Short Term Carbon Cycle

Question 1

Photosynthesis and the biological pump

Answer 1

Phytoplankton absorb atmospheric carbon dioxide, sequestering carbon into the ocean. It is then converted into organic matter such as glucose. Carbon passes up the food chain and is also released into the ocean and atmosphere as organisms respire.

The biological pump also transports carbon from surface oceans to deep oceans. As organisms die, their organic matter sinks to the bottom. Shells and exoskeletons containing carbonate ions also sink to the bottom. Microorganisms respire dead matter, releasing carbon dioxide into deep ocean water.

Question 2

Physical pump and thermohaline circulation

Answer 2

Water near the poles is cooler and more saline (as seawater freezes, forming ice). This makes polar water dense, causing it to sink to the bottom. Cold water also absorbs more carbon dioxide. This results in downwelling at the poles, transporting carbon to the deep ocean. At the tropics, warmer, less saline water rises due to lower density. As it can’t store as much carbon dioxide as cold water, upwelling near the tropics not only transports carbon to surface oceans but also release carbon dioxide into the atmosphere.

Question 3

The carbonate pump

Answer 3

Carbon dioxide, which is sequestered by the ocean, dissolves, forming a weak carbonic acid, which dissociates into hydrogen ions and bicarbonate ions. Bicarbonate ions then dissociate, forming hydrogen ions and carbonate ions, which marine organisms such as shellfish and coral use to make their exoskeletons and shells. When these organisms die, carbonate ions sink to the sea bed where they undergo sedimentation and lithification, forming carbonate rocks like lime stone. This locks carbon into the long-term carbon cycle.