Magnesium is the third most abundant element dissolved in seawater (on a molar basis). It has a generally conservative distribution that varies only due to differences in evaporation or precipitation in the water that sank to form intermediate or deep waters in the sea .
Concentrations are higher in the Atlantic because that ocean is saltier. Recent work ,using high precision analyses with isotope dilution mass spectrometry, suggests that there is a magnesium minimum near 2500 m depth that is produced by consumption in hydrothermal systems (de Villiers and Nelson, 1999).
Magnesium is present in seawater as the Mg2+ ion. The magnesium residence time is from Tracers in the Sea (Broecker, W. S. and Peng, T.-H., 1982. Eldigo Press, Palisades, NY). It was calculated from the global average river input.