Why are the Oceans Salty? – One Foot in the Mudd

My son asked me yesterday “why are the oceans salty” and I literally didn’t have an answer as to why they are salty and where their salt comes from- so decided to do some research. This is why the oceans are salty

Ocean salt primarily comes from rocks on land

mussels growing in brine pool — Some areas of the ocean are saltier than others. This image shows methane mussels living at the edge of a underwater brine pool in a cavern at a depth of 650 feet in the Gulf of Mexico. The pool of brine in the foreground is nearly four times as salty as seawater and is so dense that a submarine can float on the pool (in fact, this photo was shot from a submarine).

Salt in the ocean comes from rocks on land.

The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid (which forms from carbon dioxide and water).

As the rain erodes the rock, acids in the rainwater break down the rock. This process creates ions, or electrically charged atomic particles. These ions are carried away in runoff to streams and rivers and, ultimately, to the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not used up and are left for long periods of time where their concentrations increase over time.

Two of the most prevalent ions in seawater are chloride and sodium. Together, they make up over 90 percent of all dissolved ions in the ocean. Sodium and Chloride are ‘salty.’

The concentration of salt in seawater (salinity) is about 35 parts per thousand, on average. Stated in another way, about 3.5 percent of the weight of seawater comes from the dissolved salts.

By some estimates, if the salt in the ocean could be removed and spread evenly over the Earth’s land surface it would form a layer more than 500 feet thick, about the height of a 40-story office building.

In the beginning, the primeval seas were probably only slightly salty. But over time, as rain fell to the Earth and ran over the land, breaking up rocks and transporting their minerals to the ocean, the ocean has become saltier.

Rain replenishes freshwater in rivers and streams, so they don’t taste salty. However, the water in the ocean collects all of the salt and minerals from all of the rivers that flow into it.

It is estimated that the rivers and streams flowing from the United States alone discharge 225 million tons of dissolved solids and 513 million tons of suspended sediment annually to the ocean. Throughout the world, rivers carry an estimated four billion tons of dissolved salts to the ocean annually.

About the same tonnage of salt from ocean water probably is deposited as sediment on the ocean bottom and thus, yearly gains may offset yearly losses. In other words, the ocean today probably has a balanced salt input and output (and so the ocean is no longer getting saltier).

https://oceanservice.noaa.gov/facts/whysalty.html