Chemical Properties of Ocean Water

I. Composition of Seawater

• Seawater is a solution of various salts dissolved in water.
• Average salinity of seawater: 35‰ (parts per thousand).
• Major dissolved salts:
  • Sodium chloride (NaCl) – ~77% of total salts.
  • Magnesium chloride (MgCl₂)
  • Magnesium sulfate (MgSO₄)
  • Calcium sulfate (CaSO₄)
  • Potassium chloride (KCl)
• These six salts account for 99% of the total dissolved salts.
• Chloride and sodium ions are the most dominant.
• Other minor elements: iodine, fluorine, bromine, boron, etc.
• Sources: weathering of continental rocks, volcanic activity, and hydrothermal vents.

II. Dissolved Gases in Ocean Water

Oceans contain several gases in dissolved form, primarily derived from the atmosphere and biological activities.

a. Oxygen and Carbon Dioxide Balance

• Oxygen (O₂):
  • Essential for marine organisms.
  • Enters water through diffusion from atmosphere and photosynthesis by phytoplankton.
  • Higher concentrations near the surface; decreases with depth.
• Carbon Dioxide (CO₂):
  • Oceans act as a major sink for atmospheric CO₂.
  • CO₂ dissolves in water and forms carbonic acid (H₂CO₃).
  • Higher solubility than oxygen, especially in cold water.
  • Marine organisms use dissolved CO₂ for photosynthesis and formation of calcium carbonate (CaCO₃) shells.
  • Deeper ocean layers store large quantities of CO₂.

III. Role of Oceans in the Carbon Cycle

• Oceans absorb about 25–30% of human CO₂ emissions annually.
• Carbon exchange processes:
  • Air-sea gas exchange.
  • Biological pump: marine organisms fix CO₂ in organic matter and calcium carbonate.
  • Solubility pump: cold, dense water at high latitudes sinks, taking dissolved CO₂ with it (deep ocean storage).
• Oceans help regulate global temperature and buffer climate change.

IV. pH Levels and Ocean Acidification

• Normal pH of seawater: ~8.1 (slightly alkaline).
• Ocean acidification: decrease in ocean pH due to increased absorption of atmospheric CO₂.
• Reaction:
  • CO₂ + H₂O → H₂CO₃ (carbonic acid)
  • H₂CO₃ → H⁺ + HCO₃⁻ → causes pH to drop
• Impacts:
  • Hinders formation of calcium carbonate shells in corals, mollusks, and some plankton.
  • Affects marine biodiversity and food webs.
  • Threatens fisheries and ocean-based economies.