286
Pyrite in such placer deposits has been quoted as evidence
for a reducing atmosphere, but since this mineral can easily
form during diagenesis, it appears less useful.
Rock weathering and the formation of carbonates were
very efficient in binding carbon dioxide during this
early stage of atrnospheric evolution and thus did not
allow the maintenance of a high atrnospheric carbon
dioxide pressure for long. Temporal variations in the
contents of oxygen and carbon dioxide cannot be excluded during this stage.
6.5.3 Evolution of the Hydrosphere and Climate
The water of the hydrosphere is derived from the differentiation and outgassing of magma and late phases
of meteorite accretion (e.g., chondrites and comets
containing ice). It was estimated that the Earth's mantle
contains approximately three times the water mass
present in the modem oceans. The initial high partial
pressure of CO 2 in the atmosphere must have raised
significantly the CO 2 content of sea water. The salt
content and the nature of the early ocean are less clear.
It was probably hot and rich in both carbonic acid and
stronger acids and therefore capable of rapidly extracting alkali and earth alkali ions, iron, and silica from
volcanic rocks. For this reason, the various acids became neutralized, and the pH of ocean water was presumably higher than assumed by some authors. The
total salt concentration of the early ocean may have
been higher than that of the modem ocean. Sodium
delivered by rocks and chlorine provided as hydrogen
chloride by volcanic exhalations were important constituents. Most ofthe other major ions, such as calcium
and magnesium, tended to form less soluble mineral
phases which precipitated. Potassium released from
primary rock minerals was largely used to form clay
minerals.
Some workers pointed out that the early ocean might
have been a "soda ocean" in analogy to modem soda
lakes, which occur in volcanic regions. Since volcanoes commonly deliver more CO 2 than HC1, not only
contained the early atrnosphere more CO 2 , but also the
initial ocean may have had higher concentrations in
HC0 3 - and cot than in chloride. This, in turn, may
have caused a situation similar to that in modem soda
lakes, which are highly alkaline and contain little calcium and magnesium due to the precipitation of these
ions as carbonates. If this is correct, the early ocean
water must have been rich in sodium carbonate and
have had a high pH, ranging between 9 and 11 (Fig.
6.18). Such a sea water, particularly if it was hotter
than the present ocean, could dissolve large amounts of
silica.
As long as only little free oxygen was available, the
conditions in sea water were reducing and allowed, in
addition, the solution of considerable amounts of ferChapter 6 Special Depositional Environments
rous iron (approximately 1000 times the mass offerric
iron dissolved in the modem ocean). Thus, the early
"soda ocean" may have been rich in silica andferrous
iron, but relatively poor in calcium and magnesium,
and devoid of sulfate. There was not sufficient oxygen
for the oxidation of hydrogen sulfide and sulfur.
Later, as a result of the growing continents, the time
span necessary for the recycling of marine sediments,
including carbonates and organic carbon, increased. At
the same time, sodium-rich pore waters were incorporated into subduction complexes where sodium was
used to form sodium feldspars (albite and plagioclas)
in the growing granodioritic continental crust. Thus,
the soda-dominated ocean was gradually transformed
into a halite-dominated ocean (Fig. 6.18).
With the establishment of a hydrosphere, the climate
on Earth probably became rather stable. On the one
hand, the young Sun had a lower luminosity than today, but on the other hand, the early oceans probably
covered a much higher proportion of the Earth's surface, generating a thalassocratic epoch. As a result, the
Earth sent back a smaller fraction ofthe received solar
energy to space than at present. The effect of the following slow increase in solar luminosity on the surface
temperature of the Earth was probably counteracted by
a decrease in atrnospheric carbon dioxide (and possibly
other gases) diminishing the "supergreenhouse" effect
(cf. Sects. 5.6 and 7.8). Thus, a reasonable temperate
climate and the persistence of the oceans were maintained for at least the last 3500 Ma. Glacial periods in
the early and late Proterozoic testify that the Earth
cannot have been significantly hotter than today.
The above mentioned views on the early evolution of the
atmosphere and hydrosphere are derived from the sparse
Precambrian fossil record and rocks, which are commonly
strongly affected by repeated periods of metamorphism and
tectonism. In spite of these difficulties, Precambrian rocks
including sedimentary sequences have been studied intensively in various aspects and have provided significant results. Chemical sediments in particular have shed some light
on the nature of the early atmosphere and hydrosphere.
The "soda ocean" theory was proposed by Kempe and
Degens (1985); Degens 1989; Kempe et al. 1989.; see ~Iso
Warren 1997). For kinetic and mass balance conslderatlOns
related to this problem see Kempe and Degens (1985). The
pH of the early ocean is discussed, e.g., by Walker and
Drever (1988); climate aspects have been addressed .by
Henderson-Sellers and Henderson-Sellers (1989), Kastmg
(1989).
6.5.4 Early Life
The earliest life forms are microfossils representing
possibly cell walls and having shapes and sizes like
bacteria. They occur in dark cherts and shales as old as
about 3500 Ma. They were capable of either utilizing
inorganic compounds for synthesis of organic mole-
Pyrite in such placer deposits has been quoted as evidence
for a reducing atmosphere, but since this mineral can easily
form during diagenesis, it appears less useful.
Rock weathering and the formation of carbonates were
very efficient in binding carbon dioxide during this
early stage of atrnospheric evolution and thus did not
allow the maintenance of a high atrnospheric carbon
dioxide pressure for long. Temporal variations in the
contents of oxygen and carbon dioxide cannot be excluded during this stage.
6.5.3 Evolution of the Hydrosphere and Climate
The water of the hydrosphere is derived from the differentiation and outgassing of magma and late phases
of meteorite accretion (e.g., chondrites and comets
containing ice). It was estimated that the Earth's mantle
contains approximately three times the water mass
present in the modem oceans. The initial high partial
pressure of CO 2 in the atmosphere must have raised
significantly the CO 2 content of sea water. The salt
content and the nature of the early ocean are less clear.
It was probably hot and rich in both carbonic acid and
stronger acids and therefore capable of rapidly extracting alkali and earth alkali ions, iron, and silica from
volcanic rocks. For this reason, the various acids became neutralized, and the pH of ocean water was presumably higher than assumed by some authors. The
total salt concentration of the early ocean may have
been higher than that of the modem ocean. Sodium
delivered by rocks and chlorine provided as hydrogen
chloride by volcanic exhalations were important constituents. Most ofthe other major ions, such as calcium
and magnesium, tended to form less soluble mineral
phases which precipitated. Potassium released from
primary rock minerals was largely used to form clay
minerals.
Some workers pointed out that the early ocean might
have been a "soda ocean" in analogy to modem soda
lakes, which occur in volcanic regions. Since volcanoes commonly deliver more CO 2 than HC1, not only
contained the early atrnosphere more CO 2 , but also the
initial ocean may have had higher concentrations in
HC0 3 - and cot than in chloride. This, in turn, may
have caused a situation similar to that in modem soda
lakes, which are highly alkaline and contain little calcium and magnesium due to the precipitation of these
ions as carbonates. If this is correct, the early ocean
water must have been rich in sodium carbonate and
have had a high pH, ranging between 9 and 11 (Fig.
6.18). Such a sea water, particularly if it was hotter
than the present ocean, could dissolve large amounts of
silica.
As long as only little free oxygen was available, the
conditions in sea water were reducing and allowed, in
addition, the solution of considerable amounts of ferChapter 6 Special Depositional Environments
rous iron (approximately 1000 times the mass offerric
iron dissolved in the modem ocean). Thus, the early
"soda ocean" may have been rich in silica andferrous
iron, but relatively poor in calcium and magnesium,
and devoid of sulfate. There was not sufficient oxygen
for the oxidation of hydrogen sulfide and sulfur.
Later, as a result of the growing continents, the time
span necessary for the recycling of marine sediments,
including carbonates and organic carbon, increased. At
the same time, sodium-rich pore waters were incorporated into subduction complexes where sodium was
used to form sodium feldspars (albite and plagioclas)
in the growing granodioritic continental crust. Thus,
the soda-dominated ocean was gradually transformed
into a halite-dominated ocean (Fig. 6.18).
With the establishment of a hydrosphere, the climate
on Earth probably became rather stable. On the one
hand, the young Sun had a lower luminosity than today, but on the other hand, the early oceans probably
covered a much higher proportion of the Earth's surface, generating a thalassocratic epoch. As a result, the
Earth sent back a smaller fraction ofthe received solar
energy to space than at present. The effect of the following slow increase in solar luminosity on the surface
temperature of the Earth was probably counteracted by
a decrease in atrnospheric carbon dioxide (and possibly
other gases) diminishing the "supergreenhouse" effect
(cf. Sects. 5.6 and 7.8). Thus, a reasonable temperate
climate and the persistence of the oceans were maintained for at least the last 3500 Ma. Glacial periods in
the early and late Proterozoic testify that the Earth
cannot have been significantly hotter than today.
The above mentioned views on the early evolution of the
atmosphere and hydrosphere are derived from the sparse
Precambrian fossil record and rocks, which are commonly
strongly affected by repeated periods of metamorphism and
tectonism. In spite of these difficulties, Precambrian rocks
including sedimentary sequences have been studied intensively in various aspects and have provided significant results. Chemical sediments in particular have shed some light
on the nature of the early atmosphere and hydrosphere.
The "soda ocean" theory was proposed by Kempe and
Degens (1985); Degens 1989; Kempe et al. 1989.; see ~Iso
Warren 1997). For kinetic and mass balance conslderatlOns
related to this problem see Kempe and Degens (1985). The
pH of the early ocean is discussed, e.g., by Walker and
Drever (1988); climate aspects have been addressed .by
Henderson-Sellers and Henderson-Sellers (1989), Kastmg
(1989).
6.5.4 Early Life
The earliest life forms are microfossils representing
possibly cell walls and having shapes and sizes like
bacteria. They occur in dark cherts and shales as old as
about 3500 Ma. They were capable of either utilizing
inorganic compounds for synthesis of organic mole-