The sedimentary rock carbon pool is larger still than the ocean, land, or atmospheric pools. Carbon exists in the form of limestones, CaCO3, and to a lesser extent as organic carbon. These carbon reservoirs together contain about 500 times as much carbon as the atmosphere and the landscape combined. Most of the organic carbon in sedimentary rocks is in a form called kerogen. Kerogen is useless as a fossil fuel because it is dilute, usually less than 1% by weight of sedimentary rocks, and because it is in a solid form making it difficult to extract. We will come back to fossil fuel forms of carbon in Chapter 9.
Carbon is exchanged between the atmosphere and the sedimentary CaCO3 rocks by means of a chemical reaction called the Urey reaction, which is
and as shown in Fig. 8.7. CaSiO3 on the left-hand side is a simplified chemical formula for a rock formed at high temperature called a silicate rock. Silicate rocks are formed by cooling and freezing melted rock. Melted rock is called lava if it is found at the Earth's surface and magma if it is in the subsurface. Real silicate rocks have other elements in them and a wide range of chemical formulas, but the simple formula CaSiO3 works for conveying the essential idea. CaCO3 and SiO2, the solid phases on the right-hand side of the reaction, are typical sedimentary rocks, which form at cold temperatures from elements that were dissolved in water.
The Urey reaction running from left to right, producing sedimentary rocks from silicate rocks, is called weathering. A silicate rock weathers by dissolving its calcium and silica into river water, ultimately to be delivered to the ocean. Left to right is the preferred direction for this chemical reaction, under the relatively cold, wet conditions of the surface of the Earth. If weathering were allowed to continue to its equilibrium, it would pull nearly all of the CO2 out of the atmosphere. Organisms like corals and shell-forming plankton extract the dissolved ions from seawater and construct solid CaCO3 and SiO2 from them.
The Urey reaction runs as we have written it backward, from right to left, producing silicate rocks from sedimentary rocks, deep in the Earth's interior where it is hot. This
chemical reaction is called metamorphic decarbonation. This direction is generally favored at high temperature. CO2 released by metamorphic reactions may find its way to the surface in volcanic gases or in hot water springs at the bottom of the ocean. Some of the carbon degassing from the Earth maybe juvenile carbon, which has spent the last 4.5 billion years of the Earth history bound up in the deep Earth, only to emerge now. We will refer to CO2 fluxes from the Earth, juvenile and metamorphic, as volcanic CO2 degassing.
The fluxes of CO2 by weathering and degassing are small compared with the other fluxes in Fig. 8.2, but if they were to be out of balance, that is, if you were to stop all degassing for example, you could use up all the CO2 in the atmosphere in a few hundred thousand years. The Earth is much older than this, so if we average over a million years or longer, the weathering and degassing fluxes of CO2 must balance. CO2 that escapes from the Earth must eventually be consumed by weathering, since what comes in must go out.
The way the Earth manages to balance the degassing and weathering fluxes is by finding the CO2 concentration and the climate at which the rate of weathering balances degassing. The rate of weathering depends on the availability of fresh water, from rainfall and runoff, that rocks can dissolve into. The rate of fresh water runoff depends, in turn, upon the climate of the Earth. If the climate gets too cold, then weathering slows down, allowing CO2 in the atmosphere to build up. If the climate is too warm, CO2 will be consumed by weathering faster than it is degassed from the Earth.
This need to balance the degassing and weathering CO2 fluxes acts to stabilize the CO2 concentration of the atmosphere and the climate of the Earth. This climate-stabilizing mechanism is called the silicate weatheringthermostat. We can recycle the sink analogy from Chapter 3, only now the faucet is CO2 degassing, the drain is silicate weathering, and the water level in the sink is CO2 in the atmosphere. If CO2 is too high, it will be drawn down by weathering, or if it is too low, it will accumulate from volcanic degassing, until the fluxes of CO2 balance.
The catch to this mechanism is that it takes hundreds of thousands to years to stabilize the CO2 concentration and climate at their equilibrium values. On timescales shorter than that, it is perfectly possible for perturbations, such as the glacial cycles or the fossil fuel CO2 release. This theory helps explain the stability of Earth's climate through geologic time.
The silicate weathering thermostat leads to variations in atmospheric CO2 concentration, on timescales of millions of years. CO2 changes occur because other factors besides CO2 may affect the weathering rate of silicate rocks. A mountainous terrain weathers more quickly than a flat plain covered in thick soil, because the soil isolates the silicate bedrock from the rain water that weathering requires. Plants affect the rate of silicate rock weathering by pumping CO2 into the soil. There may also be variation through time in the rate of CO2 degassing from the Earth, driven by variation in plate tectonics.
Some intervals of the Earth's history, such as the Cretaceous period when dinosaurs ruled the Earth, and Early Eocene optimum which followed, apparently needed more CO2 in their atmosphere in order to balance the degassing and weathering CO2 fluxes at that time. The breathing of the rocks, via the silicate weathering thermostats, takes place on timescales of millions of years (Fig. 8.8).
The terrestrial planets Venus, Earth, and Mars provide a perfect Goldilocks parable to end this section. The sin of Venus (the planet) was to be too close to the Sun. The water that must have originally been packed with Venus evaporated and its hydrogen lost to space forever, a runaway greenhouse effect discussed in Chapter 7. With no water, silicate weathering reactions do not go. With weathering out of the picture, degassing
Today End of the dinosaurs
Fig. 8.8 History of the temperature of the deep ocean, which tells us about the temperature of the high latitude surface, since 65 million years ago.
wins. Venus' carbon allotment ended up as CO2 in the atmosphere, 70atm of CO2. At that pressure, CO2 is no longer strictly speaking a gas; the atmosphere of Venus is actually more like an ocean. And of course Venus is very hot.
Mars messed up by being small, about half the diameter of Earth, so that its interior cooled faster than Earth's. Mars boasts the largest volcano in the solar system, Olympus Mons, but today Mars is geologically dead. With degassing out of the picture, weathering wins and the carbon allocated to Mars has ended up as CaCO3 rocks. And Mars is cold.
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