15.2 Seawater
Salinity – is the amount of dissolved salts in seawater. Salinity is expressed as grams of salt per kilogram
of water or parts per thousand (ppt). The total salt content of seawater averages 35 ppt or 3.5 percent.
The most abundant salt in seawater is sodium chloride. Others include chlorides, sulfates of magnesium,
potassium, and calcium.
Variations in salinity – even though the average salinity
in the oceans is 35 ppt, actual salinity varies from place
to place. In subtropic regions, the salinity can be as high
as 37 ppt because evaporation exceeds precipitation
thus leaving salt behind. In equatorial regions, where
there is more precipitation, salinity is lower. In polar
waters the ppt is 32 or 33 because melting sea ice
dilutes the seawater. The lowest salinity occurs where
large rivers empty into the oceans creating areas of
water called estuaries. Even though salinity varies
from place to place, the overall types of sea salts is
constant because of the intermingling of the ocean
water.
Sources of sea salt – Volcanic gases such as sulfur dioxide and chlorine dissolve in water forming sulfate
and chlorine ions. Other sources include the decaying of organisms that have magnesium in their calcium
carbonate shells, and sodium and calcium from the weathering of crustal rock and iron and magnesium
come from the weathering of other rocks rich in these elements. These ions enter rivers and then are
transported to oceans.
Removal of sea salts – Salinity does not increase because marine organisms use the salt left behind from
evaporation to build shells, bone, and teeth. When these animal die, their solid parts settle on the ocean
floor and become sediments. Wind will also pick up tiny salt droplets and deposit them inland. Thus
there is a balance between the adding and removal of sea salt.
Density – seawater is denser than fresh water because of the salt in it. The more salinity the water has,
the denser it is. Temperature also effects density. The warmer the water, the lese dense it is, the colder,
the denser. The density of seawater ranges form 1.02 g/cm3 to 1.03
g/cm3.
Freezing point – salinity cause the water to freeze at a lower temp.
Fresh water freezes at 32oF whereas seawater freezes at 28oF.
Ocean Layering – ocean surface temps range from -2oC (28oF) in polar
waters to 30oC (86oF) in equatorial waters. The average surface
temp is 15oC (59oF). The deeper you go in the ocean, the colder
the water no matter where you are.
The ocean can be divided into three layers according to
temperature. First layer is relatively warm because of the sun.
This layer extends to about 100m (328 ft.). The tropic surface
water is warmer than the polar water though. Next is a transitional
layer called the thermocline, which is characterized by rapidly
decreasing temps. The bottom layer is cold and dark with temps near
freezing. Polar waters don’t have the warm surface water or the
thermocline because the water there is cold from top to bottom. Ocean layering is caused by density.
Cold more dense water sinks to the bottom, and warm, less dense water is near the surface.
Deepwater Masses – Cold water comes from Earth’s polar seas. When seawater freezes during the arctic
and Antarctic winter, sea ice forms. Salt ions will accumulate beneath the ice during the freezing process;
therefore the cold water beneath the ice becomes saltier and denser than the surrounding water. This
saltier water will then sink. It will then migrate towards the equator as a cold, deepwater mass along the
ocean floor.
Three water masses account for most of the deepwater masses in the oceans. They are – Antarctic
Bottom Water, North Atlantic Deep Water, and Antarctic Intermediate Water. The Antarctic Bottom
Water is the coldest and densest of all water masses. The North Atlantic Deep Water is the second coldest
and the Antarctic Intermediate Water is the “warmest” of the three.
AABW = Antarctic Bottom Water
NADW = North Atlantic Deep Water
Intermediate is the Antarctic Intermediate