It's been 550 million years since the Earth has had an ocean composition quite like today's, according to a Johns Hopkins researcher. Contradicting the idea that the composition of ocean water has not changed much since the dawn of life, geochemist Lawrence Hardie asserts that the amounts of certain ingredients in seawater change over geologic time.
Studying ancient limestones and salt deposits, Hardie has observed that the concentration of salts in seawater fluctuates over time because of fluctuations in the two primary sources feeding the oceans: rivers and hydrothermal vents. Since hydrothermal water is 100 times as salty as river water, any change in the flow from these vents above mid-ocean volcanic ridges can cause a radical change in seawater composition. The variations in heat and water released from hydrothermal vents lead to changes in concentrations of magnesium, calcium, potassium, and sulfates.