Sea water
Density
-2
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Depth (m)
Pacific Ocean (167.5°W)
0
1000
• Density of Ocean is determined by temperature, the quantity of dissolved
salts (also known as salinity), and the pressure to which a parcel of
seawater is exposed.
• Density is lighter at the depth than at the surface. The average density of
ocean is 1.03 g/cm3.
2000
3000
4000
5000
Temperature
Depth (m)
Atlantic Ocean (17.5°W)
0
• 75% of the water in the oceans has temperatures ranging between 0° and
6°C; the average temperature is 3.5°C.
1000
2000
3000
4000
• Water is cold at the depth and gets warmer towards surface. Similarly, water
at poles is cold and gets warmer towards equator.
5000
Depth (m)
Indian Ocean (64.5°E)
0
• Surface temperature in the Bay of Bengal is usually between 22ºC and 31ºC,
whereas 1 2ºC in the Arabian
1000
2000
3000
4000
5000
80° S
40° S
0°
40° N
80° N
North-south vertical sections of temperature (oC)
in the Pacific, Atlantic, and Indian Oceans.
33.25
34.25
33.75
34.75
35.25
35.75
36.75
36.25
Pacific Ocean (167.5°W)
• Salinity is measured as the ratio of weight of dissolved salts to total weight
and is usually expressed as parts per thousand (ppt).
• 75% of seawater has a salinity ranging between 34-35 ppt. The average
salinity in the oceans is 34.7 ppt i.e., on an average there is 34.7 g of salt in
every kg of seawater.
Depth (m)
• Salinity near the surface in the northern Bay of Bengal can be as low as 31
ppt because the bay receives lots of freshwater in the form of rain and from
runoff of surrounding rivers (Ganga, Brahmaputra, Irrawaddy, Godavari, and
others).
Atlantic Ocean (17.5°W)
Depth (m)
• Salinity near the surface in the Arabian Sea is much higher than in the Bay
of Bengal because evaporation over the Arabian Sea is much greater and it
receives relatively less river runoff.
Pressure
Indian Ocean (64.5°E)
Depth (m)
• We live at 1 atmosphere pressure. In the ocean, pressure increases by 1
atmosphere for every 10 m increase in depth. Hence, the pressure at the
bottom of the ocean of an average depth of 3700 m will be 370
atmospheres.
North-south vertical sections of salinity (ppt) in
the Pacific, Atlantic, and Indian Oceans.
0
5
10
15
20
25
30
0
5
10
15
0
150
20
S
50
100
Arabian Sea
200
25
30
T
Bay of Bengal
S
T
250
500
1000
31
32
33
34
35
Salinity
36
37 31
32
33
34
35
36
37
Salinity
20 21 22 23 24 25 26 27 28 20 21 22 23 24 25 26 27 28
Density
• Pressure has an influence on physical (density), chemical, and biological
(decomposition of shells) properties of the ocean.
Why seawater composition does not change?
Temperature
Depth (m)
Salinity
Density
Vertical profiles of temperature, salinity, and
density in the northern Arabian Sea and Bay of
Bengal. The much lower surface salinity in the
latter leads to a strong near-surface density
gradient (high stratification). Because of the
small variability of density in the ocean, the
scale marked density gives density in (g/cm3-1)
x 1000.
Just as the salt content of seawater does not change, the concentration of its
components also remains unchanged. To maintain the constant composition,
it is necessary that dissolved ions are removed at the same rate as they are
added. Processes by which ions are removed from seawater include the
following:
• Evaporative precipitation: In hot, dry climates where the sea is shallow and
enclosed, solid deposits of salts such as rock salt (sodium chloride) and
gypsum (calcium sulphate) are formed.
• Chemical precipitation: When the concentration of a salt becomes too great,
that salt forms a solid precipitate. For example, calcium ions and carbonate
ions combine together to form insoluble limestone (calcium carbonate).
• Biochemical removal: Organisms remove ions by scavenging them from
seawater. Some animals like coral and bivalves make shells of calcium
carbonate. Many organisms concentrate ions in their body tissues by a
factor of 105 or more. For example, sea squirts concentrate vanadium,
other tunicates concentrate niobium, oysters concentrate zinc, lobsters
concentrate copper, and other shellfish concentrate mercury.