Seawater

Seawater• Seawater is a solution of about 96.5% water and

3.5% dissolved salts. • The most abundant salt in seawater is sodium

chloride (NaCl). • Most elements on Earth are present in seawater. • Because these substances are dissolved, they

are in the form of ions (positive or negative

atoms).

Chemical Properties of Seawater

Salinity is a measure of the amount of dissolved salts in seawater that is expressed as grams of salt per kilogram of water, or parts per thousand (ppt).

Seawater

• The total salt content of seawater is, on average, 35 ppt, or 3.5 percent.

• Seawater also contains dissolved gases and nutrients.

Chemical Properties of Seawater

Variations in Salinity

Seawater

– The actual salinities of the oceans vary from place to place.

• Salinities may be as high as 37 ppt in subtropical regions where rates of evaporation exceed those of precipitation.

• Salinities are lower in equatorial regions where precipitation is abundant.

• Salinities of 32 or 33 ppt occur in polar regions where seawater is diluted by melting sea ice.

• The lowest salinities often occur where large rivers empty into the oceans.

Chemical Properties of Seawater

Sources of Sea Salt

Seawater

– Geological evidence indicates that the salinity of ancient seas was not much different from that of today’s oceans.

– The proportion of magnesium in the calcium-carbonate shells of some marine organisms depends on the overall salinity of the water in which the shells form.

– Present-day shells contain about the same proportion of magnesium as similar shells throughout geologic time.

Chemical Properties of Seawater

Sources of Sea Salt

Seawater

– The sources of sea salts has remained the same over time.

• Chlorine and sulfur dioxide dissolve in water and form the chlorine and sulfate ions of seawater.

• The weathering of crustal rocks generates most of the other abundant ions in seawater.

• These ions are then flushed into rivers and transported to oceans.

Chemical Properties of Seawater

Removal of Sea Salts

Seawater

– Salts are removed from the ocean at the same rate as they are added. – The removal of sea salts involves several processes.

• Some sea salts precipitate from seawater near arid, coastal regions. • Salty spray droplets from breaking waves are picked up by winds and deposited inland. • Marine organisms remove ions from seawater to build their shells, bones, and teeth.

Physical Properties of Seawater

• Freshwater has a maximum density of 1.00 g/cm3.

Seawater

• Seawater is denser than freshwater because salt ions are heavier than water molecules.

• The density of seawater ranges from about 1.02 g/cm3 to 1.03 g/cm3 depending on its salinity and temperature.

• Because salt ions interfere with the formation of hydrogen bonds, the freezing point of seawater is –2°C.

Physical Properties of SeawaterAbsorption of Light

Seawater

– Water absorbs light, which gives rise to another physical property of oceans—they are dark.

– In general, light penetrates only the upper 100 m of seawater.

– Red light penetrates less than blue light.

– Light sufficient for photosynthesis exists only in the top 100 m of the ocean.

Ocean Layering• Ocean surface temperatures range from –

2°C in polar waters to 30°C in equatorial regions, with the average surface temperature being 15°C.

Seawater

• Ocean water temperatures decrease significantly with depth.

Ocean LayeringA typical ocean temperature profile plots changing water temperatures with depth.

Seawater

Ocean Layering• Based on temperature variations, the ocean

can be divided into three layers.

Seawater

– The first layer is a relatively warm, sunlit, surface layer some 100 m thick.

– The thermocline is a transitional layer which is characterized by rapidly decreasing temperatures with depth.

– The bottom layer is cold and dark with temperatures near freezing.

Ocean Layering• Both the thermocline and the warm surface

layer are absent in polar seas, where water temperatures are cold from top to bottom.

Seawater

• In general, ocean layering is caused by density differences of warm and cold water. – In warm water the molecules are spaced further apart

less dense– In cold water the molecules are spaced closer

together more dense– More dense fluids tend to sink, less dense fluids tend

to rise or float.

Water Masses• Cold water migrates toward the equator as a

cold, deep water mass along the ocean floor.

Seawater

– To start, sea ice that forms in the polar regions does not incorporate salt ions into growing ice crystals, causing them to accumulate beneath the ice.

– As the cold water beneath the ice becomes saltier and denser than the surrounding seawater, it sinks.

– Surface currents in the ocean also bring relatively salty midlatitude or subtropical waters into polar regions where they cool and sink.

– The dense, salty water then migrates toward the equator as a cold, deep water mass along the ocean floor.

Water Masses• Three water masses account for most of the

deep water in the Atlantic Ocean.

Seawater

1.Antarctic Bottom Water forms when antarctic seas freeze during the winter and water temperature drops below 0°C.

2.North Atlantic Deep Water forms in a similar manner offshore from Greenland.

3.Antarctic Intermediate Water forms when the relatively salty waters of the Antarctic Ocean decrease in temperature during winter and sink.

Water Masses

Seawater

• The Indian and Pacific Oceans contain only the two deep antarctic water masses.

Ocean Currents

• Ocean water properties like salinity, density, and temperature play a crucial part in how currents are formed.

• We will explore this later in the unit