Which property change defines a halocline in the ocean?

• A. Temperature change
• B. Density change due to pressure
• C. Salinity change with depth
• D. Light attenuation with depth

Answer: (C)

A halocline is a layer in the ocean where salinity changes rapidly with depth. The defining feature is the vertical gradient in salinity—from fresher surface waters to saltier waters below, or vice versa. This salinity boundary often interacts with temperature and density, since saltier water is denser, but the term specifically describes the salinity change, not temperature or light. Thermoclines describe temperature changes with depth, and light attenuation with depth refers to how quickly light fades as you go down, which are separate concepts.