SEDIMENTARY CYCLES: CHARACTERISTICS, STAGES AND EXAMPLES - GEOGRAPHY - 2025

The sedimentary cycles refer to the set of stages that pass certain mineral elements present in the earth 's crust. These phases involve a sequence of transformations forming a circular time series that is repeated over long periods.

These are biogeochemical cycles in which the storage of the element occurs mainly in the earth's crust. Among the mineral elements that are subject to sedimentary cycles are sulfur, calcium, potassium, phosphorus and heavy metals.

Lithological cycle. 1 = magma; 2 = crystallization (cooling of the rock); 3 = igneous rock; 4 = erosion; 5 = sedimentation; 6 = sediments and sedimentary rocks; 7 = tectonics and metamorphism; 8 = metamorphic rock; 9 = fusion. Source: Woudloper / Woodwalker

The cycle begins with the exposure of rocks containing these elements from deep within the crust to or near the surface. These rocks are then subjected to weathering and undergo erosion processes due to the action of atmospheric, hydrological and biological factors.

The eroded material is transported by water, gravity or wind to later sedimentation or deposition of the mineral material on the substrate. These layers of sediment accumulate over millions of years and undergo compaction and cementing processes.

In this way the lithification of the sediments occurs, that is, their transformation back into solid rock at great depths. In addition, in the intermediate phases of sedimentary cycles, a biological phase also occurs that consists of solubilization and absorption by living organisms.

Depending on the mineral and the circumstances, they can be absorbed by plants, bacteria or animals, passing to the trophic networks. Then the minerals will be excreted or released by the death of the organism.

characteristics

Sedimentary cycles constitute one of the three types of biogeochemical cycles and are characterized because the main storage matrix is ​​the lithosphere. These cycles have their own discipline of study, called sedimentology.

Cycle time

Sedimentary cycles are characterized because the time it takes to complete the different stages is very long, even measured in millions of years. This is because these minerals remain embedded in rocks for long periods at great depths in the earth's crust.

Stages of sedimentary cycles

It is important not to lose sight that it is not a cycle whose stages follow a strict sequence. Some phases can be interchanged or presented multiple times throughout the process.

- Exposition

Rocks formed at certain depths in the earth's crust are subjected to different diastrophic processes (fractures, folds and elevations) that end up taking them to or near the surface. In this way, they are exposed to the action of environmental factors, whether edaphic, atmospheric, hydrological or biological.

Diastrophism is the product of convection movements of the Earth's mantle. These movements also generate volcanic phenomena that expose rocks in a more dramatic way.

- Weathering

Once the rock is exposed, it undergoes weathering (decomposition of the rock into smaller fragments) with or without changes in chemical or mineralogical composition. Weathering is a key factor in soil formation and can be physical, chemical, or biological.

Physical

In this case, the factors that cause the rock to break do not alter its chemical composition, only physical variables such as volume, density and size. This is caused by different physical agents such as pressure and temperature. In the first case, both the release of pressure and its exercise are causes of rock ruptures.

Weathering. Source: Prince Roy, Taipei

For example, as rocks emerge from deep within the crust, they release pressure, expand, and crack. For its part, the salts accumulated in the cracks also exert pressure when recrystallizing, deepening the fractures.

In addition, daily or seasonal temperature variations cause cycles of expansion and contraction that end up breaking the rocks.

Chemistry

This alters the chemical composition of the rocks in the disintegration process because chemical agents act. Among these chemical agents involved are oxygen, water vapor and carbon dioxide.

They cause various chemical reactions that affect the cohesion of the rock and transform it, including oxidation, hydration, carbonation and dissolution.

Biological

Biological agents act by a combination of physical and chemical factors, including pressure, friction and others among the former. While as chemical agents are the secretions of acids, alkalis and other substances.

For example, plants are very effective weathering agents, breaking up rocks with their roots. This is thanks to both the physical action of radical growth and the secretions they emit.

- Erosion

Erosion acts both directly on the rock and on the products of weathering, including the formed soil. On the other hand, it involves the transport of the eroded material, the same eroding agent being the means of transport and can be both wind and water.

Erosion. Source: Carl Wycoff

Gravitational erosion is also noted, when material displacement and wear occurs on steep slopes. In the erosive process the material is fragmented into even smaller mineral particles, susceptible to transport over long distances.

Wind

The erosive action of the wind is exerted both by drag and wear, which in turn exerts the entrained particles on other surfaces.

Water

Water erosion acts both by the physical action of the impact of rainwater or surface currents, as well as by chemical action. An extreme example of the erosive effect of precipitation is acid rain, especially on calcareous rocks.

- Transportation

Mineral particles are transported by agents such as water, wind or gravity over long distances. It is important to take into account that each means of transport has a defined load capacity, in terms of size and quantity of particles.

By gravity, even large, even slightly weathered rocks can move, while the wind carries very small particles. In addition, the environment determines the distance, since gravity transports large rocks over short distances, while the wind displaces small particles over enormous distances.

Water, for its part, can transport a wide range of particle sizes, including large rocks. This agent can carry the particles short or extremely long distances, depending on the flow rate.

- Sedimentation and accumulation

It consists of the deposition of the transported material, due to a decrease in the speed of the means of transport and to gravity. In this sense, fluvial, tidal or seismic sedimentation can occur.

Sedimentation. Source: Calogerogalati

As the Earth's relief consists of a gradient that goes from maximum altitudes to the seabed, this is where the greatest sedimentation occurs. As time passes, layers of sediment build up one on top of the other.

- Solubilization, absorption and biological release

Once the weathering of the rocky material has occurred, the dissolution of the released minerals and their absorption by living beings is feasible. This absorption can be carried out by plants, bacteria or even directly by animals.

Plants are consumed by herbivores and these by carnivores, and all by decomposers, the minerals becoming part of trophic networks. Likewise, there are bacteria and fungi that directly absorb minerals and even animals, such as macaws that consume clay.

- Lithification

The cycle is completed with the lithification phase, that is, with the formation of new rock. This happens when minerals settle forming successive layers that accumulate exerting enormous pressure.

The strata deeper in the crust are compacted and cemented to form solid rock and these layers will again be subjected to diastrophic processes.

Compaction

Product of the pressure exerted by the sediment layers that are piling up in the successive sedimentation phases, the lower layers are compacted. This implies that the pores or spaces that exist between the sediment particles are reduced or disappear.

Cementation

This process consists of the deposit of cementitious substances between the particles. These substances, such as calcite, oxides, silica and others, crystallize and cement the material into solid rock.

Examples of sedimentary cycles

- Sedimentary sulfur cycle

Sulfur is an essential component of certain amino acids such as cystine and methionine, as well as vitamins such as thiamine and biotin. Its sedimentary cycle includes a gas phase.

This mineral enters the cycle due to the weathering of rocks (slate and other sedimentary rocks), decomposition of organic matter, volcanic activity and industrial contributions. Also mining, oil extraction and the burning of fossil fuels are sources of sulfur in the cycle.

The forms of sulfur in these cases are sulfates (SO4) and hydrogen sulfide (H2S); sulfates are both in soil and dissolved in water. Sulfates are absorbed and assimilated by plants through their roots and pass to the trophic networks.

When organisms die, bacteria, fungi and other decomposers act, releasing sulfur in the form of hydrogen sulfide gas that passes into the atmosphere. Hydrogen sulfide is quickly oxidized by mixing with oxygen, forming sulfates that precipitate to the ground.

Sulfur bacteria

Anaerobic bacteria act in swamp sludge and in the decomposition of organic matter in general. These process SO4 generating gaseous H2S that is released into the atmosphere.

Acid rain

It is formed due to precursors such as H2S, emitted into the atmosphere by industry, sulfur bacteria and volcanic eruptions. These precursors react with water vapor and form SO4 which then precipitates.

- Sedimentary calcium cycle

Calcium is found in sedimentary rocks formed on the seabed and lake beds thanks to the contributions of organisms provided with calcareous shells. Similarly, there is free ionized calcium in water, as in the oceans at depths greater than 4,500 m where calcium carbonate is dissolved.

Calcium-rich rocks such as limestone, dolomite and fluorite among others, are weathered and release calcium. Rainwater dissolves atmospheric CO2, resulting in carbonic acid that facilitates the dissolution of limestone rock, releasing HCO 3– and Ca 2+.

Calcium in these chemical forms is carried by rainwater into rivers, lakes, and oceans. This is the most abundant cation in the soil from which plants absorb it while animals take it from plants or directly dissolved in water.

Calcium is an essential part of shells, exoskeletons, bones and teeth, so when it dies it is reintegrated into the environment. In the case of oceans and lakes, it sediments at the bottom and the lithification processes form new calcareous rocks.

- Sedimentary potassium cycle

Potassium is a fundamental element in cell metabolism, because it plays a relevant role in osmotic regulation and photosynthesis. Potassium is part of the minerals in the soil and rocks, being clay soils rich in this mineral.

Weathering processes release water-soluble potassium ions that can be absorbed by plant roots. Humans also add potassium to the soil as part of crop fertilization practices.

Through vegetables, potassium is distributed in trophic networks, and then with the action of decomposers it returns to the soil.

- Sedimentary phosphorus cycle

The main phosphorus reserves are in marine sediment, soils, phosphate rocks and guano (seabird excrement). Its sedimentary cycle begins with phosphate rocks that, when they weather and erode, release phosphates.

Likewise, humans incorporate additional amounts of phosphorus into the soil by applying fertilizers or fertilizers. The phosphorous compounds are carried along with the rest of the sediments by the rain towards the water currents and from there to the ocean.

These compounds partly sediment and another part is incorporated into marine food webs. One of the loops of the cycle occurs when phosphorus dissolved in seawater is consumed by phytoplankton, this in turn by fish.

The fish are then consumed by seabirds, whose excreta contain large amounts of phosphorus (guano). Guano is used by humans as organic fertilizer to provide phosphorus to crops.

The phosphorus that remains in the marine sediment undergoes lithification processes, forming new phosphate rocks.

- Sedimentary cycle of heavy metals

Heavy metals include some that perform essential functions for life, such as iron, and others that can become toxic, such as mercury. Among the heavy metals there are more than 50 elements such as arsenic, molybdenum, nickel, zinc, copper and chromium.

Some, like iron, are abundant, but most of these elements are found in relatively small amounts. On the other hand, in the biological phase of their sedimentary cycle they can accumulate in living tissues (bioaccumulation).

In this case, as they are not easy to dispose of, their accumulation increases along the food chains causing serious health problems.

Sources

Heavy metals come from natural sources, due to rock weathering and soil erosion. There are also important anthropic contributions through industrial emissions, burning of fossil fuels and electronic waste.

General sedimentary cycle

In general terms, heavy metals follow a sedimentary cycle that starts from their main source, which is the lithosphere, and they pass through the atmosphere, the hydrosphere and the biosphere. Weathering processes release heavy metals to the ground and from there they can pollute the water or invade the atmosphere through wind-blown dust.

Volcanic activity also contributes to the emission of heavy metals into the atmosphere and rain carries them from the air to the ground and from this to bodies of water. Intermediate sources form loops in the cycle due to the aforementioned human activities and the entry of heavy metals into food webs.

References

1. Calow, P. (Ed.) (1998). The encyclopedia of ecology and environmental management.
2. Christopher R. and Fielding, CR (1993). A review of recent research in fluvial sedimentology. Sedimentary Geology.
3. Margalef, R. (1974). Ecology. Omega editions.
4. Márquez, A., García, O., Senior, W., Martínez, G., González, A. and Fermín. I. (2012). Heavy metals in surface sediments of the Orinoco River, Venezuela. Bulletin of the Oceanographic Institute of Venezuela.
5. Miller, G. and TYLER, JR (1992). Ecology and Environment. Grupo Editorial Iberoamérica SA de CV
6. Rovira-Sanroque, JV (2016). Contamination by heavy metals in the sediments of the Jarama River and its bioassimilation by Tubificids (Annelida: Oligochaeta, Tubificidae). Doctoral Thesis. Faculty of Biological Sciences, Complutense University of Madrid.
7. Odum, EP and Warrett, GW (2006). Fundamentals of ecology. Fifth edition. Thomson.