Magnesium was estimated from the formula 2748.4*Salinity/1.80655/(1000-1.81578*Salinity/1.80655) where 1.80655 is the Salinity/Chlorinity ratio and 2748.4 is the number of mmoles of Chlorinity - Millero, F., 1974. Seawater as a multicomponent electrolyte solution, in The Sea, Vol. 5, E. Goldberg, ed., Wiley, pp. 3-80.
An original reference on magnesium in seawater is:
Culkin, F., and Cox, R. A., 1966. Sodium, potassium, magnesium, calcium and strontium in sea water. Deep-Sea Res., 13: 789-804.
Recent work includes:
de Villiers, S., and Nelson, B. K., 1999. Detection of low-temperature hydrothermal fluxes by seawater Mg and Ca anomalies. Science 285: 721-723.