Every mouthful of seawater is a sip of Earth’s entire geological history — about 35 grams of salt per litre, enough that the oceans hold roughly 50 million billion tonnes of it. But why is the ocean salty while rivers and lakes are (mostly) fresh? The answer spans billions of years, from the planet’s fiery birth to the rain falling on Britain today. And the saltiness, remarkably, stays almost exactly constant.
Where the salt comes from: two sources
Source 1: the land (the ongoing delivery). Rainwater is slightly acidic — it absorbs carbon dioxide from the air, forming weak carbonic acid. As this rain falls on rocks, it weathers them chemically, dissolving tiny amounts of minerals including sodium and chloride ions. Rivers carry these dissolved minerals to the sea, delivering around 4 billion tonnes of dissolved salts every year. Every river on Earth is a slow conveyor belt of salt to the ocean.
Source 2: the seafloor (the ancient inheritance). When the oceans first formed billions of years ago, volcanic outgassing and the weathering of the young crust loaded them with salts from the start. Additionally, hydrothermal vents on the seafloor — superheated water circulating through oceanic crust — continuously leach minerals from rocks and inject them into the ocean. Underwater volcanoes contribute their share too.
Rivers are fresh because they are throughput systems — water flows through, carrying salts away. The ocean is the end of the line: water arrives, but the only way out is evaporation, which leaves the salt behind. Imagine a bathtub with the taps running and only evaporation draining it — over billions of years, the salt accumulates.
Why doesn’t it keep getting saltier?
This is the elegant part. The ocean’s salinity has been roughly stable for at least hundreds of millions of years, because removal processes balance the inputs. Salt leaves the ocean through: sea spray depositing salt on land; minerals precipitating onto the seafloor; ions being absorbed into seafloor rocks and sediments; and subduction carrying salty crust back into the mantle at plate boundaries. Input ≈ output, and the great salty equilibrium holds — one of Earth’s quiet regulatory miracles.
Not all seawater is equal
Average ocean salinity is about 35 parts per thousand, but it varies. The Red Sea and Mediterranean are saltier (high evaporation, little river input); the Baltic Sea is nearly fresh in places (huge river input, narrow connection to the ocean); polar regions freshen when ice melts and saltify when ice forms (freezing squeezes salt out of the ice). Around Britain, the Atlantic water is close to average, while the North Sea is slightly less salty due to river inflow.
Salinity drives the planet’s climate engine: salty water is denser and sinks, powering the great ocean conveyor — the thermohaline circulation the Met Office tracks in its climate monitoring that carries warm water to Britain (which is why the UK is far warmer than Labrador at the same latitude) and cold water back south. Changes in salinity from melting ice are one of climate science’s serious concerns — the National Oceanography Centre monitors the Atlantic circulation directly., because slowing that circulation would reshape European weather.
Could we drink it? (Desalination)
Removing the salt is possible — reverse osmosis pushes seawater through membranes fine enough to block salt ions — and countries like Saudi Arabia and Israel rely on it. But it is energy-intensive (roughly 3–4 kWh per cubic metre) and produces hypersaline brine that harms marine life if dumped carelessly. For Britain, with abundant rainfall, desalination remains a niche backup — the Thames Gateway plant exists for drought emergencies. The sea’s salt is staying put.
Taste the history
Next time you are at the seaside — and Britain has no shortage of coastline to choose from, whether the best coastal walks in Wales or the best beaches in Cornwall for families — consider what you are tasting: sodium from ancient rocks, chloride from primeval volcanoes, delivered by every river on Earth across four billion years, held in perfect balance by the planet’s own chemistry. The ocean is salty because the Earth is a closed system with an open tap — and the salt, having nowhere else to go, became the sea itself.
The Dead Sea: salt taken to extremes
If the ocean is salty, the Dead Sea is salinity unleashed — around 34% salt, nearly ten times the ocean, so dense that swimmers float effortlessly. It is a terminal lake: the River Jordan flows in, water leaves only by evaporation, and 3 million years of accumulation have created water so mineral-rich it stings cuts and coats skin in salt crystals. Nothing macroscopic lives in it (the name is literal), though microbes and algae survive. The Dead Sea is the ocean’s logic taken to its endpoint — a preview of what any enclosed sea becomes given time and evaporation. It is also shrinking: mineral extraction and diverted inflows have dropped its level over a metre a year in places, a slow-motion environmental warning. The ocean avoids this fate only through its removal processes — the great equilibrium that keeps the seas at 35 parts per thousand instead of 340.
Salt from the sea: how sea salt is made
That equilibrium is also an industry: evaporating seawater in shallow ponds leaves sea salt — practised since antiquity and still done at places like the Anglesey Sea Salt Company in Wales. Different evaporation rates and mineral mixes create the texture differences between flaky sea salt, coarse crystals and table salt. Every pinch is the ocean’s 4-billion-year accumulation, harvested in a summer.
Why lakes differ: from freshwater to salt lakes
Lakes are the perfect illustration of the ocean’s logic. Through-flow lakes (like the Lake District’s Windermere, fed and drained by rivers) stay fresh — salts pass through. Terminal lakes with no outflow (like the Dead Sea, or Utah’s Great Salt Lake) accumulate salt exactly like mini-oceans, some reaching ocean-beating salinity. This is why neighbouring lakes can differ wildly: it is all about whether water — and therefore salt — can leave. The Caspian Sea, Earth’s largest lake, is brackish for precisely this reason: part through-flow, part terminal, caught between the two fates.
Frequently Asked Questions
Why is the ocean salty but rivers are not?
Rivers constantly carry dissolved minerals from weathered rocks to the sea, but water only leaves the ocean by evaporation, which leaves salt behind. Rivers are throughput systems; the ocean is the end of the line where salt accumulates.
Where does ocean salt originally come from?
Two sources: chemical weathering of rocks on land (rain dissolves minerals that rivers carry to sea) and the seafloor itself (ancient volcanic outgassing plus minerals leached by hydrothermal vents).
Why doesn’t the ocean keep getting saltier?
Removal processes balance the inputs: sea spray deposits salt on land, minerals precipitate to the seafloor, ions are absorbed into rocks, and subduction recycles salty crust into the mantle. The system is in long-term equilibrium.
How salty is seawater exactly?
About 35 grams of dissolved salts per litre on average (35 parts per thousand). It varies: saltier in the Red Sea and Mediterranean, fresher in the Baltic and near melting ice.
Does ocean salinity affect climate?
Profoundly. Salty water is denser and sinks, driving the thermohaline circulation that warms Britain and regulates global climate. Freshening from melting ice could slow this circulation — a major climate concern.
