Answer: the ratio of magnesium to calcium in the shells
Explanation: During the formation of the shell, Mg is incorporated and may weaken the shells. In some instances, foraminifer expend energy to control the incorporation of Mg. The substitution of Mg into calcite depends on the temperature of the sea water, so that the amoung of magnesium in the shell exponentially increases from cold to warm water. In turn mean that the magnesium and calcium ratio of the shell is expected to rise with increasing temperatures.
