From Randy's article on magnesium
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One other comment on seawater concentrations of magnesium. The magnesium content of seawater has not been constant since the oceans formed. Specifically, the magnesium content has often been lower, as in the late cretaceous period. As is discussed below, the amount of magnesium getting into calcium carbonate skeletons is a function of how much magnesium is in the water. Consequently, the magnesium content of ancient sediments can be significantly lower than more modern ones from similar organisms.<SUP>1</SUP> In addition to being an interesting fact, this result may also play a role in the suitability of certain limestone deposits in maintaining magnesium in aquaria. For example, such limestone is sometimes used in CaCO<SUB>3</SUB>/CO<SUB>2</SUB> reactors or as the raw material for making calcium hydroxide (lime). If it is low in magnesium, one may find additional supplements necessary to maintain modern seawater magnesium concentrations. These issues will be detailed more in future articles. "