Geochemical Cycles: Water, Sodium, and Carbon Cycles

Geochemical cycles refer to the transport, cycling, and transformation of chemical elements through various reservoirs or spheres within the Earth system, including the atmosphere, lithosphere, hydrosphere, and biosphere. These cycles operate through diverse processes and timescales across different reservoirs, maintaining a closed flow within the fully defined Earth system. A key principle is material balance: each element remains constant in total abundance but moves between locations via sequential processes. The classic analogy is the rock cycle, where molten magma rises from Earth’s deep interior, crystallizes into igneous rock, erodes to sedimentary rock, becomes buried and transforms into metamorphic rock, and eventually melts back into magma that rises again.

All geochemical cycles have a characteristic completion time. The longest cycle transports material from the deep Earth to form mid-ocean ridges, creating oceanic crust that later subducts and returns to the deep mantle before rising again. Estimates for this full cycle range from several hundred million years to approximately 4.5 billion years. The material balance of chemical elements can be complex, involving transfers among geological, biological, atmospheric, and liquid systems. These cycles are largely controlled by, and serve as indicators of, past conditions such as continental configuration and elevation, landmass and vegetation distribution, large volcanic eruptions, climate, and biological productivity. Geochemical cycles are generally divided into two types: exogenic cycles (occurring near Earth’s surface) and endogenic cycles (occurring in the deep interior).

Water Cycle. The global water cycle is a primary driver and reservoir for many other geochemical cycles on Earth. It describes all processes operating in the hydrosphere—a dynamic mass of liquid continuously moving among land, oceans, and atmosphere. The hydrosphere includes all water in oceans, lakes, streams, glaciers, the atmosphere, and groundwater, though the vast majority resides in the oceans. The hydrologic cycle encompasses both long- and short-term changes in Earth’s hydrosphere, powered by heat from the Sun that drives evaporation and transpiration. Water serves both as a transport medium for other chemical components and as a reactive agent that removes elements from continental rocks and soils, moving them into oceanic reservoirs.

The water cycle can be envisioned as beginning in the ocean, where solar energy causes surface waters to evaporate, changing from liquid to gas. Evaporation absorbs heat from the ocean and transfers it into the atmosphere. An estimated 102 cubic miles (425 km³) of water evaporate from the ocean annually, leaving salts behind. Water vapor condenses into droplets in clouds and eventually falls as precipitation. Most (92 cubic miles; 383 km³) falls directly back into the ocean, but about 26 cubic miles (108 km³) falls as rain or snow on continents, transforming salty ocean water into freshwater on land. Nearly three-fourths of this continental precipitation (17 cubic miles or 71 km³ per year) evaporates back to the atmosphere or is transpired by plants. The remaining estimated 10 cubic miles (42 km³) per year becomes runoff—some merging into streams and rivers that return to the ocean, and other portions seeping into the ground to recharge groundwater systems (a process called infiltration). Humans now intercept approximately half of the fresh surface water for drinking, agriculture, and other uses, significantly impacting the natural hydrological cycle.

Water in the atmosphere is a major greenhouse gas that helps regulate global temperature and climate. Changes in atmospheric water content can alter the erosion rate of chemical elements on land, the evaporation rate from the ocean, and the balance among many other geochemical cycles.

Sodium Cycle. The sodium cycle is one of the most important geochemical cycles. Sodium is a major constituent of crustal rocks, sediments, and ocean water, moving among these reservoirs over long geological timescales. Rainwater dissolves sodium from crustal rocks such as granite; streams and rivers then carry it in solution to the sea. Sodium (Na) and chlorine (Cl) are the two most abundant dissolved elements in ocean water, combining to form the mineral halite (NaCl) upon evaporation. This conversion from dissolved sodium to solid halite occurs continuously in areas of strong evaporation along seashores worldwide. At times in the geological past, large sections of ocean basins (the Mediterranean Sea, Red Sea, juvenile Atlantic Ocean) have evaporated, leaving thick salt deposits. Stream waters re-erode some of these deposits and carry them back to the sea, completing one circuit of the geochemical cycle.

When salt deposits become buried on the seafloor, the salt may interlayer with oceanic muds; sodium is then removed from the salts and transferred into clay minerals. Replenishing ocean sodium via river flow from continents takes an estimated 65–100 million years. If the concentration of sodium (or another element in a different cycle) remains constant in a reservoir such as the ocean basins over time, there exists a balance between input and extraction. The time required to replenish that amount reflects this balance and is known as the residence time, calculated by dividing the mass of the element in the reservoir by the input rate. Sodium in seafloor sedimentary deposits can react with oceanic crust basalt, forming veins and replacing other elements in the basalt. Ultimately, these basalts and sodium-bearing sediments are subducted into the mantle. Some remelt to form igneous rocks that rise to the surface, containing sodium-rich minerals that are then eroded by rivers, leaching sodium back to the ocean. Other sodium atoms are carried deeper into the mantle, representing the longest residence time arm of the sodium cycle.

Carbon Cycle. The carbon cycle preserves a record of many processes throughout Earth’s history, integrating geologic, biologic, ocean, and atmospheric systems. During the early Archean, volatile substances including water and carbon dioxide were degassed from Earth’s deep interior, with additional contributions from cometary and meteorite impacts. The early atmosphere was rich in carbon dioxide (CO₂). Since the Archean, this CO₂ has been progressively removed by the precipitation of limestones (composition close to CaCO₃) and by photosynthesis, which converts CO₂ (along with nitrogen, phosphorus, and sulfur) into organic matter while releasing free oxygen. The development of life enhanced limestone and other carbonate formation, as many organisms secrete calcium carbonate for their shells and tissues. Inorganic processes since the Archean have also formed limestones.

Over long geologic timescales, carbon dioxide returns to the atmosphere through decomposition of limestones that are subducted into Earth’s deep interior, releasing CO₂ via gases dissolved in magmas that rise to the surface. Plate tectonics and the supercontinent cycle also play large roles in cycling carbon between the atmosphere and the rock sphere. When many continents collide to form a supercontinent, the passive margins containing thick limestone sequences are uplifted above sea level. This tectonic uplift exposes carbonate rocks to the atmosphere during continental collisions. Calcium carbonate (CaCO₃) then combines with atmospheric CO₂, depositing it in the oceans. Thus, continental collisions and supercontinent formation are associated with drawdown and reduction of CO₂ from the atmosphere, leading to global cooling and sea-level changes.

The mass of carbon stored in limestone and organic matter reservoirs on Earth is enormous—about 2,000 times greater than all carbon presently in the atmosphere and oceans combined. Living plants contain approximately the same amount of carbon as the atmosphere. Therefore, human activities such as deforestation, which alter the planet’s vegetation balance, may significantly change the balance between atmospheric and living organic carbon reservoirs, increasing atmospheric CO₂ and altering global climate. Living plants remove CO₂ from the atmosphere and release one molecule of oxygen for every molecule of carbon dioxide used to produce organic matter. When these plants die, much of the organic matter oxidizes and returns to CO₂, but some escapes this process and becomes buried in organic sediments, storing carbon in another reservoir. A delicate balance exists between the carbon cycle and the oxygen cycle; the amount of oxygen released, indicated by the mass of the present-day organic carbon reservoir, is 30 times the current atmospheric oxygen level. This demonstrates recycling of both carbon and oxygen on geological timescales. Similar relationships among biological, geological, and atmospheric processes govern the geochemical cycles of nitrogen, phosphorus, and sulfur. Plants absorb these elements in fixed proportions from different environments, store them in organic soils, where groundwater can leach them into the hydrological system, transport them to the ocean, and there they form building blocks for new life.

Further Reading: Berner, Elizabeth Kay, and Robert Berner. Global Environment: Water, Air and Geochemical Cycles. Upper Saddle River, N.J.: Prentice Hall, 1994.
Brantley, Susan, James D. Kubicki, and Art White. Kinetics of Global Geochemical Cycles. New York: Springer, 2008.