Salinity of Seawater and Its Constituents

Salinity of Seawater

• Average salinity is 3.5% (96.5% water)
• Salinity varies between 30-38 psu (Practical salinity units)
• Units previously determined by evaporation, now grams per kilogram or parts per thousand

Dissolved Substances in Seawater

• Major Constituents: Primarily salt (NaCl)

• Minor Constituents: Lithium, iodine, and other trace elements

• Trace Components: Concentrations less than 1 part per billion (e.g., copper, radionuclides)

• Nutrients: Nitrogen, phosphorus, silicon compounds

• Gases: Nitrogen, oxygen, carbon dioxide, inert gases

• Organics: Dissolved and particulate material, biological origin

• Other elements: Found in trace amounts (e.g., Alkali metals like sodium, magnesium, calcium, potassium, strontium; halogens like chloride, bromide, fluoride)

• Sodium concentration is 10.76

• Potassium concentration is 0.39

• Strontium concentration is 0.008

• Borate is removed and replaced by boron concentration 0.004

• Sodium chloride (NaCl) accounts for 85% of dissolved salts.

• Salts dissociate into cations and anions.

Minor Constituents and Units

• Lithium concentration: 0.18-24 ug/L

Trace Constituents

• Heavy metals (zinc, chromium, copper)
• Concentrations in the nanomolar or picomolar range
• Elements typically found in rows 5, 6, and 7 of the periodic table
• Nutrients (various forms, examples given include nitrate, nitrite, ammonia, and urea)
• Organic compounds (dead and living matter): Contains elements C, H, O, N, P, S and other nonmetals.
• Inorganic particles
• Pollutants : xenobiotic and natural compounds.

Other Elements

• All elements are present in seawater, some at very low concentrations.
• These elements may be dissolved or in particles
• Concentrations in seawater can fluctuate globally, depending on inputs and outputs of water systems, but major salts' concentrations remain constant.

How Salts Get into the Sea

• Rivers (weathering of rock, carrying dissolved cations)
• Atmosphere (volcanic emissions, biogenic materials, windblown dust, rainfall, dry depositions)
• Hydrothermal vents.

Composition of Earth's Crust

• Key elements in order of abundance in the crust include Silicon, Aluminum and Iron.

What Happens to River Salts in Seawater

• Some salts (conservative like sodium and magnesium) are unchanged by biological/chemical processes
• Other salts (non-conservative like nitrate, sulphate) undergo chemical and biological reactions (e.g., precipitation, oxidation-reduction)

Residence Time

• Assumes oceans are in a steady state.
• Calculated by dividing the total amount of substance in the reservoir by the sum of inflow and outflow fluxes.