Pycnocline definition.

• A. Depth with rapid change in density with depth
• B. Layer with constant temperature
• C. Layer with no light
• D. Boundary between ocean and atmosphere

Answer: (A)

The concept tested is how density in the ocean changes with depth. A pycnocline is the layer where density increases rapidly with depth, creating a distinct boundary between lighter near-surface water and denser deep water. This rapid density change is due to cooling and/or higher salinity with depth, and it helps limit vertical mixing, contributing to ocean stratification. It can coincide with a thermocline, but it’s defined by density, not temperature alone. The other ideas describe a layer with constant temperature, a no-light zone, or the air–sea boundary, none of which define a rapid density change with depth.