OCEAN

SALINITY

OCEAN

SALINITYby Robert Perry

DISCLAIMER:

The images, maps and diagrams inthis presentation were taken from the public domain on the world

wide web. Any privately owned images not available for this non-

commercial, educationalproject will gladly be removed.

E-mail: zalophus@ucla.edu

SALINITY

The amount of saltsdissolved 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 open-ocean 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.

Modified “Chemistry NaCl,”courtesy of the artist Craig A. Finseth, http://www.finseth.com/~fin

Na = sodium Cl = chlorine

NaCl

The polarity of the water molecule and its relationshipwith 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.

THERMOMETER

HYDROMETER

Here’s an easy way to measurethe salinity of a water sample…

SST and DENSITY determine

the SALINITY

22.3

John Jones 6Sept. 21, 2000 9:10 am

0.02

310green

7

90 %

20.0 2 4 5

LET’S FIND SALINITY:If our SST was 20.0OCand our density was

1.0245 g/cm3.

The salinity is:34.5 o/oo

Here’s wherethe linesintersect.

22.3

John Jones 6Sept. 21, 2000 9:10 am

0.02

310green

7

90 %

20.0 2 4 5 34.5

The amount of dissolved salts inthe ocean is controlled by temp-erature, which increases evaporation and causes thesalinity to increase. Also, during the winter rainyseason, large volumes of fresh water (especially fromthe Pacific Northwest region) can dilute the saltcontent and lower salinity values.

SALINITY

A rosette or clusterof water sampling

bottles is ready to belowered into the sea.

This is one wayocean salinities are

determined atdifferent depths

below the surface.

Su

rfac

e sa

lin

ity

of t

he

wes

tern

Pac

ific

.

Most oceansalinities arebetween 34.6and 34.7 ppt.

d

Regions ofhigher

evaporation

Regions ofhigher

evaporation

Regions ofhigher

precipitation

Regions ofhigher

precipitation

Regions ofhigher

precipitation

The red line shows surface salinity vs. latitude. The dashed lineshows evaporation minus precipitation. (Areas above zero are dry and those below zero are wet). Note the similarity in the lines.

North AtlanticDeep Water

AntarcticBottom Water

Higher evaporationthan precipitation

Higher evaporationthan precipitation

Atlantic Ocean Salinity Cross Section

T H E E N D