Transcript powerpoint

OCEAN
OCEAN
SALINITY
SALINITY
by Robert Perry
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E-mail: [email protected]
SALINITY
The amount of salts
dissolved in the
ocean water.
Why is
the ocean
salty ?
Why is the ocean salty?
The ocean is salty because of the weathering of rocks in the
early stages of the formation of the oceans and continents.
The ocean probably is continuing to become saltier very
slowly (on geologic time scales) as rivers bring dissolved
material to the sea. Even though the concentrations of
dissolved salts in "fresh" river water is tiny, evaporation from
the ocean surface (which is how water gets back to the rivers)
removes only pure water so whatever salt enters from rivers
remains in the sea and slowly builds up.
Sea water is 35o/oo or 3.5% salt by weight, which means the
total quantity of dissolved salt, if dried, would form a layer
45m thick over the entire earth, or 153m thick over the present
land area.
The salt in sea water is composed of a variety of
components, primarily sodium chloride (table salt),
magnesium chloride, magnesium sulfate and calcium
carbonate, along with many trace elements. One of the
remarkable aspects of ocean salinity is how constant the
relative ratios of these constituents are in sea water, no
matter where in the oceans one looks. This suggests that the
oceans are "well-mixed" on the time scale of salt input. Total
salinity, however, varies from about 3.3% to 3.8% in openocean surface waters, depending on the rainfall, evaporation,
and river runoff in different regions. Salinity in bays or near
the mouths of rivers is less, for example the salinity of Puget
Sound near Seattle is about 2.9-3.2%. Deep-ocean salinity is
much less variable.
The origin of ocean salts.
NaCl
Na = sodium
Cl = chlorine
Modified “Chemistry NaCl,”courtesy of the artist
Craig A. Finseth, http://www.finseth.com/~fin
The polarity of the water molecule and its relationship
with the sodium and chlorine ions.
Sea salts being extracted by evaporation.
SALINITY MEASUREMENTS:
Oceanographers report salinity (total salt content)
and the concentrations of individual chemical
constituents in sea water -- chloride, sodium, or
magnesium for example -- in parts per thousand,
for which the symbol o/oo (or ppt) is used. That is, a
salinity of 35 o/oo means 35 pounds of salt per
1,000 pounds of sea water. Similarly, a sodium
concentration of 10 o/oo means 10 pounds of
sodium per 1,000 pounds of water.
Here’s an easy way to measure
the salinity of a water sample…
THERMOMETER
HYDROMETER
John Jones
Sept. 21, 2000
22.3
0.0
green
90 %
20.0
6
9:10 am
310
2
245
7
SST and DENSITY
determine
the SALINITY
LET’S FIND SALINITY:
If our SST was 20.0OC
and our density was
1.0245 g/cm3.
Here’s where
the lines
intersect.
The salinity is:
34.5 o/oo
John Jones
Sept. 21, 2000
22.3
0.0
green
90 %
20.0
245
6
9:10 am
310
2
34.5
7
The amount of dissolved salts in
SALINITY
the ocean is controlled by temperature, which increases evaporation and causes the
salinity to increase. Also, during the winter rainy
season, large volumes of fresh water (especially from
the Pacific Northwest region) can dilute the salt
content and lower salinity values.
A rosette or cluster
of water sampling
bottles is ready to be
lowered into the sea.
This is one way
ocean salinities are
determined at
different depths
below the surface.
Surface salinity of the western Pacific.
Most ocean
salinities are
between 34.6
and 34.7 ppt.
d
Regions of
higher
precipitation
Regions of
higher
precipitation
Regions of
higher
evaporation
Regions of
higher
evaporation
Regions of
higher
precipitation
The red line shows surface salinity vs. latitude. The dashed line
shows evaporation minus precipitation. (Areas above zero are
dry and those below zero are wet). Note the similarity in the lines.
Higher evaporation
than precipitation
Higher evaporation
than precipitation
North Atlantic
Deep Water
Atlantic Ocean Salinity Cross Section
Antarctic
Bottom
Water
THE
END