What is the thermocline? beneath 200 meters, ocean water cools at a constant rate of 1°C per 100 meters depth
the ocean area near the polar regions where water temperatures cool
where magma is forced up through the ocean floor along mid-ocean ridges
the area off the continental shelf that declines to the ocean floor

Beneath 200 meters, ocean waters cools at a constant rate of 1°C per 100 meters depth.

Explanation:

The ocean water is with the same characteristics throughout all of the oceans. The differences come in differences in temperature, density, salinity, etc. When it comes to temperature, it is what influences the other characteristics, and it is what causes the movement of water in the oceans.

The movement of ocean water occurs because in some parts, the ocean water gets cold, while in others it gets warm. The cold water drops deeper and moves toward the less dense warmer water, and as it gradually warms up itself it rises, pushing the warm water in opposite direction. The water is mixing at the surface, but this is not the case below 200 meters of depth, where water mixing stops. This zone, where the water is not mixing, is known as the thermocline and exists as such throughout all of the oceans.