Oceanography

Quantity of Materials Dissolved in the World Ocean

If we assume that the amount of dissolved material in the ocean remains unchanged even today, we must also assume that the amount of dissolved material entering the sea each year is equivalent to the amount lost in sediment. Actually, the assumption cannot be validated, because scientists have measured the composition of the oceans for only a few decades. If no ions were removed from the sea, the ions added by rivers in 100 years would increase the concentrations of substances dissolved in the world ocean by only 0.0005 per cent. Rivers transport approximately 2.5 x 10¹⁵g of dissolved material (5x 10²²g) already present in the world ocean (this is 20,000,000 times the amount added every year). In any case, the rate of change is so small that the assumption of a steady state is reasonable.

Every kilogram of seawater in the open ocean contains about 35g of ions. In oceanography, this concentration is expressed as parts per thousand and can be written 35%. The measure of the concentration of total ions in seawater is called the salinity. A salinity of 35 parts per thousand is an average value for water in the open ocean.

In near shore coastal regions, bays, and especially in river estuaries, salinity is highly variable. Near river mouths the salinity of surface water may vary from nearly zero to 34 parts per thousand, but low values generally prevail. These variations are also seasonal and depend on river conditions. During summer months when river carry less water, near shore salinities may be close to 34 parts per thousand. During the winter rainy season or spring thaw, the seawater in the near shore region frequently becomes diluted by flood water.

Exactly the opposite conditions prevail in hot, dry regions of the world where excess evaportation causes salinities to be higher than average. Salinity values as high as 40 parts per thousand may be found in places like the Mediterranean and Red seas.

The surface layer of the ocean is separated from the deeper layers of the ocean by a permanent thermocline 100 to 700m deep that persists between the 50° to 60° north and south latitudes. It is shallow and steep at the equator but deeper and more gentle in mid-latitudes. It rises to the surface at 50° to 60° latitude and marks the largest continental rises which extend seaward more than 500km.