Oceanography

Residence time of constituents in sea water

A useful concept for characterizing substances in seawater is residence time. It is the time required to replace the amount of a given substance in the ocean completely. This concept can work in two ways, using either the rate of addition of elements or the rate of removal of elements incorporated in sediments deposited on the ocean bottom. Using the second option, we can define residence time as follows

Residence time = Amount of element present - Removal rate (in years)

Sodium’s calculated residence time in the ocean of 68 million years is one of the longest residence times for an element.

An element’s residence time in the sea is related to its chemical behavior. Elements like sodium are little affected by sedimentary or biological processes. They have residence times of millions of years. Elements used by organisms or readily incorporated in sediments, such as aluminium or iron, have much shorter residence times, ranging from a few hundred to a few thousand years.

We can also calculate residence times for water. There is a net removal, due to evaporation (which does not fall back on the ocean surface as rain) of a layer of water about 10 centimeters thick from the ocean surface each year. This water falls on the land and returns to the ocean through river discharge. Recall that the ocean has an average depth of about 4000 meters. From these figures we find the residence time of 40,000 years for water.