Properties of Seawater Oceans 11 Ms. Bowie

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). 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 The actual salinities of the oceans vary from place to place. Salinity 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, thereby diluting the ocean in that region.

Seawater Sources of Sea Salt Geological evidence indicates that the salinity of ancient seas was very similar to 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. The composition of shells has not changed much since the earliest shelled creatures.

Seawater Sources of Sea Salt The sources of sea salts include: The weathering of rocks on land and underwater generates most of the other abundant ions in seawater. These ions are then flushed into rivers and transported to oceans.

Seawater Removal of Sea Salts 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 is denser than freshwater because salt ions are heavier than water molecules. The density of seawater ranges from about 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 (freshwater freezes at 0oC).

Seawater Absorption of Light 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 Surface Temperature Seawater Ocean Surface Temperature 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 Layering Ocean water temperatures decrease significantly with depth. Water temperatures at the bottom are closer to 4oC.

Seawater Ocean Layering Based on temperature variations, the ocean can be divided into three layers. The top layer is a relatively warm, sunlit, surface layer some 100 m thick. The 2nd layer is known as a thermocline. This is a transitional layer which is characterized by rapidly decreasing temperatures with depth. The bottom layer is cold and dark with temperatures near freezing.

Seawater Ocean Layering Both the thermocline and the warm surface layer are absent in polar seas, where water temperatures are cold from top to bottom. 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.

Seawater Water Masses Cold water migrates toward the equator as a cold, deep water mass along the ocean floor. 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.

Simulation of Density of Water

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