The calcium carbonate cliffs of Dover and the petrol at motorway service stations both represent large geological reservoirs of carbon and, as such, potentially very long-term carbon sinks. Ridgwell and Edwards (Chapter 6, this volume) examine the key determinants of these sinks and their role in the global carbon budget.
Marine sediments provide the ultimate long-term 'geologic' sink for CO2 emitted to the atmosphere. For instance, carbon extracted from the surface ocean and transformed into organic matter by photosynthesizing organisms can be 'locked away' by burial in accumulating sediments. Sediments, mainly those of the oceans, can provide long-term sinks for carbon. The remains of oceanic plants and animals sink to the sea bed as 'marine snow', with productive areas of the world's oceans producing huge amounts of such particulate organic carbon. In coastal areas, dissolved and particu-late carbon carried by rivers can also provide a significant source of carbon to marine and estuarine sediments. Through the sedimentation of dissolved and particulate organic carbon, sediments provide a global carbon sink of ~10 million tonnes per year. We are familiar with the end result of this - fossil fuel deposits such as natural gas and oil.
Carbon is also taken up by plants on land: the anoxic conditions prevailing in swamps and peatlands allow for efficient preservation of this plant material, ultimately forming coal measures. However, the rate of formation of new fossil fuel deposits is more than 100 times slower than the rate at which we are burning them (~6 Pg C/year). It is also unlikely that there would be any significant increase in their rate of formation in the future. This means that the geologic organic carbon sink may not be of any appreciable help in removing the greenhouse gases we are adding to the atmosphere.
Carbon is also buried as an inorganic solid - carbonate, the common reservoirs of which include chalks and limestones. Precipitation of calcium carbonate and its deposition in sediments lead to a further 200 Tg C, ending up in sediments annually. This deposition of carbon is made even more important by the very long periods (thousands of years) during which such carbon is generally taken out of the atmosphere. The global burial rate of carbonate carbon is several times faster than for organic carbon but, like fossil fuel deposits, formation of rock carbonate is unlikely to match the current rate of use of this carbon sink by humans in industries such as cement production. At present, 100-200 Tg C/year is released to the atmosphere from use of this carbonate carbon reservoir.
More importantly, accumulation of carbonate in the deep ocean is very sensitive to environmental conditions, particularly ocean pH. Geochemical interactions between dissolved CO2 and marine sediments will act to enhance the capacity of the ocean to sequester CO2 from the atmosphere on a timescale of thousands of years.
Ultimately, virtually all the fossil fuel carbon that humankind can find to burn is likely to become locked up in geological formations as carbonates. Unfortunately, the timescale for this slowest removal process is hundreds of thousands of years. In contrast, rising greenhouse concentrations in the atmosphere and associated climate change are occurring at a much quicker, century-scale pace.
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