chlorides, sulfates, and carbonates were present. Indeed, salts of these anions have been leached from
primitive meteorites such as the carbonaceous
chondrites as well as some other meteorites that have
undergone some reheating. On the basis of theoretical calculations, similar compounds should have
been present in the accreting Earth. With leaching of
these compounds by the original water released from
the Earth early in its history, it can be assumed that
the oceans were always salty.
Indeed, the argument that oceans over several
hundred million years ago might actually have been
twice as saline as the contemporary ocean is based on
the fact that there are many geological salt deposits
and deep brines, often associated with oil fields. The
assertion is based on a reasonable premise that salt
deposits became important about 400 million years
ago. The salt prior to that time had to be stored in
the oceans, thus increasing its salinity by a factor of
about two higher than the contemporary ocean.
The Future of the Oceans
If most of the water found on Earth was primarily in
the oceans, with some dissolved in a molten mantle
existing in the early days of the history of the Earth,
then as we have seen, there was a decrease in the size
of the original oceans. This decrease occurred because of photolysis of water vapor and subsequent
loss of hydrogen from the atmosphere or the entrapment of water in hydrated minerals that were
then subducted into the mantle.
The rate of photolytic loss must be considerably
smaller at present compared to that on the early Earth
because of the decrease in the extreme ultraviolet flux
from the Sun. Also, the rate of subduction of the hydrated crust must be less now than it was early in the
Earth’s history because the driving forces for mantle
convection and thus plate tectonics are gravitational
heat from accumulation and fractionation and heat
production by radioactive nuclides, both of which are
waning with time. Therefore, the rate of supply of
water to the mantle is now diminishing and there may
actually be a release of the water stored in the mantle
from previous times. As in the case of carbon dioxide,
we may be in a steady-state of water supply from the
mantle and return of water to the mantle, thereby
maintaining the size of the oceans. At any rate, changes in the volume of water will probably not be large
in future.
See also
Conservative Elements. Elemental Distribution:
Overview. Hydrothermal Vent Fluids, Chemistry
of. Mid-Ocean Ridge Geochemistry and Petrology.
Volcanic Helium.
Further Reading
Craig H (1963) The isotopic geochemistry of water and
carbon in geothermal areas. In: Tongiorgi E (ed.)
Nuclear Geology on Geothermal Areas, Proceedings of
the First Spoleto Conference, Spoleto, Italy, pp. 17--53.
Pisa: V. Lischi Figli.
Farley KA and Poreda RJ (1993) Mantle neon and atmospheric contamination. Earth and Planetary Science
Letters 114: 325--339.
Kump LR, Kasting JF, and Crane RC (1999) The Earth
System. London: Prentice-Hall.
Lupton JE and Craig H (1981) A major helium-3 source at
151S on the East Pacific Rise. Science 214: 13--18.
Lupton JE and Rubey WW (1951) Geologic history of sea
water: an attempt to state the problem. Geological
Society of America Bulletin 62: 1111--1147.
Turekian KK (1990) The parameters controlling planetary
degassing based on
40
Ar systematics. In: Gopalan K,
Gaur VK, Somayajulu BLK, and Macdougall JD (eds.)
From Mantle to Meteorites, pp. 147--152. Delhi: Indian
Academy of Science.
6 ORIGIN OF THE OCEANS
primitive meteorites such as the carbonaceous
chondrites as well as some other meteorites that have
undergone some reheating. On the basis of theoretical calculations, similar compounds should have
been present in the accreting Earth. With leaching of
these compounds by the original water released from
the Earth early in its history, it can be assumed that
the oceans were always salty.
Indeed, the argument that oceans over several
hundred million years ago might actually have been
twice as saline as the contemporary ocean is based on
the fact that there are many geological salt deposits
and deep brines, often associated with oil fields. The
assertion is based on a reasonable premise that salt
deposits became important about 400 million years
ago. The salt prior to that time had to be stored in
the oceans, thus increasing its salinity by a factor of
about two higher than the contemporary ocean.
The Future of the Oceans
If most of the water found on Earth was primarily in
the oceans, with some dissolved in a molten mantle
existing in the early days of the history of the Earth,
then as we have seen, there was a decrease in the size
of the original oceans. This decrease occurred because of photolysis of water vapor and subsequent
loss of hydrogen from the atmosphere or the entrapment of water in hydrated minerals that were
then subducted into the mantle.
The rate of photolytic loss must be considerably
smaller at present compared to that on the early Earth
because of the decrease in the extreme ultraviolet flux
from the Sun. Also, the rate of subduction of the hydrated crust must be less now than it was early in the
Earth’s history because the driving forces for mantle
convection and thus plate tectonics are gravitational
heat from accumulation and fractionation and heat
production by radioactive nuclides, both of which are
waning with time. Therefore, the rate of supply of
water to the mantle is now diminishing and there may
actually be a release of the water stored in the mantle
from previous times. As in the case of carbon dioxide,
we may be in a steady-state of water supply from the
mantle and return of water to the mantle, thereby
maintaining the size of the oceans. At any rate, changes in the volume of water will probably not be large
in future.
See also
Conservative Elements. Elemental Distribution:
Overview. Hydrothermal Vent Fluids, Chemistry
of. Mid-Ocean Ridge Geochemistry and Petrology.
Volcanic Helium.
Further Reading
Craig H (1963) The isotopic geochemistry of water and
carbon in geothermal areas. In: Tongiorgi E (ed.)
Nuclear Geology on Geothermal Areas, Proceedings of
the First Spoleto Conference, Spoleto, Italy, pp. 17--53.
Pisa: V. Lischi Figli.
Farley KA and Poreda RJ (1993) Mantle neon and atmospheric contamination. Earth and Planetary Science
Letters 114: 325--339.
Kump LR, Kasting JF, and Crane RC (1999) The Earth
System. London: Prentice-Hall.
Lupton JE and Craig H (1981) A major helium-3 source at
151S on the East Pacific Rise. Science 214: 13--18.
Lupton JE and Rubey WW (1951) Geologic history of sea
water: an attempt to state the problem. Geological
Society of America Bulletin 62: 1111--1147.
Turekian KK (1990) The parameters controlling planetary
degassing based on
40
Ar systematics. In: Gopalan K,
Gaur VK, Somayajulu BLK, and Macdougall JD (eds.)
From Mantle to Meteorites, pp. 147--152. Delhi: Indian
Academy of Science.
6 ORIGIN OF THE OCEANS