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What is in seawater? Ask a scientist

By Marla Vacek Broadfoot
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Dr. Carrie Thomas is a research associate professor of marine, earth and atmospheric sciences at N.C. State. Here she explains the chemical makeup of the deep blue sea. Questions and answers have been edited.

Q: Why is the ocean salty?

Salts in the ocean come from two sources. Mineral salts from the Earth’s crust (rocks at the surface) run down rivers and streams into the ocean. Other salts come from deeper in the planet, through volcanic vents on the seafloor or on land (washed into the sea by precipitation). Over time, water evaporates from the ocean and leaves the salts behind. You might expect this process to make the oceans saltier and saltier, but today there is a balance between the amount of salt added and the amount lost through removal processes at the bottom of the sea.

Q: How much salt is contained in the world’s oceans?

Scientists estimate that there are 50 million billion tons of salt in the ocean. About 3.5 percent of the weight of seawater comes from dissolved salts.

Q: What else makes up the chemical composition of our ocean?

In addition to sodium and chloride – the chemicals we typically think of as salt – we find sulfate, magnesium, calcium, potassium and carbonate in large concentrations in seawater. There are also other chemicals that are not as concentrated, but are very important to human health and maintaining ocean life. Those would include nutrients, heavy metals – such as mercury and cadmium – and organic pollutants. It is no longer unusual for scientists to find chemicals such as pesticides, prescription drugs and food additives in our rivers and coastal ocean (like artificial sweeteners, cinnamon, vanilla, caffeine).

Q: How do storms, hurricanes and climate change affect ocean chemistry?

During storms, including hurricanes, the chemistry of the coastal ocean can be affected by precipitation and run-off and by mixing from the winds. Runoff (water flowing over land, as in floods) and locally heavy rain can temporarily lower the salinity of the surface of the ocean. The effect does not last long, because the ocean is large and well-mixed. Hurricanes and nor’easters are strong enough to stir up sand, silt and mud in shallow areas along our coast. This makes the water cloudy and releases chemicals such as nutrients from the seafloor.

The amount of carbon dioxide in our atmosphere is increasing and creating global climate change. Some of it enters the ocean. As the gas dissolves in seawater, carbonic acid is created and the ocean becomes more acidic. Scientists theorize that increasing the acidity of the ocean will negatively impact animals that form carbonate shells – such as shellfish and corals – and may positively impact some organisms that photosynthesize, such as algae and sea grasses.
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