Radiocarbon cycle, 11-year

Throughout this chapter many of the arguments are based on an assumption of steady state. Before the agricultural and industrial revolutions, the carbon cycle presumably was in a quasi-balanced state. Natural variations still occur in this unperturbed environment the Little Ice Age, 300-400 years ago, may have influenced the carbon cycle. The production rate of varies on time scales of decades and centuries (Stuiver and Quay, 1980,1981), implying that the pre-industrial radiocarbon distribution may not have been in steady state. 

Burchuladze, A. A., Pagava, S. V., Povinect, P., Togonidze, G. I., Usacevt, S., Radiocarbon variations with the 11-year solar cycle durinq the last century. Nature, 287, 320-322 (1980). 

These changes in soil C stocks are relatively rapid, due to rapid turnover of soil organic matter (SOM) in these tropical soils. Using radiocarbon derived from atmospheric testing of nuclear weapons in the 1960s as a tracer, Trumbore et al. (1995) estimated that the mean residence time of C in the top 10 cm of soil is about 3 years for 30% of the SOM and 10-30 years for 60% of the SOM. Only about 10% of the C in the top 10 cm of soil cycles on a millennial time scale. This very old C fraction increases to 40-80% in the... 

Radiocarbon dating has attracted considerable attention. Carbon-14 is produced in the upper atmosphere by cosmic-ray bombardment of nitrogen-14. It is oxidised to carbon dioxide and eventually absorbed and incorporated in the tissues of plants and animals. The time taken for a carbon atom to complete such a carbon-cycle and return to the upper atmosphere is, on average, about 500 years. As the half-life of is 5568 years, the specific activity of carbon in the carbon cycle is roughly constant. But carbon removed from this life-embracing cycle by conversion to, and retention in, a solid such as wood, bone or shell loses activity at a rate determined by the decay constant for Thus the specific activity of carbon in a rock, a fossil plant or bone, or ancient artifact gives its age (Libby, 1951). Measurements are not easy because of the low specific activities but are of considerable and improving accuracy. 

C-atom on a global scale. However, the actual cycling and turn-over rates of carbon in biological systems within various micro-environments have to be considered short-circuited and much more rapid, as numerous radiocarbon labeling experiments show. Life processes also stimulate and regulate multiple subcycles of carbonate dissolution and reprecipitation (see Chapters 2.2 and 2.4). On the other hand, some carbon deposited in stable continental interiors (cratons) remains protected from dissolution (or oxidation) and recycling from the time of deposition billions (10 ) of years ago up to the present. 

According to Broadbent (1960), pollen analysis and radiocarbon dating indicate that the rate of accumulation of organic soils is about one foot in SOO years. This value would vary widely from place to place with climatic conditions. It is not unusual for peat beds to experience cycles of gains and losses as climatic conditions change. 

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