What Makes the Ocean Salty?

One undeniable truth about the oceans is that they are salty, but why? In short: rocks.

The water cycle is something most people are generally familiar with. Clouds rain down water, which end up on land, in lakes, or in the ocean. The hot rays from the sun heat up this water, causing evaporation. The water is now in gas form, where it can return again to the clouds. This cycle continues day in and day out. But what does this have to do with the ocean being salty?

When rain falls on the continents, it flows downstream into rivers, which eventually wash out into the ocean. Because rainwater is slightly acidic, when the rain hits rocks, it is able to dissolve small amounts of salts and minerals. These newly collected components are carried with the water into the river. Rivers, being a much stronger force of water, also cut through land and rocks in the same way rainwater does. As river water enters the ocean, any dissolved minerals it picked up along the way are mixed in as well.

On average, the oceans contain 35 grams of salt in each liter of seawater; that’s about 2.3 tablespoons of salt for every 4.2 cups of water. Sea salt is made up of six major ions, with most of it coming from two: sodium and chlorine (Na and Cl). Most food contains table salt, which has a simple chemical formula of… drumroll please… NaCl! If you were to add 2.3 tablespoons of salt to 4.2 cups of water, you would get a mixture almost as salty as the ocean. This is because it is still missing the other four ions, which account for about 14% of the salts in seawater.

Wait… if the rivers move salts into the ocean, why aren’t rivers salty? Rivers replenish with rainwater and continuously flow into the ocean, and they are considered freshwater because the amount of salt they hold at any given time is really low. Their salinity is typically less than 1 ppt (part per thousand), where sea water is 35 ppt. Rivers are constantly moving, so they don’t build up large amounts of salt like the ocean does. The freshwater being churned into the ocean doesn’t stay there like salts do, this is where the evaporation from the water cycle comes in. The sun heats up the surface of the ocean, which pulls water into its gas form and the salts are left behind.

As civilization constantly evolves, humans are always coming up with new, innovative ideas. One that plays into this topic is salting roadways during the wintertime. It keeps the sidewalks and roads from icing over, but have you ever stopped to think about where the salt ends up? One study in 2005 looked at the salinity of rivers in the northeastern United States during a cold, snowy winter. The chloride concentrations of streams in Maryland, New York, and New Hampshire were up to 100 times greater than remote forest streams during summers. The authors warned that if no changes were made, the freshwater in this area could become unusable for humans and unlivable for marine life in the next century. Instead of salt, we could use something the ocean is already giving us: sand. It’s one of the most common alternatives for rock salt and doesn’t pose significant risks to the environment. However, it does leave cleanup issues once the snow melts. Further research on deicers should be a focus for environmental agencies worldwide, but with so many other issues, it gets swept under the rug. When something as simple as salting our driveways can have such a massive impact on the environment, it exemplifies how human nature’s footprint is changing the world as we know it.

References

Durack, P. (2015). Ocean salinity and the global water cycle. Oceanography, 28(1), 20–31. https://doi.org/10.5670/oceanog.2015.03

Kaushal, S. S., Groffman, P. M., Likens, G. E., Belt, K. T., Stack, W. P., Kelly, V. R., Band, L. E., & Fisher, G. T. (2005). Increased salinization of fresh water in the Northeastern United States. Proceedings of the National Academy of Sciences, 102(38), 13517–13520. https://doi.org/10.1073/pnas.0506414102

Lockwood, D. (2019). For healthier lakes, rivers, and drinking water, hold the salt. Cen.acs.org. https://cen.acs.org/environment/water/healthier-lakes-rivers-drinking-water/97/i6

Water Science School. (2019). Why is the Ocean Salty? U.S. Geological Survey. https://www.usgs.gov/special-topics/water-science-school/science/why-ocean-salty#:~:text=From%20precipitation%20to%20the%20land%20to%20the%20rivers%20to%20the%20sea&text=The%20rain%20physically%20erodes%20the,and%20then%20to%20the%20ocean