Transcript Seawater Chemistry

Seawater
and Ocean
Chemistry
Seawater Chemistry



Water
Seawater
Salts in seawater
Water


Composition
Properties
Water is …

a chemical compound
(H2O) comprising two
atoms of hydrogen and
one atom of oxygen, in
liquid state ...
Ques The fusion of how many deuterium atoms would create
tion: a carbon atom? nitrogen atom? oxygen atom?
Two hydrogen atoms share their electrons
with one oxygen atom
to form the electrically
polarized water
+
+
molecule
-
-
-
+ +
+ +
-
-
+ +
+ +
-
-
+
-
+
-
-
+ +
++
-
When water freezes
to ice, the angle of
hydrogen bonding
expands from 105° to
109°.
+
105°
+
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.
+
109°
+
Hydrogen bonding of these electrically
polarized
molecules
gives water
the
properties
of
cohesion,
adhesion
and
dissolving
power.
Common
salt (NaCl)
thus
dissolves
in water
most
readily. It is
the ocean’s
most
abundant
constituent,
therefore.
Atomic Number
Atomic Weight (or Mass)
Number of Electrons
Number of Protons
Number of Neutrons
Sodium
(Na)
Chlorine
(Cl)
11
23
11
11
12
17
35
17
17
18
Fresh water comprises
Oxygen: 857.8g (=85.8%)
Hydrogen: 107.2g (=10.7%)
Dissolved gases (O2 and CO2 contents here) tell us much
about biological activity in the ocean:
0.0
 Deep waters
show
increasing
2
gas content
0.5
with depth.
 An oxygen
minimum
1.0
2
layer is often
seen at
intermediate
1.5
depths.
 Surface
1.5 3.0
4.5 6.0
7.5
O2 waters often
2.0
have excess
90
94 98 102 106 110 CO2 O2 and lack
CO2 .
Concentration of dissolved gases
Why?
(parts per million, by weight)
Depth (km)
O
CO
Most of carbon from the
Earth’s atmosphere eventually
ended up in the ocean
Dissolved Gas
% of gas, by volume in
atmosphere seawater
Nitrogen (N2) 78.08%
Oxygen (O2) 20.95%
Carbon dioxide (CO2) 0.035%
48%
36%
15%
Major constituents of Seawater at
3.5% Salinity
Constituent
Water:
Oxygen (O)
Hydrogen (H)
85.8%
10.7%
The most abundant ions
Chloride (Cl-)
Sodium (Na+)
Sulfate (SO42-)
Magnesium (Mg2+)
Calcium (Ca2+)
Potassium (K+)
Bicarbonate (HCO3-)
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 and
Conservative Constituents
1. Some constituent salts in seawater occur in
constant proportions, e.g.,
Dissolved Salts in Seawater (‰)
Chlorine in Seawater (‰)
= 1.80655
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.
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?
 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, compositionally, seawater has remained
remarkably uniform through the
earth’s history
Annual volcanic output x Age of the Earth
Estimated existing quantity
10,000
Quadrillion (1015)
Metric Tons
Water
1,000
100
10
Sulfur
Chlorine
Carbon
Nitrogen
1
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)
The case for extraterrestrial origin of the
oceans
Deuterium
 Comets are >40% water.
 The deuterium/hydrogen ratio
of comets and oceans overlap.
Total Hydrogen
ratio
Comet Halley 0.06-0.48 ppt
Earth’s Oceans
0.16 ppt
 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).
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 50100 billion gallons of water vapor into
the atmosphere,
Compare this to the
and that
• this rate has been
constant through
geological history
(~4.5 billion years).
This amounts to 225450 x 1018 gallons of
water.
total amount of
water in the oceans
= 1370x106 Km3
(volume)
109 m3/Km3
264.2
gallons/m3
= 362 1018 gallons
of water
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.