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Seawater and Ocean Chemistry. Seawater Chemistry. Water Seawater Salts in seawater. Water. Composition Properties. Water is …. a chemical compound (H 2 O) comprising two atoms of hydrogen and one atom of oxygen, in liquid state .

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slide2

Seawater Chemistry

  • Water
  • Seawater
  • Salts in seawater
slide3

Water

  • Composition
  • Properties
water is
Water is …
  • a chemical compound (H2O) comprising two atoms of hydrogen and one atom of oxygen, in liquid state ...
slide6

Two hydrogen atoms share their electrons with one oxygen atom to form the electrically polarized water molecule

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slide7

When water freezes to ice, the angle of hydrogen bonding expands from 105° to 109°.

105°

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109°

As the space taken by 27 water molecules is now used by 24 molecules, the density of ice is less than the density of water, i.e.,

water freezes over.

slide8

Hydrogen bonding of these electrically polarized molecules gives water the properties of cohesion, adhesion and dissolving power.

slide9

Common salt (NaCl) thus dissolves

in water most readily. It is the ocean’s most abundant constituent, therefore.

slide10

Sodium

(Na)

11

23

11

11

12

Chlorine

(Cl)

17

35

17

17

18

Atomic Number

Atomic Weight (or Mass)

Number of Electrons

Number of Protons

Number of Neutrons

slide12

Fresh water comprises

Oxygen: 857.8g (=85.8%)

Hydrogen: 107.2g (=10.7%)

slide13

Dissolved gases (O2 and CO2 contents here) tell us much about biological activity in the ocean:

0.0

  • Deep waters show increasing gas content with depth.
  • An oxygen minimum layer is often seen at intermediate depths.
  • Surface waters often have excess O2 and lack CO2 .

Why?

O2

0.5

CO2

Depth (km)

1.0

1.5

1.5

3.0

4.5

6.0

7.5

O2

2.0

90

94

98

102

106

110

CO2

Concentration of dissolved gases

(parts per million, by weight)

slide14

Most of carbon from the Earth’s atmosphere eventually ended up in the ocean

% of gas, by volume in

Dissolved Gas

atmosphere

seawater

Nitrogen (N2)

Oxygen (O2)

Carbon dioxide (CO2)

78.08%

20.95%

0.035%

48%

36%

15%

slide15

Major constituents of Seawater at 3.5% Salinity

Constituent

Water:

Oxygen (O)

Hydrogen (H)

The most abundant ions

Chloride (Cl-)

Sodium (Na+)

Sulfate (SO42-)

Magnesium (Mg2+)

Calcium (Ca2+)

Potassium (K+)

Bicarbonate (HCO3-)

85.8%

10.7%

1.9%

1.1%

0.3%

0.1%

0.04%

0.04%

0.01%

only 2% of Cl in seawater could have come from land sources

only 20% of sulfur in seawater could have come from land sources

slide16

Constant Proportions and

Conservative Constituents

1. Some constituent salts in seawater occur in constant proportions, e.g.,

Thus, for average seawater with Cl = 19.2‰,

salinity = 1.80655 × 19.2‰ = 34.7‰

2. The constituents of seawater that occur in constant proportion (i.e., change very slowly) are known as conservative constituents or elements.

Dissolved Salts in Seawater (‰)

Chlorine in Seawater (‰)

= 1.80655

slide18

Why is seawater salty?

  • Hydrothermal activity on the seafloor can provide the
    • sources for excess Cl and S and
    • sinks to balance the continued enrichment in sulfates and magnesium by streams.
  • We should also note that, compo-sitionally, seawater has remained remarkably uniform through the earth’s history
slide19

Annual volcanic output x Age of the Earth

Estimated existing quantity

10,000

Water

Sulfur

1,000

Chlorine

Quadrillion (1015)

Metric Tons

Carbon

100

Nitrogen

10

This comparison of the total quantities of selected substances in the oceans and atmosphere with what could have come from volcanism favors the volcanic origin of these substances.

1

Adapted from Robert Decker & Barbara Decker: VOLCANOES (W.H. Freeman, New York, 1996)

slide20

The case for extraterrestrial origin of the oceans

Deuterium

Total Hydrogen

ratio

  • Comets are >40% water.
  • The deuterium/hydrogen ratio of comets and oceans overlap.

Comet Halley

Earth’s Oceans

0.06-0.48 ppt

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  • During the initial 2 Ga of its history, Earth may well have received 2 x 108 to 1 x 1017 metric tons of cometary matter by way of bombardment episodes.
  • Comets may well have contributed significantly, therefore, to the hydrospheric mass of 1.4-1.7 x 1018 metric tons. Indeed, all this water could have been produced by either ~10% of the cometary mass or entirely by the asteroidal source if initial bombardment was of carbonaceous chondrites.

Adapted from C.F. Chyba & C. Sagan in COMETS AND THE ORIGIN AND EVOLUTION OF LIFE

(Ed: P.J. Thomas, C.F. Chyba & C.P. McKay; Springer-Verlag, New York, 1997).

slide21

A trio of frames from Polar's Visible Imaging System (VIS), taken 6 seconds apart last December 31st, captures an object rapidly descending toward northern Europe. Because the camera's filter isolates emission from hydroxyl (OH) radicals, the incoming object must have contained abundant water

(the map of Earth is intended for reference).

slide22

Suppose

  • annual influx from outer space is 50-100 billion gallons of water vapor into the atmosphere,
  • and that

Compare this to the

total amount of

water in the oceans

= 1370x106 Km3 (volume)

 109 m3/Km3

 264.2 gallons/m3

= 362  1018 gallons of water

  • this rate has been constant through geological history (~4.5 billion years).
  • This amounts to 225-
  • 450 x 1018 gallons of
  • water.
slide23

Therefore,

  • there is no problem in accepting the
  • extraterrestrial possibility, except that
  • this requires
  • a much smaller ocean and
  • an appreciably weaker hydrological cycle
  • in the past, than at the present.
  • But there is no geological evidence to
  • support this possibility.