Seawater Seawater and Ocean and Ocean ChemistryChemistry

Seawater Seawater ChemistryChemistry

WaterWater SeawaterSeawater Salts in seawaterSalts in seawater

CompositionComposition PropertiesProperties

WaterWater

Water is …Water is …

a chemical compound (H2O) comprising two atoms of hydrogen and one atom of oxygen, in liquid state ...

Question: 

The fusion of how many deuterium atoms would create a carbon atom? nitrogen atom? oxygen atom?

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

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105°

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109°

When water freezes When water freezes to ice, the angle of to ice, the angle of hydrogen bonding hydrogen bonding

expands from 105° to expands from 105° to 109°. 109°.

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

water freezes overwater freezes over..

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

Common salt (NaCl) thus dissolves in water most readily. It is the ocean’s most abundant constituent, therefore.

Atomic NumberAtomic Weight (or Mass)

Number of ElectronsNumber of Protons

Number of Neutrons

Sodium(Na)

1123111112

Chlorine(Cl)

1735171718

Fresh water comprises

Oxygen: 857.8g (=85.8%)Hydrogen: 107.2g (=10.7%)

0.0

0.5

1.0

1.5

2.0

90 94 98 102 106 110

1.5 3.0 4.5 6.0 7.5

Dep

th (

km)

Concentration of dissolved gasesConcentration of dissolved gases(parts per million, by weight)(parts per million, by weight)

COCO22

OO22

OO22

COCO22

Dissolved gases (ODissolved gases (O22 and CO and CO22 contents here) tell us much contents here) tell us much

about biological activity in the ocean:about biological activity in the ocean: Deep waters Deep waters

show show increasing increasing gas content gas content with depth.with depth.

An oxygen An oxygen minimum minimum layer is often layer is often seen at seen at intermediate intermediate depths.depths.

Surface Surface waters often waters often have excess have excess OO22 and lack and lack COCO22 . .

Why?Why?

Nitrogen

(N2)

Oxygen (O2)

Carbon dioxide

(CO2)

78.08%

20.95%

0.035%

48%

36%

15%

atmosphere

seawater

% of gas, by volume inDissolved Gas

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

Major constituents of Seawater at 3.5% SalinityConstituent

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

Constant Proportions andConstant Proportions andConservative ConstituentsConservative Constituents

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

Thus, for average seawater with Cl = 19.2‰, Thus, for average seawater with Cl = 19.2‰, salinity = 1.80655 × 19.2‰ = 34.7‰salinity = 1.80655 × 19.2‰ = 34.7‰

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

Dissolved Salts in Seawater (‰)

Chlorine in Seawater (‰)= 1.80655

Salt brought in by run-off from land can explain only ~2% of Cl and ~20% of S in the seawater

Why is seawater salty?Why is seawater salty? Hydrothermal activity on the seafloor Hydrothermal activity on the seafloor

can provide thecan provide the

– sources for excess Cl and S andsources for excess Cl and S and– sinks to balance the continued enrichment sinks to balance the continued enrichment

in sulfates and magnesium by streams.in sulfates and magnesium by streams.

We should also note that, compo-We should also note that, compo-sitionally, seawater has remained sitionally, seawater has remained remarkably uniform through the remarkably uniform through the earth’s historyearth’s history

1,000

1

10

100

10,000Q

uad

rilli

on

(10

15)

Met

ric

To

nsEstimated existing quantity

Annual volcanic output x Age of the Earth

Water

Sulfur

Nitrogen

Carbon

Chlorine

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

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.

The case for extraterrestrial origin of the oceans

DeuteriumTotal Hydrogen ratio

Comet HalleyEarth’s Oceans

0.06-0.48 ppt0.16 ppt

Comets are >40% water.

The deuterium/hydrogen ratio of comets and oceans overlap.

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).

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).

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 ofwater 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 ofwater.

Therefore,

there is no problem in accepting the extraterrestrial possibility, except that

this requiresthis requires • a much smaller ocean anda much smaller ocean and

• an appreciably weaker hydrological an appreciably weaker hydrological cyclecycle

in the past, than at the present.in the past, than at the present.

But there is no geological evidence toBut there is no geological evidence tosupport this possibility.support this possibility.