Radiocarbon carbon production rates

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. 

The ratio is fairly uniform within each atmospheric hemisphere. There is a small-amplitude annual oscillation of the order of 0.4 % in the preindustrial era (Kromer et al. 2001) with Imiger-term fluctuations due both to changing production rates and changes in the carbon cycle itself (particularly ocean circulation and ventilation). The Southern Hemisphere is typically depleted in radiocarbon compared to the Northern Hemisphere by about 0.5 %. 

Obviously this wide distribution of the 14C formed in the atmosphere lakes time it is believed to require a period of 500-1000 years. This time is not. however, a deterrent to radiocarbon dating because of two factors die long half-life of I4C and the relatively constant rate of cosmic-ray formation of l4C in the earth s atmosphere over the most recent several thousands of years. These considerations lead to the conclusion that the proportion of 14C in the carbon reservoir of the earth is constant, and that the addition by cosmic ray production is in balance with the loss by radioactive decay. If this conclusion is warranted, then the carbon dioxide on earth many centuries ago had the same content of radioactive carbon as the carbon dioxide on earth today, Thus, radioactive carbon in the wood of a tree growing centuries ago had the same content as that in carbon oil earth today. Therefore, if we wish to determine how long ago a tree was cut down to build an ancient fire, all we need to do is to determine the relative 14C content of the carbon in the charcoal remaining, using the value we have determined for llie half life of 14C. If the carbon from Ihe charcoal in an ancient cave has only as much 14C radioactivity as does carbon on earth today, then we can conclude that the tree which furnished llie firewood grew 5730 30 years ago. 

The dominant source of organic carbon in seawater is the photosynthetic fixation of C02 by unicellular algae (phytoplankton) in the photic zone. Their growth by cell division is rapid, but the population is kept in balance by grazing species (zooplankton). DeVooys (1979) has discussed the state of the art for determining the rate of primary production of marine biomass. Recent estimates, all based on the take-up of radiocarbon, fall into the... 

If it can be assumed that the rate of production has not varied over time, and thus that a dynamic equilibrium has formed, and if it is possible to extract clean sample carbon, unaltered apart from the decline in and to measure its current concentration, it is possible using eqn [1] to calculate the elapsed time since the death of the organism. In practice, the process is far more complicated than this brief description indicates. Principally, one of the basic assumptions, that the rate of formation is constant, is known to be incorrect. The rate has, in fact, varied over time in response to a number of effects, principally fluctuations in the cosmic-ray flux with changes in the geomagnetic field and in solar activity. Because of this, no radiocarbon measurement equates directly with a calendar date, and all such measurements must be calibrated before use. 

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