Question

SEP Construct an Explanation Look at Figure 2. In winter, ice forms on the lake surface. There is a layer of highly saline water below the ice. Suggest how this layer has formed and why there are areas where the salty layer is descending toward the bed of the lake in some places.

Answer 1

When freshwater freezes it excludes most of the dissolved salts: the growing ice lattice is nearly pure water, so the salt is left behind in the liquid just under the ice (this is the same “brine rejection” process that happens when sea ice forms). That raises the salinity and therefore the density of the thin water layer immediately below the ice.

If that near-surface water becomes denser than the water below, the stratification is gravitationally unstable and the dense saline water will sink. The sinking does not happen as a uniform sheet but in localized plumes or “fingers” where brine has concentrated (for example where freezing is fastest, where there are cracks or brine channels in the ice, or where small-scale flow concentrates the salty water). Those plumes can penetrate all the way to the bottom in places and spread out along the bed as a dense saline bottom layer.

So: ice formation → salt expelled into the water beneath the ice → formation of a dense saline layer → gravitational instability and convective sinking in localized plumes (guided by local freezing patterns and bathymetry), which explains the areas where the salty layer is observed to descend toward the lake bed.