Geology and Climate
ACS Climate Science Toolkit | Oceans, Ice, and Rocks
The CO2 bubbles in this photograph contain carbon that is completing (or just beginning) its several-million-year journey from the atmosphere to the ocean to marine organisms’ carbonate structures to ocean sediment to limestone subducted beneath a tectonic plate to release by magma heating and return to the surface by volcanism to begin the cycle once more. The diagram further down the page is a schematic representation of this pathway. Along the way, the carbon’s journey might have been interrupted by more rapid reactions, such as incorporation into organic molecules via photosynthesis, but as the organics decay most of the carbon on the oxygen-rich Earth ends up in its most stable oxidized form, as CO2 and carbonate.
This photo shows an ocean acidification experiment the Earth has been carrying out in a few localized areas for a very long time. The bubbles are essentially pure CO2 being emitted from the shallow floor of the Mediterranean Sea off the volcanic island of Ischia in Italy’s Bay of Naples. Unlike the mixture of hot gases and liquids emitted by the thermal vents at the juncture of tectonic plates in the deep ocean, the vents here emit the CO2 at ambient temperature. The pH of the sea in the vent area can be as low as 7.3, increasing to the usual 8.2 about 150 m from the vents. Studies of the biodiversity in this setting can help us understand the consequences of ocean acidification by increasing fossil fuel CO2 emissions.
“Venting of volcanic CO2 at a Mediterranean site off the island of Ischia provides the opportunity to observe changes in the community structure of a rocky shore ecosystem along gradients of decreasing pH close to the vents. Groups such as sea urchins, coralline algae and stony corals decline in abundance or vanish completely with decreasing pH. Sea grasses and brown algae benefit from elevated CO2 availability close to the vent by increasing their biomass. Similar high CO2/low pH conditions are on the verge of progressively developing ocean-wide through the uptake of fossil-fuel CO2 by the surface ocean.” (U. Riebesell, Nature 2008, 454, 46-47)
research interests webpage: http://www3.geosc.psu.edu/~jfk4/PersonalPage/ResInt2.htm.
Geologists estimate that about 90% of the Earth’s crust is made up of silicates—quartz, clays, and zeolites are among the very large number of complex mineral structures … Exposure of the silicates to CO2 in Earth’s humid atmosphere leads to the weathering reaction shown in the diagram.
CaSiO3 + 2 CO2 + H2O → Ca2+ + 2 HCO3– + SiO2 . . . . . . . . . . . . . . . . . . . . . . . . . (1)
Carbonate rocks—limestone and marble, for example—also react with CO2 and H2O.
CaCO3 + CO2 + H2O ⇔ Ca2+ + 2 HCO3– . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2)
The products of these reactions are ultimately washed into the ocean where marine organisms use them to build shells and other structures by the reverse of reaction (2) or analogous reactions by other organisms that build silicate structures. When these organisms die, their remains either dissolve or sink to the bottom of the ocean. Some of this sediment forms sedimentary rocks that may be uplifted to the surface (to begin the weathering process all over) or, as the diagram shows, subducted beneath tectonic plates as the plates move over one another.
Beneath the plate, under high pressure and heated by the magma (the molten rock on which the plates float), the subducted carbonates and silicates can undergo metamorphism (change in form). For our purposes, this complex process can be characterized by this reaction.
CaCO3 + SiO2 → CaSiO3 + CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (3)
The CO2 from this reaction can escape through cracks and fissures in the crust, especially where it is thin and the magma is close to the surface, as in areas of volcanic activity. Thus, the CO2 finishes its journey through the rock cycle by reentering the atmosphere via volcanoes and surrounding vents, as in the photograph that began this discussion.
Note that reaction (3) can be obtained as the combination of forward reaction (1) and reverse reaction (2) and it is sometimes shown with arrows in both directions. This is unfortunate because it obscures the role of water in the weathering processes [“reverse” of reaction (3)] that occur at atmospheric pressure and ambient surface temperature. This contrasts with the high pressure and temperature conditions required for metamorphism. It also obscures the essential role of the marine biological chemistry that produces the sediments from which the reactants are formed.
As indicated above, over a couple of billion years, essentially all the carbon on Earth has been oxidized to carbonate. The graphic below shows that about 99.6% of the carbon is now sequestered in the rock reservoir. The rock cycle briefly outlined above has been the long-term control on the carbon in the atmosphere, the oceans, and the land surface of the Earth, including the biosphere—represented by the blue area in the graphic.
Chemistry, Atmospheric CO2, and Ocean Acidification,” Annu. Rev. Earth Planet. Sci.,
2012, 40, 141-165.
Human activity has increased the atmospheric level of CO2 in this system to levels unprecedented in at least a million years and done so essentially instantaneously on a geological time scale. Given another million years or so and assuming that the marine biological chemistry continues to work in a more acidic ocean, the rock cycle could probably bring the carbon cycle back into balance. Although a mere blink of an eye in the time scale of life on Earth, a million years is five times longer than humans have been part of that life. On this time scale, it is unlikely that increased rock weathering will play much of a role in mitigating any other effects of increased atmospheric CO2 levels.