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Material World
Material · Natural

Seawater

The largest chemical reservoir on the planet — where magnesium, bromine and most of the world's salt come from, and where every soluble element eventually ends up.

Ore

Seawater is about 3.5 per cent dissolved solids, and the remarkable thing about that figure is how constant the *proportions* are. Total concentration varies with evaporation and rainfall, but the ratio of sodium to chloride to magnesium to sulfate is essentially the same in every ocean on Earth — a uniformity that follows from the sea being mixed on a timescale of a thousand years and supplied over hundreds of millions.

Six ions account for almost all of it: chloride, sodium, sulfate, magnesium, calcium and potassium. Everything else in the periodic table is in there too, at concentrations from parts per million down to parts per quadrillion, which is why the sea is both the largest mineral resource on the planet and almost entirely uneconomic to mine.

How it forms

The salt came from rock. Rain is faintly acidic, weathering releases ions, rivers carry them to the sea, and water leaves by evaporation while the dissolved solids do not. That has been running for most of the planet's history.

It is not simply accumulation, though, or the sea would be far saltier than it is. Elements are removed as fast as they arrive: calcium is taken out by organisms building shells, silicon by diatoms, and sodium and chloride are buried in evaporite basins and cycled through hydrothermal circulation at mid-ocean ridges. What looks like a static composition is a balance between supply and removal, and each element's residence time — a hundred million years for sodium, a few hundred for iron — is a measure of how reactive it is.

Economic significance

Three things are extracted from the sea at scale, and they are the ones present in high enough concentration to pay for moving the water: salt, magnesium and bromine. Magnesium in particular is effectively unlimited — the ocean holds enough to supply demand for as long as anyone need consider — which makes it one of the very few metals with no resource constraint at all.

Everything else is a recurring bad idea. Seawater contains roughly four billion tonnes of uranium and about twenty million tonnes of gold, and both are dispersed at a few parts per trillion. Extracting them means processing enormous volumes of water for a trace, and every scheme proposed since Fritz Haber tried to pay German war reparations with seawater gold has failed on that arithmetic.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

contains

  • Chlorine — element · the most abundant dissolved ion by mass, and rather more than half the dissolved solids
  • Sodium — element · with chloride, and the pair make up something like eighty-five per cent of what is dissolved
  • Sulfur — element · as sulfate, the third most abundant ion
  • Magnesium — element · the most abundant metal in the sea, and the one extracted from it directly — the resource is effectively unlimited
  • Calcium — element · constantly removed by organisms building shells, which is why the concentration stays where it is rather than rising
  • Potassium — element · the fourth metal, and too dilute against evaporite potash to be worth recovering
  • Bromine — element · at sixty-five parts per million, which is dilute and still the source of most of the world's bromine
  • Oxygen — element · as water, and dissolved as gas — the dissolved fraction is what everything in the sea breathes
  • Hydrogen — element · as water, which is the other 96.5 per cent of it
  • Carbon — element · as dissolved carbonate and bicarbonate, in a reservoir far larger than the atmosphere's — which is why the ocean takes up so much of what is emitted, and why it is acidifying
  • Uranium — element · at three parts per billion, which is four billion tonnes in total and has defeated every extraction scheme proposed for it
  • Gold — element · at a few parts per trillion. Fritz Haber spent years trying to pay German war reparations with it and found the concentration was a thousand times lower than the figure he had started from
  • Iodine — element · at sixty parts per billion, which is dilute even by the sea's standards — seaweed concentrates it by a factor of thousands, and that is how it was first isolated and how it reached the caliche brines that supply it now

is an input to

  • Brine evaporation — process · the feedstock for solar salt and, in the salars, for the far stronger brines that lithium comes from

is a source for

  • Evaporite — rock · what is left when a body of seawater evaporates faster than it is replenished — the salts come out in order of solubility, which is why the sequence is readable
  • Limestone — rock · precipitated from it, mostly by organisms building shells and skeletons out of dissolved calcium and carbonate
  • Magnesium — element · extracted directly from the sea, precipitated as the hydroxide with lime before reduction — the one metal with no resource constraint worth discussing
  • Bromine — element · displaced from concentrated seawater and from the connate brines pumped out of oil wells, which are richer
  • Nacre — material · built from calcium and carbonate the animal takes out of the water it is sitting in, which is why nacre quality tracks water quality directly
  • Pearl — material · for the saltwater cultures; freshwater pearls are the same process in rivers, and now most pearls by number
  • Precious coral — material · secreted by a colonial animal at 50 to 1,000 metres, in the dark and without symbiotic algae — which is why it is not a reef organism and does not behave like one
  • Water — compound · by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it
  • Salt — compound · by solar evaporation, which needs a dry sunny coast and is the cheapest route there is
  • Chalk — rock · built from dissolved calcium and carbonate by algae in warm shallow Cretaceous seas, and accumulated at a few centimetres a millennium

is composed of

  • Water — compound · about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Questions this page answers

Where it comes from, and what it becomes

Follow Seawater back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water

Downstream — what it becomes

  • Seawater → is an input to (the feedstock for solar salt and, in the salars, for the far stronger brines that lithium comes from) → Brine evaporation → produces (the majority of world supply, concentrated over a year or more in ponds on the Andean salars from a few hundred parts per million to a few per cent) → Lithium → is used in (the ion that shuttles, and the only part of the cell the name mentions) → Lithium-ion cell → is used in (and portable computing came first by two decades) → Electronics manufacture → is associated with (and the supply chain it produced is the deepest and most concentrated in the world economy) → The semiconductor era complete chain
  • Seawater → is a source for (extracted directly from the sea, precipitated as the hydroxide with lime before reduction — the one metal with no resource constraint worth discussing) → Magnesium → is an input to (the reducing agent, recovered afterwards by electrolysing the magnesium chloride by-product) → Kroll process → produces (as sponge, which must then be crushed, melted and cast before it is usable metal) → Titanium → is a component of (the other half of the strengthening phase) → Nickel superalloy → is associated with (the jet engine created the industry, because no existing material survived the turbine inlet) → The wartime materials programmes complete chain
  • Seawater → is a source for (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Water → is a component of (and it is consumed rather than dried off: cement sets by hydration, which is a chemical reaction with this water, and concrete cured under water is stronger than concrete left in the air) → Concrete → is a component of (with steel put where the tension is) → Reinforced concrete → is associated with (after it rather than during — Portland cement is 1824 and reinforcement is the 1860s onward, and the Romans had concrete with no reinforcement at all) → Industrial Revolution complete chain
  • Seawater → is a source for (what is left when a body of seawater evaporates faster than it is replenished — the salts come out in order of solubility, which is why the sequence is readable) → Evaporite → is a source for (mined as rock salt from bedded deposits, or dissolved in place and pumped up as brine — which is what the chemical industry wants anyway) → Salt → is an input to (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Chlor-alkali electrolysis → produces (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Sodium hydroxide → is used as (in the Bayer process and in metal cleaning before coating) → Corrosion protection
  • Seawater → is a source for (precipitated from it, mostly by organisms building shells and skeletons out of dissolved calcium and carbonate) → Limestone → is an input to (heated until the carbonate decomposes to lime and carbon dioxide) → Calcination → produces (the solid residue once carbon dioxide has been driven off) → Quicklime → is a component of (as calcium oxide within the clinker phases, not as free lime) → Portland cement → is used in → Construction
  • Seawater → is a source for (by solar evaporation, which needs a dry sunny coast and is the cheapest route there is) → Salt → is an input to (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Chlor-alkali electrolysis → produces (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Sodium hydroxide → is an input to (hot concentrated caustic soda dissolves the aluminium hydroxides) → Bayer process → produces (crystallised from the liquor and calcined, ready for the smelter) → Aluminium oxide

These are the most distinct paths onward. Seawater ends up in others besides.