Question

SEP Construct an Explanation In Figure 2, a “brine pool” of denser salt water forms on the lakebed after ice forms on the surface. Suggest why the water in the brine pool does not mix with the water above.

Answer 1

When the lake freezes, most of the salt is rejected from the ice and remains in the remaining liquid. That salty water is much denser than the fresher water, so it sinks to the bottom and establishes a strong density increase with depth (a halocline or pycnocline). That density stratification is gravitationally stable: heavy water stays below light water, so there is no buoyancy-driven overturning to mix the layers.

Two factors make the stratification long‑lived:
- The ice cover acts like a lid, preventing wind and waves from stirring the water column and supplying the energy needed to overcome the buoyancy barrier.
- Molecular diffusion of salt is very slow, so vertical mixing across the sharp salinity/density gradient happens only on long timescales.

In short, the dense brine sits stably at the bottom and cannot mix upward unless substantial mechanical energy or very long diffusive processes act to break down the density gradient.