Sedimentary Cycle
Certain essential elements, such as phosphorus, calcium, and magnesium, circulate in nature through the sedimentary cycle.
Unlike gaseous cycles (e.g., carbon and nitrogen), the sedimentary cycle does not pass through the atmosphere. Instead, these elements follow a pattern of movement involving processes like erosion, sedimentation, mountain building, volcanic activity, and biological transport — such as the excreta of marine birds.
1. Phosphorus Cycle
Phosphorus plays a crucial role in aquatic ecosystems and water quality. Unlike carbon and nitrogen, which are largely atmospheric in origin, phosphorus is found in large quantities as minerals in phosphate rocks. It enters the ecosystem primarily through erosion and mining activities.
- On land, phosphorus is mostly present as phosphates in the earth’s crust.
- Through weathering and erosion, phosphates dissolve and are carried by rivers and streams into oceans.
- In oceans, phosphorus accumulates on continental shelves as insoluble deposits.
- Over geological time, crustal movements and uplift expose these deposits on land, restarting the cycle.
- Phosphorus is the key nutrient responsible for excessive growth of microscopic and rooted plants in lakes (a process called eutrophication).
Thus, the phosphorus cycle is primarily geological and much slower than gaseous cycles, yet vital for biological productivity.
2. Sulphur Cycle
The sulphur cycle describes the movement of sulphur through the environment and living organisms. The main reservoir of sulphur is present in the soil and sediments, where it is stored in both organic forms (such as coal, oil, and peat) and inorganic forms (such as pyrite and sulphur rocks) in the form of sulphates, sulphides, and elemental sulphur.
- Sulphur is released from these reservoirs through weathering of rocks, erosional runoff, and decomposition of organic matter, and is carried into terrestrial and aquatic ecosystems dissolved in water.
- The sulphur cycle is primarily sedimentary, but it also has a gaseous component contributed by two compounds — hydrogen sulphide (H₂S) and sulphur dioxide (SO₂). Sulphur enters the atmosphere from various natural and human-made sources such as volcanic eruptions, combustion of fossil fuels, ocean surface emissions, and gases from decomposition. Once in the atmosphere, hydrogen sulphide is oxidized to sulphur dioxide, which then dissolves in rainwater to form weak sulphuric acid and is deposited back to the soil.
- Plants absorb sulphur from the soil in the form of sulphates, which are incorporated into sulphur-containing amino acids and proteins in their tissues. Sulphur then passes through the grazing food chain as herbivores and carnivores consume plants and each other. Finally, sulphur returns to the soil and sediments through excretion and decomposition of dead organisms, completing the cycle.