Seawater Composition, Water Properties, and Oceanic Physical-Chemical Systems

Seawater Composition

96.5% liquid water with dissolved salts (chloride, sodium), organic molecules, gases, and elements.

Unique Properties of Water

Water molecules are dipolar (two H atoms on one side of O atom), allowing hydrogen bonding, high heat capacity, and universal solvent ability.

Atom

The smallest unit of a substance retaining all its properties.

Subatomic Particles

Protons (+), neutrons (neutral) in the nucleus; electrons (-) orbit the nucleus.

Ion

An atom with unequal numbers of protons and electrons.

Cation

Positive ion.

Anion

Negative ion.

Isotopes

Atoms of the same element with different numbers of neutrons, affecting weight but not charge.

Molecule

Two or more atoms chemically bonded together.

Heat

Kinetic energy from vibrations of atoms and molecules.

States of Matter

Solid → liquid (melting); Liquid → gas (evaporation).

Density

Mass per unit volume.

Water Molecule Structure

Two hydrogen atoms covalently bonded to one oxygen atom; dipolar.

Unusual Properties of Water

High boiling/melting points, high heat capacity, universal solvent.

Dissolving Salts

Water attracts cations and anions; this separation is called hydration.

Ice Density

Hydrogen bonding forms hexagonal crystals, spacing H atoms farther apart.

Average Salinity of Seawater

~35 ppt (3.5% salt).

Major Seawater Solutes

Sodium (Na+) and Chloride (Cl-) (~86%). Other major ions: sulfate, magnesium, calcium, potassium, bicarbonate, bromide, boric acid, strontium, fluoride.

Nonconservative Ions

Nutrients (N, P, Si) and gases (O₂, CO₂) whose concentrations vary with time and location.

Ocean Salinity vs Freshwater

Long residence times allow salt accumulation in the ocean; freshwater minerals have short residence times.

Salinity Effects on Water Properties

Freezing point lowers as salinity increases; Density increases with salinity; Vapor pressure decreases with salinity.

Latitude and Temperature

Lower insolation at poles → colder water.

Isotherms

Contour lines connecting points of equal water temperature.

Ocean Currents and Temperature

Western boundary currents → warm water poleward; Eastern boundary currents → cold water equatorward.

Thermocline

Boundary between warm surface water and cold deep water.

Salinity Variation with Latitude

Highest at 20-35° latitude (evaporation > rainfall); Lower at equator and poles (high rainfall, low evaporation).

Halocline

Boundary layer where salinity changes with depth.

Density Factors

Salinity, temperature, and (at great depth) pressure. Density increases with salinity and decreasing temperature.

Water Column Layers

Surface layer: thin, affected by weather and climate, allows photosynthesis; Pycnocline layer: transition zone; combines thermocline/halocline; Deep layer: ~80% of ocean; dense water sinks from high latitudes.

Gas Solubility Factors

Decreased salinity/temperature and increased pressure.

Gas Regulation in Subsurface Waters

Photosynthesis, respiration, decomposition.

Oxygen Distribution

Warm, well-oxygenated surface layer; Oxygen minimum zone at 150-1500 m in pycnocline; Deep water oxygen replenished by sinking cold water (advection).

CO₂ Effect on Seawater

Dissolves → forms bicarbonate → releases H+ → lowers pH (more acidic). Seawater is buffered; pH ~7.5-8.5.

Earth's Water in Oceans

97.25%.

Water Reservoirs

Groundwater and atmosphere exceed rivers in volume.

Hydrologic Cycle

Sun evaporates water → vapor condenses → precipitation → runoff/groundwater → returns to ocean.

Ocean as Biogeochemical System

Inputs of sunlight and river ions drive energy and matter transfer.

Nutrient Recycling

Dead organisms → nutrients → biological processes or sediments → recycled by plate tectonics and weathering.