Physical processes, like biological ones, are capable of adding and removing CO₂ from the atmosphere. A key process is volcanism: Volcanoes and mid-ocean ridges release an estimated 0.1 billion metric tons of carbon (as CO₂) into the atmosphere each year. More CO₂, about 0.05 billion metric tons of carbon per year, is released by the slow oxidation of coal, oil, and other ancient organic material in sedimentary rocks exposed at the Earth’s surface. In nature, bacteria and fungi accomplish most of this oxidation by respiring old organic molecules. By burning fossil fuels, humans accelerate this process dramatically, increasing a hundredfold the rate at which sedimentary organic carbon is oxidized.

What geologic processes remove carbon from the atmosphere? About 0.43 billion metric tons of CO₂ are removed from the atmosphere by chemical reactions between air and exposed rocks, a process called chemical weathering. To understand how weathering fits into the carbon cycle, we must first understand that CO₂ in the air reacts with rainwater to form carbonic acid (H₂CO₃). This acid slowly reacts with rock-forming minerals, generating (among other substances) calcium and bicarbonate (HCO₃–) ions that are transported by rivers to the oceans. Within the oceans, the calcium and bicarbonate ions react with each other to form calcium carbonate (CaCO₃) minerals that precipitate out of seawater and accumulate on the ocean floor, forming limestone. In essence, carbon moves from the atmosphere to the sediments, where it can be stored for millions of years.

The two linked processes of chemical weathering and mineral precipitation in the oceans can be summarized by a simple chemical reaction (here, CaSiO₃ stands in for rock-forming minerals as a whole):

CaSiO₃ + CO₂ → CaCO₃ + SiO₂

In present-day oceans, the calcium carbonate minerals that accumulate on the seafloor do not precipitate directly from seawater, like salt. Instead, they are generated predominantly by organisms, as mineralized skeletons and shells. Clams, corals, and many other organisms form beautiful and functional shells of CaCO₃ (Fig. 25.7). Skeleton formation is an example of biomineralization, the precipitation of minerals by organisms. Biomineralization provides yet another link between Earth and life, and one that has varied through evolutionary history.

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The annual carbon fluxes associated with geologic processes are small relative to the yearly inputs and outputs from photosynthesis and respiration. As we see in Fig. 25.6, however, photosynthesis and respiration almost cancel each other out. In the long-term carbon cycle, it is the net result of these rapid biological processes—photosynthesis input minus respiratory output—that matters. Is CO₂ produced or consumed when all biological processes are considered together?

There is some uncertainty in estimates of reservoir and flux sizes in the present-day carbon cycle, but by most estimates the amount of carbon incorporated into organic matter by photosynthesis is a bit larger than the amount returned to the atmosphere by respiration. The difference isn’t much—about the same magnitude as volcanic and weathering fluxes. The “excess” organic carbon produced by photosynthesis is deposited in sediments as they accumulate on land or the seafloor, sustaining a small but constant leak of carbon from the short-term biological components of the carbon cycle to long-term geologic reservoirs.

One final set of geological processes completes the long-term carbon cycle. Earth’s crust is constantly in motion, propelled by heat within the Earth’s interior. Plate tectonics is the name given to this dynamic movement of our planet’s outer layer. New crust forms at spreading centers, the places where molten rock rises upward from the underlying mantle. Old crust is destroyed in subduction zones, where one slab of crust slides beneath another, returning material to the mantle. Plate tectonics provides geology’s best explanation for the formation of mountains and ocean basins, and it plays an important role in the long-term carbon cycle. The crust that descends into subduction zones carries with it sediments, including carbonate minerals and organic matter. Subduction removes carbon from Earth’s surface, but this carbon will be recycled to the surface as CO₂ emitted from volcanoes and mid-ocean ridges. Fig. 25.8 shows the physical processes at work in Earth’s long-term carbon cycle.

Quick Check 2 If plate tectonic processes form a chain of high mountains, would you expect atmospheric CO₂ to increase or decrease?