Why do the major dissolved salts occur in nearly constant proportions throughout the open ocean?

The major dissolved salts in the open ocean occur in nearly constant proportions because the time required for thorough oceanic mixing is significantly shorter than the time required for input or removal processes to substantially alter the concentration of these constituents.1

While local salinity levels can fluctuate due to surface processes—such as evaporation, which increases salinity, or precipitation and freshwater runoff, which decrease it—these processes primarily affect the total amount of water rather than the relative ratios of the major dissolved ions. Because the ocean has been mixed over such a long period, the composition of sea salt remains uniform throughout the open ocean. Consequently, while the absolute concentration of salts may vary spatially or temporally, the relative proportions of the principal inorganic constituents remain remarkably stable.21