Physical Oceanography

Salinity

Salinity

Salt has always been a valuable commodity to mankind. Its

antiseptic and preservative powers have long been known. Salt has

always been used as currency and the word ‘salary’ is derived from the

word ‘salt.’ Even today, many tropical societies use salt as

currency.

Seawater

• Average seawater is composed of three things: water (96.5%), salt (3.5%) and trace amounts of cosmic dust.

• The most abundant ions in salt are chlorine (Cl-) and sodium (Na+), which together make up about 86% of all salts in the ocean.

What is Salinity?• Salinity = total mass (grams) of

dissolved material in 1 kg of seawater

• Average salinity = 35g salt/kg of seawater

= 35 parts per 1000 = 35‰, or 3.5%

• This is equivalent to 2.5 teaspoons of salt in 1 L of water

Examples of Salinity

Bottom of Dead Sea 270 ‰

Great Salt Lake, Utah 280 ‰

Red Sea 37-46 ‰

Persian Gulf 35-40 ‰

Sargasso Sea (world’s saltiest open ocean water) 38 ‰

Black Sea 18 ‰

Baltic Sea (numerous rivers run into it) < 10 ‰

Terminology

• Hypersaline Water: salinity > 47 ‰

• Brine: salinity from 36-47 ‰

• Brackish Water: salinity < 17 ‰ (mixture of fresh and salty, mostly in bays, inlets and river mouths)

Are the following hypersaline, brine, brackish, normal, or a

combination?

Bottom of Dead Sea 270 ‰

Great Salt Lake, Utah 280 ‰

Red Sea 37-46 ‰

Persian Gulf 35-40 ‰

Sargasso Sea 38 ‰

Black Sea 18 ‰

Baltic Sea < 10 ‰

Bottom of Dead Sea Hypersaline 270 ‰

Great Salt Lake, Utah Hypersaline 280 ‰

Red Sea Brine 37-46 ‰

Persian Gulf Brine/normal 35-40 ‰

Sargasso Sea Brine 38 ‰

Black Sea Brackish/

normal

18 ‰

Baltic Sea Brackish < 10 ‰

How is Ocean Salinity Measured

1. How well it conducts electricity

A salinometer: a water sample is placed in it – you get a digital readout of the salinity

Greater Conductivity = Higher Salinity

2. Measure the refraction of light

A refractor is used to measure the

amount of ‘bending’ that occurs when

light passes through water.

Greater Refraction = Higher Salinity

3. CTD Sensor

Lowered over the side of a ship, it measures conductivity, temperature and pressure/density all at once.