Importance: Seawater Chemistry

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Recent evidence has suggested that our global climate along with our oceans is changing. However, the chemical composition of seawater—that is, the kinds and relative amounts of chemical elements in it—hasn’t changed very much over millions of years. Every year, the world’s rivers dump millions of tons of elements dissolved from rocks and sediments into the oceans. For ocean composition to remain the same every year, an equal amount of those elements must be removed by natural processes. These include organisms making shells and minerals precipitating from seawater. For some elements, scientists were able to demonstrate that the inputs equal the outputs, but for other elements, the inputs and outputs did not balance. So how could the ocean’s chemistry stay the same?

The discovery of hydrothermal vents provided a way—which had never been thought of before—to remove some of these excess elements from the ocean. Hydrothermal vents also add some elements to the ocean.

Samples from the first vents discovered in 1977 showed that fluids coming out of the vents were very different from seawater. What happens is this. Cold seawater percolating through cracks in the ocean crust is heated up by underlying magma or hot rock. This launches chemical reactions between the hot seawater and volcanic rocks in the ocean crust. The seawater gives up certain elements and takes in other elements from the rocks. After these exchanges, seawater is no longer seawater. It has chemically changed into hydrothermal fluid.

Scientists showed, for example, that the vents remove elements such as magnesium and sulfur from seawater (which are put there by rivers). These elements get incorporated into seafloor rocks. At the same time, the vents add to seawater some elements leached out of seafloor rocks.

Scientists estimate that the entire volume of the world’s oceans cycles through hydrothermal vent systems along the global Mid-Ocean Ridge every 10-20 million years or so. Hydrothermal circulation at mid-ocean ridges draws in seawater, rearranges the seawater’s chemical composition, and spews out chemically different fluids. The vents act as great chemical reactors that help maintain the balance of Earth's ocean chemistry.