Seawater Chemistry Through Phanerozoic Time

The major ion balance of the ocean, particularly the concentrations of magnesium (Mg), calcium (Ca), and sulfate (SO 4 ), has evolved over the Phanerozoic (last 550 million years) in concert with changes in sea level and the partial pressure of carbon dioxide ( p CO 2 ). We review these changes, along with changes in Mg Ca and strontium/calcium (Sr/Ca) of the ocean; how the changes were reconstructed; and the implication of the suggested changes for the overall charge balance of the ocean. We conclude that marine Mg, Ca, and SO 4 concentrations are responding to different aspects of coupled tectonic changes over the Phanerozoic and the resulting effect on sea level. We suggest a broad conceptual model for the Phanerozoic changes in Mg, Ca, and SO 4 concentrations along with the seawater 87 Sr 86 Sr and sulfur isotope composition. Marine concentrations of magnesium, sulfate, and calcium have varied over the last 550 million years in sync with changes in sea level and atmospheric carbon dioxide. Seawater chemistry and sea level both respond to supercontinent formation and breakup, age of the ocean floor, and extent of continental shelf area. Changes in plate tectonics impact the ocean's chemical balance and the carbon cycle in varied ways, resulting in cyclical changes in key climatic variables over geological time.