A-Level Marine Science — Water & Earth Processes

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Cohesion and Adhesion

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Cohesion is the attraction between water molecules (hydrogen bonding), creating high surface tension. Adhesion is the attraction between water molecules and other surfaces. These properties allow water to move against gravity in narrow spaces (capillary action) and enable waves to form.

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Solubility of Gases vs. Temperature

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Unlike most solids, gases become less soluble in water as temperature increases. Higher kinetic energy allows gas molecules to overcome dipole attractions and escape into the atmosphere. This inverse relationship is critical for understanding oxygen levels in warmer tropical waters versus polar regions.

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Salinity (Practical Salinity Scale)

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Salinity on the Practical Salinity Scale (PSS-78) is a dimensionless measure based on the conductivity of seawater, representing the concentration of dissolved salts relative to seawater. Average ocean salinity is about 35 PSU, and higher salinity increases density, affecting buoyancy and ocean circulation.

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Calculating Density

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Density (ρ) is defined as mass per unit volume (ρ = m/v). In marine science, seawater density increases with salinity and pressure but decreases with temperature. This variation drives thermohaline circulation, where cold, dense water sinks and moves along the ocean floor.

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The Pycnocline

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The pycnocline is a cline (layer) where the water density changes rapidly with depth. It encompasses the thermocline (temperature change) and halocline (salinity change). This stable layer acts as a barrier to mixing between surface waters and deep waters, affecting nutrient distribution.