The Composition of Seawater Chapter 15.1 The Composition of Seawater

Salt water has dissolved substances like sodium chloride, other salts, metals, and dissolved gases that give it a salty taste. Salt in seawater makes it unusable for drinking or for irrigating crops and it is a highly corrosive material. Seawater usually contains about 3.5% dissolved mineral substances called salts.

Salinity Salinity – the total amount of solid material dissolved in water. This is a ratio of the mass of dissolved substances to the mass of the water sample. Most of the salt in seawater is sodium chloride, table salt.

Sources of Sea Salts One source is chemical weathering of rocks. These materials come from the rivers and streams that run into the ocean. The second major source is from the Earth’s interior. Usually caused from volcanic eruptions that were in the atmosphere and have fallen into the ocean.

Processes Affecting Salinity The ocean is well mixed so the concentrations of salts are constant. There are a few places that will be more or less salty. Icebergs, sea ice, runoff and evaporation all affect the salinity. Icebergs and runoff decrease salinity. They add water to the ocean and decrease the concentration of salts. Sea ice and evaporation remove water from the ocean therefore increasing the amount of salts.

Ocean Temperature Variation The ocean’s surface water temperature will vary with the amount of sunlight received. The latitude will affect the amount of sunlight. Higher latitudes will have less sunlight than tropical latitudes That means that higher sea surface temperatures will be found in tropical latitudes.

Temperature Variation with Depth Surface water is warmed by the sun, so they will be warmer than deeper water. Thermocline is the layer of ocean water between 300 m and 1000 m where there is a rapid change in temperature with depth. High latitudes generally have colder temperatures and do not have a thermocline.

Ocean Density Variation Remember density is the mass per volume. Density determines the water’s vertical position in the ocean. Density differences can cause large areas of ocean water to sink or float.

Factors Affecting Seawater Density There are 2 main factors that affect seawater density – salinity and temperature. An increase in salinity, adding dissolved substances, increases density.

Density Variation with Depth Temperature is the most important factor affecting density. Density increases as the temperature decreases. Pycnocline – the layer of ocean water where there is a rapid change of density with depth.

Ocean Layering The ocean layers according to density. Low-density water is near the surface. Higher density water is lower. There are 3 layering structure in the open ocean: a shallow surface mixed zone, a transition zone, and a deep zone.

Surface Zone At the surface the temperature is the warmest. The mixed zone is the area of the surface made by the mixing of water by waves, currents, and tides. There is a uniform temperature here.

Transition Zone This is an area between the mixed zone and the deep zone of cold water.

Deep Zone Sunlight never reaches this area. The temperatures are very cold. The density is very high. This accounts for 80% of ocean water. In high latitudes the 3 layers do not exist. In this area there is mixing between surface and deep water. The cold high-density water forms at the surface, sinks, and makes deep ocean currents.