What controls the ratio of magnesium to calcium in the ocean? How have changes in this ratio influenced the mineralogical composition of limestone during Phanerozoic time?

The ratio of magnesium to calcium (Mg/Ca) in the ocean is controlled by factors such as weathering of rocks, hydrothermal activity, dissolution and precipitation rates of minerals, biological activity, and changes in sea temperature and salinity. During Phanerozoic time, fluctuations in the Mg/Ca ratio have influenced the mineralogical composition of limestone. Periods with higher Mg/Ca ratios favored the formation of magnesium-rich carbonates (dolomite), while lower Mg/Ca ratios led to the dominance of calcite and aragonite (calcium-rich carbonates) in limestone. These changes also relate to the evolution of marine life, as different organisms preferentially utilize different minerals in their structures.