Question

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?

Short answer

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.

Step-by-step solution

Factors controlling the Mg/Ca ratio in the ocean

The ratio of magnesium to calcium (Mg/Ca) in the ocean is controlled by several factors: weathering of rocks, hydrothermal activity, dissolution and precipitation rates of minerals, biological activity, and changes in sea temperature and salinity. Each of these factors affects the availability and concentrations of these ions in seawater.

Weathering and Hydrothermal Activity

The weathering of rocks on land weathering provides a major source of both Mg and Ca ions to the ocean. Similarly, hydrothermal circulation at mid-ocean ridges dissolves rocks and leads to changes in the concentrations of ions in seawater. Changes in the intensity of these processes can alter the Mg/Ca ratio in the ocean.

Dissolution and Precipitation Rates

Sea temperature, pressure, and salinity affect the dissolution and precipitation rates of minerals such as calcium carbonate (CaCO3) and magnesium-rich minerals. Higher temperature or increased salinity can lead to a higher saturation state of these minerals, influencing the Mg/Ca ratio in the ocean.

Biological Activity

Marine organisms such as plankton and corals use calcium to build their shells and skeletons, increasing the demand for this element. When these organisms die and their shells/skeletons begin to dissolve, it can release the calcium back into the water. Similarly, certain organisms can preferentially incorporate magnesium into their structures, affecting the Mg/Ca ratio.

Changes over Phanerozoic Time

During the Phanerozoic time (approximately 541 million years ago to the present), the composition of limestone has changed due to the fluctuations in the Mg/Ca ratio in the ocean. For example, periods of higher Mg/Ca ratios have favored the formation of magnesium-rich carbonates (dolomite). In contrast, during times of lower Mg/Ca ratios, calcite and aragonite (calcium-rich carbonates) were the dominant minerals forming limestone. These changes in mineralogical composition of limestone are also related to the evolution of marine life, as different organisms preferentially utilize different minerals in their structures.