Salinity: The Science Behind Seawater's Saltiness
Seawater's **saltiness** is the **total concentration of dissolved salts** — overwhelmingly **sodium chloride (NaCl, ~85%)** but a **complex ionic cocktail** that defines ocean chemistry, physics, and biology.
Composition (Average Seawater, 35 PSU)
| Ion | Concentration (g/kg) | % of Dissolved Salts |
|---|
| Chloride (Cl⁻) | 19.35 | 55.0% |
| Sodium (Na⁺) | 10.76 | 30.6% |
| Sulfate (SO₄²⁻) | 2.71 | 7.7% |
| Magnesium (Mg²⁺) | 1.29 | 3.7% |
| Calcium (Ca²⁺) | 0.41 | 1.2% |
| Potassium (K⁺) | 0.40 | 1.1% |
| Bicarbonate (HCO₃⁻) | 0.15 | 0.4% |
| Bromide (Br⁻) | 0.07 | 0.2% |
| Others (Sr, B, F, Si, etc.) | <0.01 each | Trace |
| Total | 35.0 | 100% |
Key Properties Driven by Salinity
- Density: ρ = 1.025 g/cm³ (at 35 PSU, 4°C) — **drives thermohaline circulation**
- Freezing point: -1.9°C (vs. 0°C fresh) — **sea ice formation rejects salt**
- Sound speed: ~1,530 m/s — **sonar, acoustics depend on S/T/P**
- Optical: Absorption, scattering — **color, visibility, photosynthesis depth**
- Corrosivity: Cl⁻ attacks passive films on metals — **materials engineering challenge**
Sources of Salt
- Continental weathering: Rivers deliver **~4 billion tons/yr** dissolved ions (CO₂ + rain → carbonic acid → rock dissolution)
- Hydrothermal vents: **High-temp water-rock reaction** — adds Mg, K, Li, metals; removes SO₄, Mg (net balance)
- Volcanic degassing: Cl, Br, F, S — atmospheric → rain → ocean
- Seafloor alteration: Basalt-seawater exchange (low-temp)
Salinity Variability
- Open ocean: 34–36 PSU (Atlantic saltier than Pacific)
- High evaporation: Red Sea (41 PSU), Persian Gulf (40+), Mediterranean (38–39)
- High precipitation/runoff: Baltic (7–10), Arctic (30–34), equatorial (34)
- Depth: Generally uniform below thermocline; surface varies
Measurement
- Practical Salinity Scale (PSS-78): **Conductivity ratio** vs. KCl standard — **unitless (PSU)**
- Absolute Salinity (TEOS-10): **g/kg** — mass fraction, thermodynamically consistent
- Chlorinity (historical): Cl⁻ titration × 1.80655
Why It Matters
- Climate: Density-driven circulation (AMOC) — heat transport
- Biology: Osmoregulation defines habitats; **stenohaline vs. euryhaline**
- Carbon cycle: Alkalinity (HCO₃⁻/CO₃²⁻) buffers CO₂
- Desalination: Energy ∝ salinity — **RO: 3–4 kWh/m³ at 35 PSU**
Salinity is **the ocean's fingerprint** — a **conserved tracer** of water masses, a **control knob on physics**, and the **chemical context for all marine life**.
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