Carbon and the basic requirements of life

What makes carbon such an important element is its ability to form an immense variety of compounds — 

Sediment is the solid material, inorganic or organic, that settles out of suspension from a fluid phase (normally water, ice or air) in which it has been transported. Over time, under the right conditions, it can undergo lithification (i.e. conversion into a solid body of rock). Various processes can be involved in lithification compaction, cementation, crystallization and desiccation. 

Inorganic sediment is supplied by erosion of material from exposed areas of high relief, and can be transported a considerable distance to the area of deposition. The composition of this detrital (or clastic) material varies, but aluminosilicate minerals are usually important. There are also biogenic sediments, resulting from the remains of organisms (e.g. calcareous and siliceous tests, peat) and chemical sediments formed by precipitation of minerals from solution (e.g. evaporites, some limestones and authi-genic infills of pores by quartz and calcite cements). 

Another prerequisite for life is liquid water, the medium in which biochemical reactions take place and usually the main constituent of organisms. Although bacteria, and even some simple animals, like the tardigrade, can survive in a dormant state without water, the processes that we associate with life can only take place in its presence. This requirement obviously imposes temperature limits on environments that can be considered suitable for life hence one of the criteria in the search for life on other planets is evidence for the existence of liquid water at some stage of a planet s life. 

Carbon is the twelfth most abundant element in the Earths crust, although it accounts for only c.0.08% of the combined lithosphere (see Box 1.2), hydrosphere and atmosphere. Carbon-rich deposits are of great importance to humans, and comprise diamond and graphite (the native forms of carbon), calcium and magnesium carbonates (calcite, limestone, dolomite, marble and chalk) and fossil fuels (gas, oil and coal). Most of these deposits are formed in sedimentary environments, although the native forms of C require high temperature and pressure, associated with deep burial and metamorphism. 

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