represents the single largest water
reservoir in exogenic cycle, because it is the principal solvent and medium of exchange for many
chemical species, and because our biochemical machinery appears to have evolved in it and
more or less in osmotic equilibrium with it.
Rock Weathering on Continents — Apart from their thin covering of sediments and
sedimentary rocks, continents are comprised mainly of igneous rocks similar to granite, which
are in turn comprised mostly of the silicate minerals quartz [SiO2], feldspar [e.g. KAlSi3O8], and
mica [e.g. KAl3Si3O10(OH)2]. Weathering turns the igneous rock into the two main components
of sedimentary rock: mechanical weathering turns the quartz into beach sand, and chemical
weathering turns the feldspar and mica into the clays of ordinary clay mud (e.g. kaolinite,
[Al2Si2O5(OH)4]) and in the process consumes acid (H+):
H+ + feldspar → clay + ions in solution (e.g. Na+)
Now the acid comes primarily from the carbon dioxide that dissolves in surface waters to form
carbonic acidcarbonic acid [H2CO3]:
H2O + CO2 → H2CO3
Carbonic acid dissociates in turn into the bicarbonate ion [HCO3-], a remarkable species that can
act either as an acid or a base:
HCO3- → H+ + CO3-2 … as an acid
HCO3- + H+ → H2CO3 … as a base
Both the bicarbonate ion and water, which also can act as an acid or a base, are important in
buffering the acidity (i.e. hydrogen ion concentration) of natural waters.
Thus our weathering reaction becomes
H2CO3 + feldspar → clay + HCO3- + ions in solution (e.g. Na+)
Note the drain on H+ and atmospheric CO2.
1
As you remember, Earth scientists find it convenient to denote thousands of years in ka’s
(kiloans), millions in Ma’s (megans), and billions in Ga’s (gigans).
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EAS 107, How the Earth Works
Class 7, Text Page 3 of 3
What Happens to the Weathering Products? The particulate weathering products are
obvious: campus buildings are built on and of them. But what about the dissolved weathering
products? They are the salt of the sea. But why hasn’t the ocean long since turned into an
antacid of plate-tectonic proportions? What controls sea water’s acid-base balance? How is the
CO2 recycled into the atmosphere?
How the CO2 Gets Recycled — About half of the CO2 is recycled through deposition of
limestone— calcium carbonate [CaCO3]— another important component of the sedimentary
record:
Ca+2 + 2 HCO3- → CaCO3 +H2O + CO2
The other half — the CO3-2 in limestone — is returned through metamorphism, which is
concentrated at plate boundaries, where CO2 is regenerated by reactions of this general form:
CaCO3 + SiO2 → CaSiO3 + CO2
But what about the acid-base balance? It used to be thought that weathering of clays suspended
in sea water was primarily responsible for regenerating the H+ and, at the same time, buffering
the ocean’s acidity. The main effect, however, turns out to be metamorphism again, particularly
hydrothermal alteration of new oceanic crust at midocean ridges. At ridges, sea water reacts
with basalt at elevated temperatures. The net result of a complex of reactions and is conversion
of HCO3- into CO2 and H+, with other ions making up the difference in electrical charge. Acid
brines emerge from hydrothermal vents rich in these species and in dissolved Fe+2 and SiO2 —
the ingredients of banded iron-formation.
Sea Salts and Sea Water’s Composition — Sea water’s content of dissolved salts represents a
balance between the influxes, mainly from rivers, and the effluxes, mainly to sediments, aerosols
(e.g. particles suspended in air, such as salt particles derived from sea spray), and altered rock.
As accompanying diagrams show, sea water is basically an undersaturated solution of
sodium chloride tinged with magnesium, potassium, and sulfate and laced more or less to
saturation with calcium carbonate. As one of these shows, the major ions in sea water are those
with the longest residence times, typically many millions of years.
As another diagram shows, chemical equilibria among the components set limits on sea
water’s composition.
As yet another diagram shows, the minerals formed in sea water — especially evaporite
minerals precipitated from evaporatively concentrated sea water (e.g. CaSO4, NaCl) — enable us
to determine how sea water’s composition has varied and thus how the exogenic cycle’s general
features have changed through time.
STUDY QUESTIONS
1. What is the exogenic cycle? … the endogenic cycle? … the water cycle? … the carbon
cycle? … the rock cycle? How does the rock cycle relate to the exogenic cycle? … the
endogenic cycle? … the water cycle? … the carbon cycle?
2. What are some important ways in which water shapes the earth-surface environment? In
what ways do sea water and its salt content reflect the character of the earth-surface
environment? … the character of life’s environment?
3. How is sea water involved in the rock cycle? How is the carbon cycle? How is the
ocean’s acid-base balance maintained? Has the acidity ever been much different from
today’s? What is the geological evidence (i.e. how do marine sedimentary rocks reflect
any changes?)? What factors have changed?
4. What are the major ions in sea water? What are their residence times? What do marine
evaporite deposits reveal about the history of sea water? …about the history of life’s
environment?
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