Ocean chemistry and deep-sea sediments

Steady state oceanDefinition: The rate of supply of dissolved constituents equals the rate of removal.This concept allows to to define a mean oceanic residence time:Total mass of a dissolved element in the ocean/rate of supply (or removal) of this elementMost of the ocean residence times for elements are long ~10 5 years.The ocean mixing time, in contrast is much shorter between 250 – 500 or 1000 years.Why can a dissolved constituent with an ocean residence time of 100 years not be mixedthroughout the ocean?Note that the life span of most marine organism is short. Thus the dissolved constituate cango through biological cycles several times.