Seawater 15.2

Seawater Seawater is 96.5% water and 3.5% salt (NaCl). Most elements on Earth are present in seawater. EX: chlorides, magnesium, potassium, and calcium

Chemical Properties of Seawater Salinity is a measure of the amount of dissolved salts in seawater. Oceanographers express salinity as grams of salt per kilogram of water, or parts per thousand (ppt). On average the total salt content of seawater is 35 ppt, or 3.5%.

Chemical Properties of Seawater Seawater also contains dissolved gases and nutrients. Oxygen, nitrogen, carbon dioxide, nitrates, phosphates, and silicates

Variations in Salinity At the equator where rain is abundant, salinities are lower. Polar regions have low salinities because of the melting freshwater sea ice. The lowest salinities occur where large rivers empty into the oceans.

Variations in Salinity Salinities vary from place to place. In subtropical regions the waters salinity would be higher due to high rates of evaporation.

Sources of Sea Salt Based on the evidence of calcium-carbonate shells of some marine organisms, scientists believe that the salinity of ancient seas are the same as today.

Removal of Sea Salts Although salt is continuously added to seawater, the salinity of seawater does not increase because salts are removed from the ocean at the same rate as they are added.

Removal of Sea Salts Sometimes salt will precipitate from seawater near arid, coastal regions which will remove immense quantities of salt. Small salty spray droplets from breaking waves are picked up by winds and deposited inland. Marine organisms also remove salt to build their shells, bones, and teeth.

Physical Properties of Seawater Because salt ions are heavier than water molecules, saltwater has a higher density. Cold water is denser than warm water. Variations in salinity cause the freezing point of seawater to be lower than that of freshwater.

Absorption of Light Water absorbs light, which is why it gets darker the deeper a body of water is. Light penetrates only the upper 100 meters of seawater.

Ocean Layering Ocean water temperatures decrease with depth. A temperature profile plots changing water temperatures with depth, which can vary on location and season.

Ocean Layering The ocean can be divided into 3 layers. The first is a relatively warm, sunlit, surface layer some 100 m thick. Under this is a transitional layer known as the thermocline, which has rapidly decreasing temperatures with depth.

Ocean Layering The bottom layer is cold and dark with temperatures near freezing. Both the thermocline and the warm surface layer are absent in polar seas, where water temperatures are cold from top to bottom. Ocean layering is caused by density differences.

Water Masses When seawater freezes during the arctic winter, sea ice forms. The cold water beneath the ice becomes saltier and denser than the surround seawater and starts to sink. This 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: Antarctic Bottom Water North Atlantic Deep Water Antarctic Intermediate Water