Supporting article DD: In Nature everything is circulated. The two broad categories of global cycles are sedimentary and biological. A short but insightful article.
The chemical elements that compose the Earth are not held in static compartments. The components of the atmosphere and oceans are transported by the fluid movements of these bodies, which determine our daily weather patterns and the major ocean currents such as the Gulf Stream. Even the solid materials of the Earth—its crust and mantle—circulate by the processes that cause mountains to rise from the sea floor and rock materials to break down, delivering dissolved constituents to the sea. For every chemical element of the periodic table, we can outline a global cycle of its movement between the land, the atmosphere, and the oceans. We can compile measurements from studies around the world to estimate the annual transport of materials between these compartments, usually in units of millions of metric tons per year.
There are two broad categories of global cycles, sedimentary and biological. Geologic processes determine the movement of elements in the sedimentary cycles. Elements circulating in sedimentary cycles typically have no gaseous forms; these are the common rock-forming elements, such as calcium, phosphorus, silicon, and most trace metals. Each year, these elements move from land to sea in the process of rock weathering, by which all crustal materials that are exposed above sea level are broken down mechanically and chemically. About 17,500 million metric tons of material are carried from land to sea each year. Some of the elemental constituents of this material, for example sodium, may spend a long time as a dissolved constituent of sea water, but eventually they are deposited on the sea floor. The deposition may be physical—for example, the deposition of river-borne sediments—or biological, as when a marine organism, with calcium carbonate (CaCO3) in its skeleton, dies and falls to the sea floor.
If it were not for the internal movements of the Earth’s crust and mantle, the movement of elements in sedimentary cycles would be unidirectional, not cyclic. However, tectonic movements of the Earth’s crust carry marine sediments deep into the Earth, where they are transformed back into rock. The average age of the exposed sea floor is only about 150 million years—far less than the age of the Earth’s oceans at 3.8 billion years. Older sediments have been recycled. Tectonic movements of the crust and mantle result in the uplift of marine sediments to form new mountainous areas on land, renewing rock weathering and rejuvenating the global sedimentary cycle of these elements. Elements moving in sedimentary cycles may spend only a fraction of their life near the Earth’s surface, where they may be incorporated in organisms. The average lifetime of an atom of phosphorus in sea water is about twenty-five thousand years. When this atom of phosphorus is incorporated in a marine sediment, it is removed from the biosphere for about 400 million years.
Elements moving in global biological cycles are typically found in one or more gaseous compounds that are produced by organisms. Carbon, oxygen, nitrogen, and sulfur are good examples. These elements are also found in rocks and also circulate in the sedimentary cycle.
For example, a small amount of nitrogen is found in rocks (as ammonium ion, NH4+), and nitrogen derived from rock weathering is carried to the sea by rivers. But in contrast with the sedimentary cycles, biotic processes dominate the annual movement of elements with biological cycles. For example, relative to the total amount of nitrogen delivered to the oceans each year, very little is sequestered in marine sediments. The bulk of it, as much as 100 million metric tons per year, is released to the atmosphere as N2 by the biological process of denitrification.
Biological processes dominate the global cycle of carbon. Each year the uptake of carbon by plant photosynthesis and the release of carbon dioxide by respiration result in a movement of carbon to and from the atmosphere that is more than 1,000 times greater than its movement in the underlying sedimentary cycle of carbon. The annual additions of oxygen to the Earth’s atmosphere by photosynthesis and its removal by respiration also dwarf the movements of oxygen in the underlying sedimentary cycle, in which oxygen is consumed by rock weathering and carried to the sea, largely as sulfate (SO42–). Thus, elements moving in global biological cycles have a relatively rapid circulation near the Earth’s surface as a result of biotic activity.
Geochemists define the mean residence time of an element in any compartment of the Earth as the mass of the material in that compartment divided by the annual exchange between that compartment and other sectors of the Earth. For instance, the mean residence time of sodium in the world’s oceans is about 75 million years—obtained by dividing the sodium content of all sea water by the annual delivery of sodium to the sea by rivers. In contrast, the mean residence time of bicarbonate (HCO3–), the major reservoir of carbon in sea water, is about 100,000 years. With its movements primarily determined by biological activity, carbon spends much less time in sea water. As a general rule, the mean residence time of elements moving in biological cycles is much shorter than those moving in sedimentary cycles at the Earth’s surface.
One advantage to quantifying the global cycles of the elements, particularly the elements of life, is that we can evaluate the impact of humans on the cycles of individual elements ( Steffen et al., 2004 ). In the mining of metals from the Earth’s crust, humans expose buried rocks much more rapidly than would normally occur by erosion and rock weathering. Thus, humans accelerate the movement of the elements in the global sedimentary cycle. The annual transport of copper in the world’s rivers is about three times greater than what one might estimate from the natural rate of rock weathering alone. The difference is largely attributed to the extraction and smelting of copper ores by humans. The mining of coal and the extraction of petroleum accelerate the natural rate at which these carbon-rich sediments would be exposed by rock weathering at the Earth’s surface. The current rate of combustion of fossil fuel releases carbon dioxide to Earth’s atmosphere about seventy times more rapidly than we would expect in nature. Humans have an enormous capacity to increase the rate of movement of materials in both the sedimentary and biological cycles.