The two largest reservoirs of carbon on Earth are the oceans, which cover the majority of Earth’s surface, and the lithosphere (the mineral fraction of Earth: soils, rocks, and sediments). Their bodies were gradually transformed by the heat and pressure of the Earth’s crust into the fossil fuels that we mine today to provide petroleum oil, natural gas, and coal (see more on this in chapter 4). Fossil fuels form over a course of 300-400 million years, forming from ancient plants and animals that decomposed slowly under very specific, anaerobic (without oxygen) conditions in wetland environments. In contrast, the residence time of carbon in the fossil pool is dramatically different. The carbon stays in the reservoir of living organisms for a relatively short time, depending on their life span, from hours and days to years and decades. When considering the flux of respiration, living organisms are the source of carbon, and the atmosphere is the sink. Aerobic (oxygen-using) organisms convert carbohydrates created by other organisms into carbon dioxide (CO 2) almost instantaneously, which they exhale into the atmosphere. Some reservoirs hold on to carbon for only a short time. DOE, Biological and Environmental Research Information System. Figure 7.1 shows a simplified version of the global carbon cycle. These locations where carbon resides are known as pools or reservoirs, and the processes that move carbon from one location to another are called fluxes. Carbon is also prevalent in soils, rocks and sediments, water bodies (dissolved), and the atmosphere. All plants, animals (including humans!), fungi, bacteria, and archaea are made of mostly carbon-based molecules such as lipids, carbohydrates, proteins, and nucleic acids. There is no new carbon in the world, rather, all carbon is continuously recycled from one form to another. In this section, as in many other pieces of scientific literature, we will periodically refer to carbon by its chemical symbol, C. Carbon, just like all other elements, cycles through the environment and is constantly in the process of changing forms and locations.
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