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Difficulty: HardWater Cycle and Oceanography

During the formation of polar sea ice, freshwater freezes while excluding dissolved salts in a process known as brine rejection. Which of the following best explains how this process impacts deep-water ocean circulation?

  1. A
    It accelerates atmospheric moisture return by releasing latent heat that triggers rapid cloud condensation above the ice pack.
  2. It increases the salinity and density of the surrounding cold surface water, causing it to sink and drive global thermohaline circulation.Answer
  3. C
    It initiates vertical mixing primarily through radiative heat transfer rather than mass density variations.
  4. D
    It converts dissolved salts into new insoluble mineral compounds through a chemical change, forming seafloor sediment layers.

Answer

It increases the salinity and density of the surrounding cold surface water, causing it to sink and drive global thermohaline circulation.
When surface seawater freezes into sea ice, the salt ions cannot incorporate into the ice crystal structure and are expelled into the surrounding liquid water. This remaining water becomes extremely cold and highly saline, giving it exceptionally high density. Because denser water sinks beneath less dense water, this heavy brine plunges toward the ocean floor, creating bottom water masses that power the global thermohaline circulation system.

Step-by-Step Solution

1
Analyze the physical mechanism of brine rejection during sea ice formation.
When seawater freezes, pure water forms ice crystals, leaving concentrated dissolved salts behind in the unfrozen liquid.
Salt does not fit easily into the crystal lattice of ice, producing cold, hyper-saline brine beneath the ice sheet.
2
Determine the effect of increased salinity and low temperature on seawater density.
Both extremely low temperatures and elevated salinity significantly increase the density of seawater.
Density of ocean water is directly proportional to salinity and inversely proportional to temperature.
3
Connect the high-density surface water to large-scale oceanographic movement.
The dense, cold, saline water sinks down the continental slope to the deep ocean floor, forming bottom water masses (such as Antarctic Bottom Water).
This downwelling acts as the primary gravitational engine for global thermohaline circulation (the global ocean conveyor belt).

Key Concept

Density-driven thermohaline circulation initiated by polar brine rejection
Estimated Time:1m 30s
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Topics

Water Cycle and Oceanography

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