Earth science - Oceans

Salinity

the amount of dissolved salt in water and is usually measured in parts per thousand (ppt). On average, the salinity of the world's oceans is 35 ppt, but ocean-surface salinities vary by small amounts around the earth. While these variations may be small, they can have significant consequences.

Differences in density drive currents in the deep ocean.

As warm surface waters cool, they become denser. The dense waters sink and set up currents in the deep ocean. These deep-water currents, in combination with the warm surface currents, set up a conveyor system of water in the oceans. This conveyor system circulates heat energy and nutrients and mixes the waters of the world's oceans.

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The properties of seawater influence how the oceans of the world—representing 70 percent of the earth's surface—behave.

Seawater has physical and chemical properties different from freshwater.

Seawater is salty. The dissolved salts in the water help give seawater physical and chemical properties different from freshwater.

Density differences cause the oceans to exist in layers.

Temperature and salinity influence the density of seawater. Since both temperature and salinity change with depth (deep waters are colder and saltier than surface waters), then the density of water also changes with depth-that is, deep waters are denser than surface waters. This sets up a layered structure of the oceans: Surface-mixed zone (0-200 m): Waters in this zone are warm and less dense with low salinity (2% of the ocean water). Transition zone (200-1,000 m): Temperature in this zone decreases from the surface-mixed zone, while salinity and density increase (18% of ocean water). Deep zone (> 1,000 m): Water in this zone is the coldest, saltiest, and densest in the oceans (80% of ocean water).

The density of seawater is influenced by temperature and salinity.

The density of pure water depends on its temperature: The colder the water, the higher the density (until it freezes, at which point it becomes less dense). The maximum density of pure water occurs at 4°C. In contrast to freshwater, the salt in seawater depresses the freezing point and alters the density of the water. So, seawater can remain liquid below the point at which pure water would freeze. The saltier the water, the denser it is. The density of seawater changes with latitude and with depth.

Temperature is an important physical property of seawater.

The high capacity of water to hold heat makes it a major source of available heat energy. Differences in water temperature help set up the structure of the oceans and drive the flow of water. Seawater absorbs solar radiation that heats the water. The sun's radiation heats the earth unevenly and this is reflected in the water temperature of the earth's oceans. The oceans are warmer at the equator and colder at the poles. Water temperatures also vary with depth.

Ocean layers and currents influence the distribution of marine life.

The layers of the ocean have different temperatures and salinities. They also have different oxygen and nutrient levels. The parameters of temperature, salinity, and available nutrients set boundaries on what types of marine life can live in an area at what depths. For example, along the coast of North Carolina, tropical fish live south of Cape Hatteras, where warm waters from the Gulf Stream hug the coast. The Gulf Stream turns at Cape Hatteras out to sea, so along the northern side of the cape, you find fish that require colder or more temperate climates. Only marine life that can tolerate cold temperatures can survive on the ocean floor.

Differences in temperature cause surface currents.

Uneven heating of the earth's surface causes temperature differences in the atmosphere and in the surface waters of the oceans. The temperature differences create winds in the atmosphere, which help move the surface waters

Physical and chemical properties of seawater determine structure and dynamics of the oceans.

Uneven heating of the earth's surface causes temperature differences in the surface waters of the oceans (temperature varies with latitude and depth). Salinity of the oceans also varies geographically and with depth. Temperature and salinity interact and influence the density of seawater. Changes in density set up a layered structure of the ocean and contribute to deep ocean currents. Changes in temperature at the ocean's surface (and in the atmosphere) cause surface currents. These currents circulate heat energy and nutrients, which influence the earth's temperature and the distribution of marine life.