4.2 Physical properties of seawater

Physical Properties of Seawater

Seawater behaves differently from pure water because of the salts dissolved in it. Its physical properties, including density, temperature, salinity, and pressure, control how the ocean circulates, how heat moves around the planet, and where marine organisms can survive. These properties also create the layered structure of the ocean and drive the large-scale currents that regulate Earth's climate.

Density, Salinity, and Temperature of Seawater

These three properties are tightly linked. A change in any one of them affects the others, and together they determine how water masses move through the ocean.

Density is the mass per unit volume of seawater, typically ranging from about 1020 to 1029 kg/m³. It depends on three factors: temperature, salinity, and pressure. Colder water is denser than warmer water, and saltier water is denser than fresher water. When surface water becomes cold and salty enough, it sinks below lighter water. These density differences are what drive thermohaline circulation, the global system of deep ocean currents.

Salinity refers to the total dissolved solids in seawater, averaging about 35 PSU (practical salinity units) globally. The major dissolved ions are:

• Sodium and chloride (together making up about 86% of dissolved salts)
• Magnesium and sulfate

These ions come primarily from rock weathering on land and from hydrothermal vents on the seafloor. Salinity varies from place to place depending on the balance between evaporation (which increases salinity) and freshwater inputs like precipitation and river discharge. For example, the Amazon River plume creates a large area of reduced salinity in the tropical Atlantic.

Temperature at the ocean surface ranges from about -2°C in polar waters to around 30°C in the tropics, varying with latitude, season, and depth. Below the surface, a thermocline forms: a layer where temperature drops rapidly with depth, separating the warm, wind-mixed surface layer from the cold deep ocean. Below the thermocline, temperatures are relatively uniform and cold (1–4°C). Temperature strongly influences marine life distribution and metabolic rates, since most marine organisms are ectotherms whose body temperature matches their surroundings.

Pressure vs. Depth in Oceans

Hydrostatic pressure is the pressure exerted by the weight of water above a given point. It increases approximately linearly with depth, adding roughly 1 atmosphere (atm) for every 10 meters of depth. At the surface you experience 1 atm from the atmosphere alone; at 100 m depth, total pressure is about 11 atm.

The formula for calculating hydrostatic pressure is:

$P = \rho g h$

where $P$ is pressure, $\rho$ is the density of seawater (approximately 1025 kg/m³), $g$ is gravitational acceleration (9.8 m/s²), and $h$ is depth. This calculation is essential for designing submersibles and understanding conditions at different ocean depths.

Effects of pressure on organisms and chemistry:

• Gas-filled spaces compress at depth, which affects buoyancy. Fish with swim bladders must regulate the gas inside them as they change depth.
• Gas solubility increases with pressure, meaning deep water can hold more dissolved gases like $O_2$ and $CO_2$ than surface water at the same temperature.
• Seawater density increases slightly at greater depths due to compression, though this effect is small compared to the influence of temperature and salinity.

Deep-sea organisms like anglerfish and giant squid have evolved bodies without large gas-filled cavities, allowing them to withstand pressures that would crush surface-dwelling species.

Properties of Water in Oceans

Water itself has unusual physical properties, mostly due to hydrogen bonding between its molecules. These properties have enormous consequences for the ocean and climate.

High heat capacity means water requires a large amount of energy to change temperature. This is why oceans warm up and cool down much more slowly than land. The ocean absorbs and stores vast amounts of solar energy, then releases it gradually, moderating coastal and global climates. This thermal inertia is also central to climate phenomena like El Niño, where shifts in ocean heat distribution alter weather patterns worldwide.

Hydrogen bonding is the attraction between the slightly positive hydrogen atoms and slightly negative oxygen atoms of neighboring water molecules. It's responsible for:

• Cohesion (water molecules sticking to each other), which creates high surface tension. This is strong enough to support small organisms at the air-water interface.
• Adhesion (water sticking to other surfaces), which enables capillary action for nutrient transport in marine plants.

The density anomaly of water is one of its most important quirks. Pure water reaches maximum density at about 4°C, not at its freezing point. This means ice is less dense than liquid water and floats. In freshwater lakes, this causes freezing from the top down, insulating the water below and protecting aquatic life in winter. In seawater, the situation is slightly different: dissolved salts lower both the freezing point (to about -1.8°C at 35 PSU) and the temperature of maximum density. Seawater at typical ocean salinity actually gets denser all the way down to its freezing point, which is why very cold, salty polar water sinks and drives deep ocean circulation.

Ocean circulation ties these properties together. The thermohaline circulation (sometimes called the global conveyor belt) is driven by density differences created by variations in temperature and salinity. Cold, salty water sinks in polar regions, flows along the deep ocean floor, and eventually rises back to the surface elsewhere. Upwelling, where deep nutrient-rich water rises to the surface, supports some of the world's most productive fisheries, such as those off the coast of Peru.

Sea Ice Formation and Impacts

Sea ice forms when seawater cools below its freezing point, which is around -1.8°C at average ocean salinity (35 PSU). The freezing point drops slightly as salinity increases.

During freezing, an important process called brine exclusion occurs: as ice crystals form, they reject most of the dissolved salt, leaving behind saltier, denser water. This brine-enriched water sinks, contributing to deep water formation and thermohaline circulation.

Sea ice develops in several structural forms:

• Nilas: thin, elastic sheets of new ice
• Pancake ice: rounded discs that form in rough water
• Pressure ridges: thick piles of ice created when floes collide

Two main categories of sea ice are distinguished by their behavior:

• Pack ice floats freely in polar seas, drifting with winds and currents.
• Fast ice is anchored to the coastline or shallow seafloor. It provides a stable platform used by marine mammals like seals and polar bears.

Properties and impacts of sea ice:

• Sea ice is less dense than liquid seawater, so it floats. This floating layer insulates the ocean surface from the frigid atmosphere above, reducing heat loss from the water.
• Sea ice has a high albedo, meaning it reflects a large fraction of incoming solar radiation back to space. This keeps polar regions cooler. When ice melts, it exposes the darker ocean surface, which absorbs more heat, causing further warming and more melting. This positive feedback loop is called the ice-albedo feedback and is a major factor in Arctic climate change.
• Seasonal ice provides critical habitat. Ice algae grow on the underside of sea ice and are a key food source for krill, which in turn support whales, seals, and seabirds.
• Spring ice melt releases nutrients and freshwater, triggering intense phytoplankton blooms that fuel polar food webs.
• Changes in sea ice extent alter ocean circulation by affecting where and how much cold, dense water forms and sinks.