Why the Ocean Is Salty: The Chemistry of Seawater

Two ions, two very different journeys

The salt in seawater is overwhelmingly two ions: chloride (Cl⁻) and sodium (Na⁺). They dominate for different reasons, and untangling them is the whole story.

• Chloride is a volcanic gift. Chlorine is not abundant in ordinary crustal rock, so rivers deliver very little of it. Instead, the ocean's Cl⁻ was largely degassed from Earth's interior as hydrogen chloride (HCl) during early volcanism and mantle outgassing, and it is topped up today at mid-ocean ridges. Being highly soluble and chemically inert in seawater, once chloride enters the ocean it essentially has nowhere to go — it accumulates.
• Sodium is a weathering product. Na⁺ is leached from continental rocks — sodium feldspars (albite, NaAlSi₃O₈) and rock salt — by rain and rivers. A representative feldspar-weathering reaction is:
  2 NaAlSi₃O₈ + 2 CO₂ + 11 H₂O → Al₂Si₂O₅(OH)₄ + 2 Na⁺ + 2 HCO₃⁻ + 4 H₄SiO₄.
  The albite is converted to kaolinite clay, releasing dissolved Na⁺ and bicarbonate that rivers carry to the sea.

Because the two ions arrive by unrelated routes, their near-1:1 pairing in the sea is a coincidence of steady state, not a molecule of NaCl marching in intact. Seawater is a solution of independent ions, not a bag of salt crystals.

Rain is a weak acid, and rock is the base it eats

The engine that strips ions off the continents is chemical weathering, driven by carbonic acid. Rainwater dissolves atmospheric CO₂ (≈ 420 ppm today):

• CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (pKₐ₁ ≈ 6.35)

This makes even pristine rain mildly acidic at pH ≈ 5.6. That weak acid attacks silicate and carbonate rock. For limestone the reaction is fast and clean:

• CaCO₃ + CO₂ + H₂O → Ca²⁺ + 2 HCO₃⁻

For silicate minerals it is slower but planet-shaping — the same reaction that also acts as Earth's long-term thermostat. Every reaction releases dissolved cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) and bicarbonate into runoff. So the ions that make the sea salty are literally the dissolved skeleton of the continents, delivered one raindrop at a time. Sulfate arrives partly from the oxidative weathering of pyrite (FeS₂), whose sulfur is oxidized all the way to SO₄²⁻.

So why doesn't the ocean keep getting saltier?

Rivers add roughly 4 billion tonnes of dissolved salt per year, yet ocean salinity has been broadly stable for hundreds of millions of years. The resolution is that the ocean is a steady-state reactor: for every ion added, a matching amount is removed. The relevant number is an ion's residence time, τ = (amount in ocean) ÷ (input or removal rate):

• Chloride: τ ≈ 100 million years — almost nothing removes it, so it piles up and dominates.
• Sodium: τ ≈ 55–75 million years — very unreactive; removed mainly by burial in sediments and sea-spray.
• Calcium: τ ≈ 1 million years — rapidly pulled out by organisms building CaCO₃ shells: Ca²⁺ + 2 HCO₃⁻ → CaCO₃(s) + CO₂ + H₂O.
• Iron/aluminum: τ ≈ 100 years — so reactive they precipitate almost immediately, which is why they are trace, not major, ions.

The rule of thumb: long residence time = abundant in seawater. Na⁺ and Cl⁻ dominate not because Earth is rich in them, but because the ocean has no fast way to spit them back out. Removal happens through carbonate burial, clay formation, evaporite deposits, and — crucially — the seafloor.

The seafloor is a giant ion-exchange column

The most under-appreciated removal valve is hydrothermal circulation. The entire volume of the ocean cycles through hot, cracked basalt at mid-ocean ridges roughly once every 10–20 million years. Seawater sinks into the crust, is heated to 350–400 °C under pressure, and reacts with basalt, chemically transforming as it goes:

• Magnesium is stripped out almost entirely — vent fluids are essentially Mg-free — as it is fixed into clay and hydroxide minerals in the crust. This is the ocean's main Mg²⁺ sink.
• Sulfate is removed, partly reduced to sulfide (SO₄²⁻ + organic/Fe → H₂S/metal sulfides), building the black-smoker mineral chimneys.
• Calcium, iron, manganese and metals are leached out of the rock and injected back into the ocean.

So the ridge system behaves like a titanic ion-exchange and titration column: it takes Mg²⁺ and SO₄²⁻ out and puts Ca²⁺ and metals in. Together with river input on the other side, this two-ended plumbing is what pins the ratios of the major ions and keeps the sea's composition remarkably constant across every ocean — the observation known as Forchhammer's Principle (or the constancy of relative proportions).

Constant proportions and how salinity is measured

A striking fact drives the whole science: while the total salt content varies from place to place (the Baltic near 7‰, the open Atlantic near 35‰, the Red Sea over 40‰), the ratios of the major ions to one another are essentially fixed throughout the open ocean. Cl⁻ is always ≈ 55% of the salt; Na⁺ always ≈ 30.6%. This is because these conservative ions mix far faster (ocean mixing time ≈ 1,000 years) than any process that could add or remove them selectively.

That constancy is what makes salinity measurable at all. Because the composition is fixed, you never have to assay each ion — you measure one proxy and scale. Modern instruments measure electrical conductivity, since more dissolved ions carry more current, and convert it via the Practical Salinity Scale 1978 (PSS-78) to Practical Salinity Units (PSU), referenced against a standard KCl solution. A CTD sensor (Conductivity–Temperature–Depth) reads salinity to about ±0.002 PSU. Older chemistry used the Mohr–Knudsen titration for chlorinity, converted by the classic relation Salinity ≈ 1.80655 × Chlorinity.

Why 35 grams matters: climate, density and life

Salinity is not trivia — it is one of the two knobs (with temperature) that set seawater's density, and density drives the global ocean. Cold, salty water in the North Atlantic sinks because it is dense, launching the thermohaline circulation — the planet-scale conveyor that moves heat from the tropics toward the poles. Melting ice and increased rainfall freshen surface water, lowering density and threatening to slow this circulation, a real climate concern.

• Freezing and boiling: dissolved salt is a colligative solute, depressing seawater's freezing point to about −1.9 °C at 35‰.
• Osmosis and life: the ionic strength (≈ 0.7 mol/kg) governs how marine cells manage water; it is why a freshwater fish dropped into the sea dehydrates through its gills.
• Buffering: the bicarbonate delivered by weathering also makes seawater a carbonate buffer at pH ≈ 8.1, which resists acidification and lets corals and plankton precipitate their shells.

Salinity, in other words, is a single number that ties the weathering of mountains, the plumbing of the seafloor, the circulation of climate, and the biology of every marine organism into one balanced equation.

The six major ions of seawater at salinity 35‰ — nearly all of the dissolved solids, in constant proportion everywhere in the open ocean.

Ion	Concentration (g/kg)	% of salt by mass	Chief source
Chloride, Cl⁻	19.35	55.0%	Mantle degassing / vents
Sodium, Na⁺	10.78	30.6%	Continental weathering
Sulfate, SO₄²⁻	2.71	7.7%	Weathering of pyrite & gypsum
Magnesium, Mg²⁺	1.28	3.7%	River input; removed at vents
Calcium, Ca²⁺	0.41	1.2%	Weathering; removed as CaCO₃
Potassium, K⁺	0.40	1.1%	Weathering of feldspars/micas