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Is the concentration of salt in the oceans gradually increasing? Will there come a point when plant and animal life in the oceans will no longer be able to tolerate it? (continued)

Alex Thomas
University of Edinburgh, UK

Short answer: no, not by enough to make the whole ocean inhospitable.

The ocean is salty because the elements that make up salt (mostly sodium and chlorine, but also sulphur, magnesium, calcium and a host of other elements) are added to the ocean by the weathering of rocks and condensation of volcanic gases. These elements are added alongside water in rivers and rain. The water is recycled via evaporation and precipitation, but the dissolved salts get left behind in the ocean. This is why the sea is saltier than the rivers that flow into it, containing, on average, 34.7 grams of salt per kilogram of water.

You might think this would mean the ocean is constantly accumulating salts and is therefore getting saltier over time. Salts are, however, removed by a range of processes. Calcium is removed by incorporation into the shells of marine organisms. Magnesium and sodium are removed by chemical reactions with hot volcanic rocks at mid-ocean ridges. Chlorine is removed at the slowest rate, as seawater gets trapped in the spaces between sediment grains of the seafloor, and this gets buried and eventually subducted into the deep Earth.

While uncommon in the modern ocean, there have also been periods in Earth’s past where regions of the ocean became isolated and evaporation could then allow salts such as gypsum and halite to precipitate, removing sodium, calcium and sulphur from the ocean. The important things to note are that these removal processes are slow, because the elements that make up salts are so soluble in water, and removal rates are also proportional to the concentration in the ocean.

So, if the concentration of salts were to go up for any reason, the removal processes would eventually increase, which would lead to the overall content of salts in the ocean staying relatively constant.

Mike Follows
Sutton Coldfield, West Midlands, UK

In a warming world, it is tempting to imagine that the oceans are becoming saltier. While this is broadly true in the tropics, the opposite is happening in polar regions, where melting ice dilutes seawater. Overall, these effects largely balance out, resulting in little net global change in average ocean salinity.

Salt is composed of charged ions. Over geological time, the weathering of rocks has steadily supplied dissolved ions (such as sodium and chloride) to the oceans, alongside inputs from hydrothermal vents. At the same time, other processes remove salt from seawater. One important pathway involves marine organisms that incorporate ions into their shells or skeletons. When they die, their remains sink to the ocean floor and may ultimately be buried at subduction zones. As a result, average ocean salinity has remained close to about 35 g of salt per kg of seawater for hundreds of millions of years.

Life is generally able to adapt to changes in salinity, even relatively rapid ones, and many species already tolerate extreme conditions. For example, brine shrimp (Artemia) can thrive in salt concentrations five to 10 times higher than those of normal seawater.

Even a mass extinction that severely reduced the number of species that incorporate ions into their shells, such as molluscs, corals and coccolithophores, would only slow the long-term removal of ions from seawater. It wouldn’t cause a rapid or dramatic increase in ocean salinity, as the ocean salt budget changes only over very long geological timescales.

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