A Question with a Deep Answer
The ocean covers over 70% of Earth's surface and contains about 97% of the planet's water. Yet almost none of it is drinkable. The average salinity of seawater is around 3.5% — which means roughly 35 grams of dissolved salts in every kilogram of seawater. But where did all that salt come from, and why doesn't the ocean get fresher over time?
The Two Main Sources of Ocean Salt
1. Rivers and Rainwater Weathering Rock
Rain is slightly acidic because it absorbs carbon dioxide from the atmosphere, forming weak carbonic acid. When this acidic rainwater falls on land, it slowly dissolves minerals — including sodium, chloride, calcium, and magnesium — from rocks and soil.
These dissolved minerals flow into streams and rivers, which carry them downhill into the ocean. While river water doesn't taste salty (the concentrations are too low), it continuously adds dissolved minerals to the sea over millions of years.
Once in the ocean, water evaporates back into the atmosphere — but the dissolved salts don't. They stay behind, gradually accumulating over geological timescales.
2. Hydrothermal Vents on the Ocean Floor
The second major source of ocean salinity is far less obvious: the ocean floor itself. Cold seawater seeps down through cracks in the ocean crust, is superheated by volcanic activity beneath, and is expelled back through hydrothermal vents.
During this process, the water reacts with surrounding rock and picks up a range of dissolved minerals. This hydrothermal activity has been contributing minerals to the ocean for billions of years.
Why Doesn't the Ocean Keep Getting Saltier?
Good question. Minerals are continuously added — so why hasn't the ocean become impossibly salty? The answer lies in a set of natural equilibrium processes:
- Marine organisms extract calcium and other minerals to build shells and skeletons, which eventually sink to the ocean floor and become sediment.
- Chemical reactions between seawater and ocean crust minerals remove certain ions from the water.
- Sea spray and evaporation play a role in cycling some salts back onto land via wind.
These removal processes roughly balance the inputs, keeping ocean salinity relatively stable over long periods — though it wasn't always exactly at today's levels.
Why Sodium Chloride Specifically?
Ocean salt isn't just table salt (sodium chloride), though that's the dominant component. Seawater contains a mix of ions:
| Ion | Approximate % of Sea Salt |
|---|---|
| Chloride (Cl⁻) | ~55% |
| Sodium (Na⁺) | ~31% |
| Sulfate (SO₄²⁻) | ~8% |
| Magnesium (Mg²⁺) | ~4% |
| Others (calcium, potassium, etc.) | ~2% |
Sodium and chloride dominate because they're highly soluble in water and aren't efficiently removed by biological or chemical processes the way calcium is.
Does Salinity Vary Across the Ocean?
Yes — significantly. Areas with high evaporation rates (like the Red Sea and Mediterranean) tend to be saltier, because water evaporates faster than it's replenished by rain. Areas near river mouths, polar ice melt zones, or heavy rainfall regions (like parts of the Pacific near the equator) are less salty.
The Bigger Picture
The saltiness of the ocean is the result of billions of years of geological and chemical processes — a slow, continuous exchange between water, rock, atmosphere, and life. It's a reminder that the world we see today is the product of deep time and interconnected systems far larger than any single human lifetime can observe.