Carbon dioxide (CO2) present in our atmosphere absorbs the infrared (IR) radiation emitted by the Earth but is transparent to incoming solar radiation. The absorbed IR radiation increases the molecular vibration of the CO2 molecule, causing a warming of the atmosphere. Due to the analogy to a greenhouse, where the glass of the greenhouse is transparent for the visible light, but blocks the IR radiation emitted from inside the greenhouse, CO2 is termed a “greenhouse gas”. The burning of fossil fuels and deforestation associated with the “industrial revolution” has led to an increase of atmospheric carbon dioxide (CO2) by 30 %, since the late nineteenth century (Houghton et al., 2001). This has caused heated debates on how rising CO2
concentrations, human activities and our climate are interrelated. In order to understand how the anthropogenically triggered increase in atmospheric CO2 can change our climate, a solid understanding of the processes that effect atmospheric CO2 and the time scales over which they occur is necessary.
Atmospheric CO2 is an important but minor reservoir in the carbon (C) cycle.
Carbon is cycled between the biosphere, geosphere, atmosphere and hydrosphere (Figure 1.1). The time scales on which the biological and geological processes occur are very different. Whereas biological processes operate on short time scales (days
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to hundreds of years), geological processes operate on much longer time scales (millions of years).
Figure 1.1: Illustration of the global carbon cycle showing the fluxes between ocean, biosphere, and atmosphere (in giga tons (Gt) C per year), as well as the different C reservoirs (in Gt). (Source:
http://earthobservatory.nasa.gov /Library/CarbonCycle/Images/carbon_cycle_diagram.jpg)
Geology
To understand the geological component of the C cycle, it is necessary to go back a few billion years in time, more precisely ~4.5 billion years. That is, when our Solar System came into existence, originating from a cloud of interstellar gas and dust that collapsed under its own gravity. Earth formed when dust particles collided with each other, merging into larger particles which subsequently joined into pebble-sized rocks and so on (Halliday, 2006). During this process heat was produced and the early Earth was probably molten and the densest material migrated toward the center of the planet, while lighter materials floated toward the surface, creating the Earth’s crust. The latter consists of the oceanic (denser) and the continental (lighter) crust, floating on the mantle (which has the highest density of all three). Since carbon represents the fourth most abundant element in the Universe, it was also present when Earth was formed. A small part of this C was released to the atmosphere in the
Earth cooled down. Since the earliest times, carbonic acid (a weak acid derived from the reaction of H2O and CO2) has been reacting with minerals (weathering) followed by the transport of the reaction products, including calcium (Ca) and magnesium (Mg), to the oceans (erosion). Some of the carbonic acid reacts with Ca or Mg to form carbonates, which eventually settle at the bottom of the oceans.
Due to convective motion of the mantle, “new” oceanic crust is formed at the oceanic ridges and drawn into the mantle at subduction zones; a process known as
“plate tectonics”. Consequently carbonates buried in marine sediments are drawn into the mantle at subduction zones. The CO2 is then released back to the atmosphere during volcanic eruptions. This “geologic” carbon cycle balances weathering, subduction, and volcanism over time periods of hundreds of millions of years (Figure 1.2). However, since ~3.5 billion years ago this cycle is influenced by another major event, the appearance of life.
Figure 1.2: Geological carbon cycle. Calcium, together with other weathering products is titrated into the ocean, where it combines with carbonate to form CaCO3. At the subduction zone the oceanic crust, together with its sediments, is drawn into the mantle. Volcanism associated with the processes at a subduction zone releases CO2 to the atmosphere (Illustration courtesy Karina Kaczmarek).
Biology
About 100 million years after the first fossil evidence of life on Earth, photosynthetic organisms had already evolved (Falkowski and Raven, 1997). These ancestors of modern plants used the radiant energy of the sun to convert simple
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inorganic molecules – CO2 and H2O – into complex organic molecules, that is, photosynthesis (Figure 1.3).
Figure 1.3: Photosynthesis; Sugar and O2 are produced within a chloroplast from CO2, H2O, and the radiant energy from the sun (Illustration courtesy Karina Kaczmarek).
In a series of reactions termed carbon fixation, CO2 molecules are converted into carbohydrates, which in turn are either converted into other organic molecules, e.g. fatty acids and amino acids, or broken down by the oxygen-utilizing process known as respiration, yielding the energy for cellular metabolism. The oxygen on which aerobic organisms depend is released as a waste product during photosynthesis. During respiration, CO2 is cleaved from the organic food molecules and returned to the atmosphere and (or) hydrosphere. Not all organic carbon is oxidized back to CO2 in the course of a plant’s life, however. Dead body parts of plants and other organisms become part of the soil organic matter, or sink to the ocean floors where, in many cases, they are consumed by decomposers – small invertebrates, bacteria and fungi – which thereby release CO2. However, some carbon is removed from the atmosphere/hydrosphere by preservation and burial of organic matter in marine sediments. On a global scale, for the present day situation it is estimated that in the modern ocean burial fluxes of inorganic C (mainly as CaCO )
Biogeochemistry
Even though geological and biological processes in the C cycle occur on significantly different time scales, they are linked to each other. A good example is CaCO3, produced by marine organisms such as coccolithophores and foraminifera (Figure 1.4), which accumulated on the sea floor some time in the geological past, and was later uplifted on land by geological processes, and currently exposed to weathering. The formation of the CaCO3 shells occurs within hours to days, the accumulation and preservation in marine sediments proceeds on time spans of thousands of years, while the formation of mountains (orogenesis) requires millions of years. The previously described scenario becomes a cycle, when the carbonated deposits on land are subject to weathering and erosion, and the carbonate is released back to the ocean, again a process which takes hundreds of thousands of years.
Figure 1.4: The left micrograph shows a coccosphere of the coccolithophore Calcidiscus leptoporus made of calcite. The right micrograph shows the test of the planktic foraminifera Globigerina bulloides also made of calcite.
If organic remains of plants and/or animals, rather than CaCO3, are buried, coal and oil (containing mainly C) may form. Once buried in the sediment, they undergo the previously described geological processes, leading to a release of the C over very long time spans (millions of years). Due to the burning of fossil fuels by humans, the time span in which C is released to the atmosphere (as CO2) is drastically decreased in comparison to natural recycling. At the moment, about 5.5 Gt (giga tons) (Houghton et al., 2001) C are released to the atmosphere per year. Some
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of this C (~3.3 Gt) remains in the atmosphere as CO2, where it contributes to the greenhouse effect. The remaining ~2.2 Gt C dissolves in the oceans where it forms carbonic acid, leading to an acidification of the oceans. In order to predict the impact of the artificially increased C cycling in the ocean-atmosphere system, it is necessary to understand how the different geological and biological processes interact. To do so, it is necessary to understand the various underlying processes. Since CaCO3
represents the largest C reservoir, the fundamental processes responsible for the formation and dissolution of CaCO3 will be discussed.