What Is the Sea Made Of? Water, Salts, Gases, Life, and Particles

The sea is a complex, dynamic solution — not just "salt water." Its composition reflects geological, biological, atmospheric, and human inputs over billions of years.

1. Water — The Solvent (96.5%)

H₂O with unique properties: high heat capacity, surface tension, density anomaly (max at 4°C), universal solvent. Isotopic variants (HDO, D₂O) trace water cycles.

2. Dissolved Salts — The Defining Feature (3.5% avg)

Major ions (conservative, constant proportions):

Minor/trace: B, Sr, F, Li, Rb, U, Mo, V, etc. Nutrients (non-conservative): N, P, Si — cycled by biology.

3. Dissolved Gases — Life Support

• O₂: 200–300 µmol/kg (surface); Oxygen Minimum Zones (OMZs) at depth
• CO₂: ~2.3 mmol/kg — ocean absorbs 25% anthropogenic CO₂ → acidification
• N₂, Ar: Inert, near saturation
• CH₄, N₂O, H₂S: Trace, microbial origin — potent greenhouse gases

4. Suspended Particles — The "Turbidity" Fraction

• Lithogenic: Clay, silt, dust (aeolian), river sediment
• Biogenic: Phytoplankton, detritus, fecal pellets, calcium carbonate, silica
• Authigenic: Precipitated minerals (Mn nodules, Fe-Mn crusts)
• Anthropogenic: Microplastics, soot, pollutants

5. Marine Life — The Active Component

From viruses (10³⁰ in ocean) to blue whales. Biomass: ~3 Gt C (mostly microbes). Life transforms chemistry — photosynthesis, respiration, calcification, nitrogen fixation, anaerobic metabolisms.

Spatial/Temporal Variability

• Salinity: 32–37 (open ocean); <5 (Baltic); >40 (Red Sea, Persian Gulf)
• Temperature: -2°C (polar) to 30°C (tropical surface)
• pH: 8.1 (preindustrial) → 8.05 (now); 7.8 projected 2100

The sea is Earth's largest chemical reactor — its composition records planetary history and determines planetary future.