It is a depositional feature, comprised of sediment that has moved down off the shelf and slope. The rise overlies the transition between the continent and the ocean basin, and it commonly receives material slumped off the steeply inclined slope. The lower drawing shows the depth change at the true scale. Its average thickness is about 100 m. Because it receives sunlight for photosynthesis, it supports the base of the marine food chain - tiny plants called phytoplankton .
Phytoplankton take up carbon dioxide and give off oxygen in the process of photosynthesis.
If they were to suddenly disappear, atmospheric concentration of carbon dioxide would almost triple. Through biological processes, the ocean takes up lots of carbon dioxide - it contains 45 times more of this gas than the atmosphere does. Where the pycnocline is well developed the water column tends to be very stable - it is hard to mix the less dense overlying waters down into the denser layers. It occurs at depths of around 2 km and deeper, except at high latitudes. In these regions, deep water is in contact with the surface.