Chem 1103: Lecture 4: Properties of Seawater and Ocean Circulation (Deep
Circulation) : finish of Chap 5 & 6
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Gases in Seawater
o The solubility and saturation value for gases in sea water increase as
temperature and salinity decrease and as pressure increases
 Solubility is the ability of something to be dissolved and go into
solution
 Solubility of gases measured in ml gas/ l for this book/course
 Saturation value is the equilibrium amount of gas dissolved in water at
an existing temperature, salinity, and pressure
 Water is undersaturated when under existing conditions it has
the capacity to dissolve more gas. Gas content is below the
saturation value
 Water is saturated when under existing conditions it contains as
much dissolved gas as it can hold in equilibrium. Gas content
is at the saturation value.
 Water is supersaturated when under existing conditions it
contains more dissolved gas than it can hold in equilibrium.
Gas content is above saturation value and excess gas will come
out of solution
 Sources of oxygen and carbon dioxide
 Atmosphere
 Volcanoes underwater
 Biology – respiration, photosynthesis, decomposition
o Respiration - O2 in CO2 out
o Photosynthesis – CO2 in O2 out
o Decomposition -CO2 in O2 out
 Sinks of oxygen and carbon dioxide
 Biology – respiration, photosynthesis, decomposition
 The surface layer is usually saturated in atmospheric gases because of
direct exchange with the atmosphere
 Below the surface layer, gas content reflects relative importance of
respiration, photosynthesis, decay, and gases released from volcanic
vents.
o Oxygen
 Tends to be abundant in the surface layer and deep layer, but lowest in
the pycnocline
 Surface layer is rich in oxygen because of photosynthesis and contact
with the atmosphere
 Oxygen minimum layer occurs at about 150 to 1500 m below the
surface and coincides with the pycnocline
 Sinking food particles settle into this layer and become
suspended in place because of the greater density of the water
below
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The food draws large numbers of organisms which respire,
consuming oxygen
 Decay of uneaten material consumes additional oxygen
 Density difference prevents mixing downward of oxygen-rich
water from the surface or upward from the deep layer
 The deep layer is rich in oxygen because its water is derived from the
cold surface waters, which sank (convected) to the bottom.
Consumption is low because there are fewer organisms and less decay
consuming oxygen
 Anoxic waters contain no oxygen and are inhabited by anerobic
organisms (bacteria)
o Carbon dioxide
 Major sources of carbon dioxide are respiration and decay
 Major sinks are photosynthesis and construction of carbonate shells
 Carbon dioxide controls the acidity of sea water
 A solution is acid if it has excess H+ (hydrogen) ions and is a
base if it has an excess of OH- (hydroxyl) ions
 pH measures how acidic or basic water is
o pH of 0 to 7 is acidic
o pH of 7 is neutral
o pH of 7 to 14 is basic
 seawater has a pH of 7.8 to 8.2
 pH is related to the amount of CO2 dissolved in water because
it combines with the water to produce carbonic acid which
releases H+ ions
 CO2 +H2O  H2CO3H+ + HCO3- 2H+ +CO3-2
 H2CO3 is carbonic acid, HCO3- is the bicarbonate ion, and CO32
is the carbonate ion
 Changing the amount of CO2 shifts the reaction to either the
right or left of the equation
o Adding CO2 shifts the reaction to the right and
produces more H+ ions making the water more acid
o Removing CO2 shifts the reaction to the left, combining
H+ ions with carbonate and bicarbonate ions reducing
the acidity
 Dissolved CO2 in water acts as a buffer, a substance that
prevents large shifts in pH
 Dissolution of carbonate shells in deep water results because
cold water under great pressure has a high saturation value for
CO2 and the additional CO2 releases more H+ ions making the
water acid
 Warm, shallow water is under low pressure, contains less
dissolved CO2 and is less acidic. Carbonate sediments are
stable and do not dissolve
The Ocean as a Physical System
o Water is recycles from the ocean to the land and returned to the sea
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The reservoirs of water include
 Oceans – cover 60% of the northern hemisphere and 80% of
the southern hemisphere and contain 97% of Earth’s water
 Rivers, lakes and glaciers
 Groundwater – contains a larger volume of water than all of the
water in the lakes and rivers
 The hydrologic cycle describes the exchange of water between
ocean, land and atmosphere Table of reservoirs
 On land precipitation exceeds evaporation
 In the ocean evaporation exceeds precipitation
The ocean is part of a biogeochemical system in which land undergoes
weathering and weathered products are transported to the sea where
they may be deposited directly or used by organisms and later
deposited as organic remains or organic wastes. Deposits are buried,
lithified, and recycled by plate tectonics into new land which is
weathered an the cycle repeats
Other Physical Properties of Water
o Light
 Amount of light entering the ocean depends upon the height of the sun
above the horizon and the smoothness of the sea surface
 65% of light entering the ocean is absorbed within the first meter and
converted into heat. Only 1% of light entering the ocean reaches
100m
 Water displays the selective absorption of light with long wavelengths
absorbed first and short wavelengths absorbed last
 In the open ocean, blue light penetrates the deepest
 In turbid coastal waters, light rarely penetrates deeper than 20 m and
the water appears yellow to green because particles reflect these
wavelengths
 The photic zone is the part of the water column penetrated by sunlight
 The aphotic zone is the part of the water column below light
penetration and permanently dark
o Speed of sound
 The speed of sound in water increases as salinity, temperature and
pressure increase, but in the ocean is mainly a function of temperature
and pressure
 Above the pycnocline, increasing pressure with depth increases the
speed of sound despite the gradual decrease in temperature
 Within the pycnocline, the speed of sound decreases rapidly because
of he rapid decrease in temperature and only slight increase in pressure
 Below the pycncline, the speed of sound gradually increases because
pressure continues to increase, but temperature only declines slightly
SOFAR channel is locate where sound speed is at a minimum. Refraction of sound
waves within the channel prevents dispersion of the sound energy and sound waves
travel for thousands of km within the channel
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Deep Ocean Circulation
o Define deep water
o Density driven, not wind driven - convection
o Thermohaline circulation is density driven flow of water generated by
differences in salinity of temperature
 Water at the surface is exposed to more rapid changes in salinity
through evaporation or precipitation and in temperature through
cooling or heating
 Once water is isolated from the atmospheric influences, salinity and
temperature are largely set for an extended period of time
 Based upon depth, surface water masses can be broadly classified as:
 Central waters – from 0 to 1 km
 Intermediate waters - from 1 to 2 km
 Deep and bottom waters – greater than 2 km
 Most deep and bottom water originated at the surface where cooling
and increased salinity raised their density until they sank
 Ocean basins interconnect and exchange water with each other and
with the surface. Inter-ocean basin circulation and exchange between
surface and deep water appears largely driven by waters of he North
Atlantic
o The major thermohaline currents appear to flow mainly equatorward, but this
is because they originate in the polar regions and their outward flow is
confined between the continents
 Warmer water (>10oC) is confined between 45o N and S latitude
 Poleward of 45o, density of water increases because of declining
temperature and increased salinity because of evaporation or ice
formation
 The water sinks to a density-appropriate level and then slowly flow
outward in all directions across the basin until they are blocked by a
continent
 Deep water gradually mixes with other water masses and eventually
rises to the surface
 Do a demo
o Explain the Conveyer Belt
o The Atlantic Ocean has the most complex ocean stratification, containing the
following layers:
 Antarctic Bottom Water forms in the Weddell Sea mainly in the winter
when water is very cold and ice formation increase salinity
 It is the densest water in the ocean and flows northward into
the North Atlantic
 Antarctic Circumpolar Water forms a less saline and slightly warmer
water mass off the coast of Antarctic where conditions are not as
severe- Note book calls this Antarctic Deep Water
 It flows northward at the surface to the Antarctic convergence,
where it sinks and flows southward along the bottom until it
encounters and rides over the denser Antarctic Bottom Water
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North Atlantic Deep water forms a cold, salty water mass off
the coast of Greenland that sinks and flows southward along
the bottom until it encounters and rides over the denser
Antarctic Bottom Water
 Arctic Intermediate Water forms in the Subarctic region and
flows southward between the surface water of the Sargasso Se
and the North Atlantic Deep Water
 Mediterranean Intermediate Water flows out of the
Mediterranean Sea as a warm, highly saline water of the
Sargasso Sea and the North Atlantic Deep Water
 The Pacific Ocean has a less complex stratification, is weakly layered,
displays sluggish circulation and is remarkably uniform below 2000m
 Common Water is the main water mass and is formed by a
blending of the Antarctic Bottom Water and North Atlantic
Deep Water in the Antarctic Circumpolar Current
o It is injected into the Pacific basin and flows northward
to 10oS where it meets Pacific Subarctic Water
 Above 2000m, the North Pacific Intermediate water and
Antarctic Intermediate water dominate
 The Indian Ocean has the simplest stratification consisting of :
 Common water which occupies most of the basin
 Antarctic Intermediate water
 Red Sea intermediate water
o It is a warm and highly saline (40 psu) water from the
Red Sea that flows into the Indian Ocean and sinks to
about 3000 m
Water flow in semi-enclosed seaways
o Most seas are indentations into continents, partially isolated from the ocean
and strongly influenced by continental climate and river drainage
 As Atlantic Ocean water flows through the Straits of Gibraltar into the
Mediterranean Sea at the surface, warm, highly saline Mediterranean
Sea water flows out through the Straits at the bottom
 As the surface water flows eastward, salinity increases because
of excess evaporation
 Sea surface slopes eastward and surface water flows down the
resulting pressure gradient
 At the eastern edge of the basin the warm but highly saline (38
psu) water sinks, flows westward and eventually out into the
Atlantic Ocean as Mediterranean Intermediate Water where it
sinks to 1000 m
 Mediterranean Sea is nutrient poor because there is no
upwelling of deep water to resupply nutrients
 German subs
Lab
1. Explain temperature, density, plotting profiles
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2. Explain water masses
Bring
1. Long tank
2. sugar water
3. sugar
4. big spoon
5. food color
6. float objects
7. soda water
8. soda
9. pH paper
10. sugar, salt, water and containers
11.
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