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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?
Mark Wareing
Seaford, East Sussex, UK
The salt concentration in the world’s oceans (around 3.5 per cent) has declined over millions of years, although it has increased more recently by a negligible 0.02 per cent on average since the 1950s as a result of human global warming. Marine life can adapt to much greater localised global warming effects on salt concentration – increased by evaporation nearer the equator and decreased by melting of ice in glaciers and near the poles.
Most marine life survived the last glacial period when mean ocean salt concentration was increased significantly as huge quantities of pure water were effectively removed from the oceans as ice. Acidification and decreased oxygen levels resulting from humanity’s activities are a much greater threat to marine life.
Tim Coles
Cambridge, UK
Despite the fact that around 4 billion tonnes of salts are delivered into the oceans annually by runoff from the land, the salt concentration of the oceans has been fairly constant over at least the past 500 million years. This is because ocean salinity is in a dynamic equilibrium, the rate of input being balanced by various mechanisms that steadily remove salt from the ocean system.
One notable removal mechanism is the formation of evaporites. These are rocks, mainly sodium chloride but also other salts, that result from the long-term evaporation of water from isolated bodies of water. A good example is the Mediterranean Sea, which dried out about 5 million years ago, leaving salt deposits 3 kilometres thick in places.
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