What's in Seawater? The 6 Major Ions + Trace Elements
Seawater is a remarkably consistent chemical cocktail. Average salinity 35 g/kg (35 ppt). Over 99% of dissolved mass is just six ions:
| Ion | Conc. (g/kg) | mmol/kg | % Total Solids |
|---|
| Chloride (Cl⁻) | 19.35 | 546 | 55.0% |
| Sodium (Na⁺) | 10.76 | 468 | 30.6% |
| Sulfate (SO₄²⁻) | 2.71 | 28 | 7.7% |
| Magnesium (Mg²⁺) | 1.29 | 53 | 3.7% |
| Calcium (Ca²⁺) | 0.41 | 10 | 1.2% |
| Potassium (K⁺) | 0.39 | 10 | 1.1% |
| Sum | 34.91 | | 99.3% |
Ratios constant globally (Forchhammer's Principle / Marcet's Law). Only total salinity varies.
Trace but Critical
- Nutrients: Nitrate, phosphate, silicate — limit primary production
- Carbon system: Bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), CO₂ — pH buffer, climate regulator
- Micronutrients: Fe, Mn, Zn, Cu, Co, Cd — enzyme cofactors (nM–pM)
- Gases: N₂, O₂, Ar, CO₂, CH₄, N₂O — solubility T/S dependent
- Isotopes: δ¹⁸O, δD, ¹⁴C, ³He — water-mass tracers
What Drives Composition
| Process | Effect on Salinity |
|---|
| Evaporation | ↑ Salinity (tropics, marginal seas) |
| Precipitation / Runoff | ↓ Salinity (high latitudes, river mouths) |
| Sea-ice formation | ↑ Salinity (brine rejection) |
| Ice melt | ↓ Salinity (freshwater lens) |
| Hydrothermal vents | Adds Mg, Fe, Mn, H₂S; removes Mg, SO₄ |
Why It Matters
- Marine physiology: osmoregulation tuned to 35 ppt
- Sound speed: c = 1449 + 4.6T – 0.055T² + 1.34(S–35) + 0.016z
- Density-driven circulation: θ-S curves define water masses
- Climate archive: foraminifera Mg/Ca, δ¹⁸O record past T/S
Seawater chemistry is Earth's most stable major solution—and its most consequential.
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