Warm surface waters get depleted of nutrients and carbon dioxide, but they enrich again as they travel through waters in the conveyor belt. The base of the world's food chain depends on the cool, nutrient-rich waters that support algae and seaweed.

saline

deep

freshwater

shallow

Option B. "Deep"

Explanation:

Thermohaline circulation helps to drive the global conveyor belt, which is a global-scale system of currents. Due to the movement of ocean currents through different polar regions, it allows depletion of nutrients and carbon dioxide at surface waters and get enrich again as move through deep waters in the conveyor belt.

Winds in the ocean helps to  drive ocean currents in the upper ocean’s surface and these ocean currents moves below the ocean surface as well. the process that controls the movement of ocean current deep in the ocean is called Thermohaline circulation that  is based on differences in the water’s density,  and controlled by temperature (thermo) and salinity (haline).

Thermohaline circulation forms in the polar regions of Earth. Ocean water in the polar region gets colds that allows the formation of sea ice, which in turn make the surrounding seawater saltier and increases the ice density and sinks. This allows the deep-ocean currents to drive the Global conveyer belt.

The conveyor belt starts from the surface of North Atlantic pole, where seawater become saltier and ice starts sinking.  This deep water heads south along the western Atlantic basin. The conveyor belt get recharged as it travels along the coast of Antarctica and carries more cold, salty, dense water. The current get divided into two different sections, one traveling to the Indian Ocean and the other moves into the Pacific Ocean. The two sections of the current get warm and rises, as they travel northward, then turn back around  westward and southward.

The warmed surface waters continue to circulate around the globe and follows depletion of nutrients and carbon dioxide. But they get enrich again when they travel through deep waters all around the the  Global conveyer belt.

Hence, the correct answer is option is B "Deep".