Questions: Salinity and Seawater Composition

Question 1

An oceanographer measures only chloride concentration in a seawater sample and confidently reports total salinity. How is a single ion measurement sufficient?

• A. Chloride dominates seawater composition so completely that other ions contribute negligibly to total salinity
• B. The principle of constant proportions means major ion ratios are fixed ocean-wide, so one ion's concentration determines total salinity
• C. She is using a regional calibration table specific to that ocean basin where the composition is locally homogeneous
• D. Modern instruments measure chloride but automatically calculate total salinity from factory-set universal constants

Answer: (B)

Marcet's principle (constant proportions) holds because the residence times of major ions (millions of years for Na⁺ and Cl⁻) far exceed the ocean's mixing time (~1,000 years), so the ocean equilibrates its composition many times over. The ratio between Cl⁻ and total dissolved solids is therefore nearly constant everywhere, allowing one measurement to stand in for all. Option 0 is a misconception: chloride constitutes about 55% of dissolved material, meaning the other ions are collectively ~45% — not negligible. It is the *constancy of ratios*, not the dominance of chloride, that makes the calculation valid.

Question 2

When seawater in polar regions freezes to form sea ice, what happens to the dissolved ions?

• A. They are incorporated into the ice crystal lattice alongside water molecules, preserving the original salinity
• B. They are excluded from the ice and concentrated in the remaining liquid water, forming cold dense brine
• C. They precipitate as solid mineral crystals and sink to the seafloor during the freezing event
• D. They are released as gas into the atmosphere as the water molecules change phase

Answer: (B)

Ice crystal formation requires a pure water lattice; dissolved ions are geometrically and energetically incompatible with the crystal structure and are expelled during freezing — a process called brine rejection. The remaining liquid water becomes cold and hypersaline (dense brine), which sinks and contributes to deep water mass formation (e.g., Antarctic Bottom Water). Option 0 is the most common misconception and explains why melting old sea ice produces nearly fresh, drinkable water — the salt was rejected long ago.

Question 3

Surface salinity tends to be highest in the subtropical gyres (around 20–30° latitude) and lower both at the equator and at high latitudes.

• T. True
• F. False

Answer: True

Surface salinity is governed by the local freshwater balance: evaporation removes fresh water (concentrating salt) while precipitation and river runoff add fresh water (diluting salt). Subtropical gyres sit beneath persistent high-pressure systems with strong solar heating and little precipitation — evaporation dominates, driving salinity to 36–37 ppt. The equatorial zone receives intense tropical rainfall, freshening the surface layer. High latitudes receive precipitation and input from melting ice. The result is a latitudinal salinity pattern with subtropical maxima, not a uniform distribution.

Question 4

When seawater freezes to form sea ice, the resulting ice has approximately the same salinity as the parent seawater it formed from.

• T. True
• F. False

Answer: False

Sea ice is nearly fresh — far less saline than the parent seawater. Dissolved ions cannot fit into the ice crystal lattice and are expelled (brine rejection), leaving the ice with only minor salt inclusions from trapped brine pockets, which themselves drain over time as the ice ages. This is why polar explorers historically melted old sea ice for drinking water. The expelled salt remains in the liquid water, raising its salinity and density — exactly the mechanism that drives dense brine to the ocean floor and initiates deep circulation.

Question 5

Explain why the principle of constant proportions holds for major seawater ions, and what oceanographic measurements this principle makes possible.

Model answer: The principle holds because the residence times of major ions in the ocean (millions of years for Na⁺ and Cl⁻, thousands for Ca²⁺) are orders of magnitude longer than the ocean's mixing time (~1,000 years). The ocean circulates and homogenizes its composition many times over before ionic ratios can drift, so any local addition (rivers, hydrothermal vents) or removal (burial, evaporite formation) is quickly smoothed out. This constancy enables salinity measurement by proxy: rather than measuring every ion individually, oceanographers measure chlorinity or electrical conductivity — both quick and precise — and calculate total salinity from known fixed ratios. This simplification underlies all seawater density calculations, which in turn drive models of ocean stratification and circulation.

The practical implication is enormous: a single conductivity sensor on a profiling float can generate continuous salinity data at every depth. Without the constancy of proportions, every salinity determination would require separate analysis of multiple ions — impractical for autonomous instruments. The principle also means that historical chlorinity measurements made before conductivity sensors can be converted to modern salinity values, giving oceanographers a century of comparable data.