geochemical cycles

geochemical cycles A great variety of processes on Earth are cyclical in nature: in them, materials are transformed from their original state into other forms and eventually return to their original state. A fundamental picture of a global cycle of Earth materials is the so-called rock cycle that represents a pathway from the molten magma in the interior crystallizing into rocks that make the oceanic and continental crust; the breaking up or erosion of the crustal surface by the physical and chemical agents of the atmosphere and running water; transport of the eroded and dissolved materials to the ocean where they are deposited on the oceanic floor; and subsequent return of oceanic sediments by crustal subduction into the deeper regions of the Earth where they can melt, completing the cycle to the original magmatic state.

Any cycle has a characteristic time that it takes to complete, and the very large cycle of materials travelling from the interior of the Earth to its surface and back into the interior takes a geologically long time, measurable in hundreds of millions to more than a billion years. This is a long time even on the scale of the Earth's age, which is about 4.5 billion years, or of the age of the oldest sediments deposited in water, about 3.8 billion years.

Although geochemical cycles, as the name implies, encompass the processes regulating the chemical composition and material balances of the solid, liquid, and gaseous parts of the Earth, they are also closely tied to many other physical, geological, or biological processes. The geochemical cycles are controlled to a varying extent by such major factors as the configuration and elevation of the continents, volcanic emanations, spreading of the ocean floor, flow of water on the Earth's surface and in the subsurface, vegetation cover of the land, biological production on land and in waters, and climate.

The distinction between the cycling processes at, or near, the Earth's surface and those in the Earth's deeper interior found its way into the scientific terminology, which distinguishes between exogenic and endogenic cycles. The historically older concept of epicycles (small cycles on a bigger cycle) has not found use in the geological literature. Early discussions of the cyclical nature of processes on the continents and in the oceans go back to James Hutton's description in 1785 of sediment erosion on land, transport to the ocean, accumulation on the ocean floor, hardening, and subsequent rising to be eroded again. Perhaps it was not a small feat of geological thinking in the late eighteenth century to deduce a picture of the global cycle of sedimentation that included processes not immediately observable on the surface.

The water cycle

The global water cycle (Fig. 1) is one of the major drivers of the geochemical cycles on the Earth's surface, in which water is both a means of transport and a substance that reacts chemically with minerals and gases in the Earth's crust and the atmosphere. Professor C. Bryan Gregor, of Dayton University, wrote in 1988 in a prologue to the volume Chemical cycles in the evolution of the Earth that one of the best of the cyclical processes that are not immediately observable as cycles is the global cycle of water: if the rivers flowed only one way, as an observer might simplistically conclude, without the water coming back somehow, then the global sea level must be rising continuously. Such a rise, at an extreme, would have been about a metre per decade, a change that could not have remained unnoticed in historical times. Since the last glaciation maximum about 18 000 years ago, melting glaciers have added water to the oceans, raising the sea level a total of some 120 metres. The rate of sea-level rise was not uniform during this period, but it translates into an average of only several centimetres per decade.

The oceans are by far the greatest reservoir of liquid water on the Earth's surface and they are the main source of water vapour in the atmosphere. Water transport from the ocean surface to the atmosphere is driven primarily by heat, and water vapour is a major greenhouse gas in the atmosphere that makes the Earth's climate much warmer than might have been expected from only the solar radiation received by the Earth. Condensation of water vapour in the atmosphere over the oceans returns it directly to its source, and its condensation over land accounts for surface flow, recharge of underground water aquifers, and plant and animal life. Glaciers and ice sheets grew in the past because the climatic conditions made the atmospheric precipitation accumulate as ice in the colder regions. Today, about 2 per cent of the volume of water in the oceans is locked in the Arctic and Antarctic ice. Ocean water circulates through the spreading zones of the oceanic lithosphere, where new material is added to the ocean floor and where water reacts with rocks and melt at high temperatures. This is an important mechanism of chemical transport between the lithosphere and the ocean.

The sodium cycle

Sodium is one of the major constituents of crustal minerals, sediments, and ocean waters. The geochemical cycle of sodium is also an example of how our thinking about the cyclical nature of geological processes has evolved since the earlier part of the nineteenth century. Sodium and chlorine are the two most abundant elements in solution in ocean water, and salt is the main mineral (halite, of composition NaCl) that precipitates by evaporation of sea water. However, areas of strong evaporation and salt precipitation near sea shores are scattered around the world, and we cannot easily observe mineral-growth reactions on the ocean floor where sodium is removed from solution into clay minerals. The earlier thinking, based on the processes and pathways more easily observable at the time, was that sodium accumulated continuously in the ocean, where it was brought by river flow from land; the age of the ocean could therefore, it was thought, be determined from the mass of sodium in the ocean and its annual import by rivers. Then, as the reasoning went, the ratio of the mass of sodium in the ocean to the mass brought by rivers every year would give the length of time needed to build up the amount in the ocean or, in other words, the age of the ocean. This figure, estimated by various authors at various times, is about 65–110 million years: much too short for the age of the oceans on Earth, where marine life began not less than 3.8 billion years ago and highly developed organisms lived in the oceans this side of 600 million years ago. This estimate, however, is not the age of the ocean, as it was once thought, but it is the time needed to replenish all the ocean-water sodium by river flow from the continents. In general, when the mass of an element in some reservoir (such as the mass of sodium in solution in ocean water) does not change with time or, allowing for inflows and outflows, it is in a steady state, then the amount brought in over some period of time must be balanced by the same amount removed from that reservoir. Therefore dividing the mass in the reservoir by the rate of input gives the length of time needed to renew the reservoir mass; this time is also called the residence time.

In reality, the picture of the geochemical cycle of sodium is considerably more complex. Starting with the continents, sodium is a constituent of crustal rocks, such as granite, where it occurs in a common rock-forming mineral, albite, composed of sodium, aluminium, silicon, and oxygen (NaAlSi₃O₈); it is subsequently leached into waters when crustal rocks become exposed to, and react with, surface and ground waters; it is transported in solution by rivers to the ocean. Rivers, however, carry sodium in solution from three sources. Some of it comes from the spray from ocean surface carrying sea salts on to the land, where they are ultimately washed back into the ocean; some of it comes from dissolution of old salt deposits that formed from ocean water in the course of the geological history of the Earth; and some of it comes from the dissolution of aluminosilicate minerals, such as albite, in the continental crust. Thus a good part of the sodium carried by rivers had already been in the ocean and it is being recycled on the Earth's surface between the ocean and the land.

Sodium is removed from ocean water by chemical reactions into sedimentary minerals on the ocean floor, into the basalt in the spreading zones, and by precipitation of mineral halite in the regions of strong evaporation; ultimately, sodium returns from the ocean and oceanic sediments into the deeper crust of the Earth. The longest paths in this process are the travel times from the sediment into the Earth's interior by subduction of the ocean floor and the time during which sodium resides in crustal rocks before it becomes ex-posed to chemical reactions with waters near the Earth's surface.

The carbon cycle and its couplings

The geochemical cycle of carbon (Fig. 2) is a fascinating story of the unique conditions on our planet during most of its history. Carbon dioxide, water, and other volatile substances are believed to have been degassed from the Earth in its early history of formation and cooling. Subsequently, two main processes were responsible for removal of carbon from the atmosphere of the early Earth: one was the deposition of limestones, made of calcium carbonate minerals, from ocean water by combinations of inorganic precipitation and extraction by organisms secreting calcium carbonate in their tissues or skeletons; the other was the process of photosynthesis that converts carbon dioxide (with nitrogen, phosphorus, and sulphur) to organic matter, releasing free oxygen in the process. Since the 1970s, James Lovelock has written extensively in the context of his ‘Gaia’ model of the Earth about the modification of the chemical composition of the Earth's atmosphere by living organisms and, in particular, their role in the removal of carbon dioxide to its present levels. Professor Robert A. Berner, of Yale University, has calculated the levels of carbon dioxide in the Earth's atmosphere, in periods of the past 500 million years, to have been up to 18 times higher than in modern times. Over the long term, carbon dioxide returns to the atmosphere through decomposition of limestones subducted from the ocean floor into the hotter interior, and through volcanic emissions in the oceans and on the continents. Behind this process is a chemical reaction between calcium carbonate and silicon dioxide minerals, occurring at high temperatures, which produces carbon dioxide and a calcium silicate mineral phase, known as the Urey reaction and named after Harold C. Urey, of the University of California, who proposed it.

The mass of carbon stored in limestones and in organic matter in sediments is very great: about 2000 times greater than all the carbon in ocean water and atmosphere combined. Today's land plants contain approximately the same amount of carbon as the atmosphere (within 20 per cent) on which they depend for carbon dioxide. This balance indicates that changes in the global distribution of vegetation on land, possibly caused by natural factors or human activities, may strongly affect the mass of carbon dioxide in the atmosphere. The mass of carbon in photosynthetic plants living in the surface layer of the ocean is only a small fraction of its mass in land plants, but because of a much faster growth of oceanic plants the flows of carbon dioxide from the atmosphere to the plant reservoirs on land and in the surface ocean waters are not too different.

Every molecule of carbon dioxide converted to organic matter by photosynthesis releases one molecule of oxygen. In fact, the occurrence of oxygen gas in the atmosphere is a direct outcome of the activity of living organisms in photosynthesizing their organic matter from carbon dioxide and water. When plants die, their carbon is ultimately oxidized back to carbon dioxide. This process, however, is not 100 per cent efficient, and a small fraction of organic matter escapes oxidation by storage in sediments. All the organic carbon preserved in sediments represents an amount of oxygen produced that is about 30 times greater than the present-day oxygen in the atmosphere. The efficient recycling of the atmospheric carbon and oxygen on the geological timescale prevented a complete drain of the atmospheric carbon dioxide and accumulation of oxygen to very high, environmentally improbable concentrations.

The connections between geochemical inorganic and biological processes in the global cycles were well recognized by the early 1920s, when Alfred J. Lotka, then at the Johns Hopkins University, wrote a book, Elements of mathematical biology, in which some of the chapters were devoted to the carbon dioxide, nitrogen, and phosphorus cycles. The conditions of life on Earth are embedded in the geochemical cycles of these chemical elements, which are among the main building blocks of organic matter. (Other main constituents of organic matter are hydrogen and oxygen in water, and sulphur.) The mechanism that essentially connects the geochemical and living worlds is that of chemical reduction and oxidation reactions: carbon, nitrogen, and sulphur are chemically reduced in the process of photosynthesis and formation of living organic matter; they are oxidized when organic matter respires or decomposes, returning them to the environment.

The biological reservoirs of plants on land and in the ocean maintain close coupling between the geochemical cycles of the four life-essential elements: carbon, nitrogen, phosphorus, and sulphur. In the process of photosynthesis, land and aquatic plants take these elements in certain nearly fixed proportions (known after their discoverer as the Redfield ratios) from different environments. For example, carbon dioxide is taken from the atmosphere, and phosphorus comes from dissolution of minerals in crustal rocks and its release into waters from dead organic matter in soils and sediments. For every atom of phosphorus available, land plants take up 500–800 atoms of carbon; in water, 106 atoms of carbon go into the formation of organic matter with every atom of phosphorus. The low abundance of phosphorus in the form of the phosphate-ion in natural waters makes its availability critically important to land and aquatic plants. Decomposition of organic matter supplies most of the phosphorus needed on a short timescale, but the slower process of leaching from crustal and soil minerals is the ultimate source of new supply of phosphorus to the biosphere.

For the past two to three centuries, the geochemical cycles of the life-essential elements have been perturbed by industrial and agricultural activities on a global scale. Observations of the effects of these perturbations are generally focused on the rising concentrations of greenhouse gases such as carbon dioxide, methane, and nitrous oxide in the atmosphere, and their future effects on the global climate. However, the coupling between the geochemical cycles ensures that global changes are unlikely to be confined only to the atmosphere, but will affect the processes and flows of materials interacting within the Earth's surface system and its major reservoirs of the atmosphere, land, biota, and waters.

A. Lerman