Radiocarbon 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. 

Houtermans, J. C., Suess, H. E., Oeschger, H., Reservoir models and production rate variations of natural radiocarbon,... 

The existence of radiocarbon (14C) was not realized until 1934, when an unknown radionuclide was formed during exposure of nitrogen to neutrons in a cloud chamber (Kurie, 1934). In 1940, Martin Kamen confirmed the existence of 14C when he prepared a measurable quantity of 14C. Over the next few decades more details on the production rate of 14C in the atmosphere and possible applications for dating archeological samples continued (Anderson et al., 1947 Arnold and Libby, 1949 Anderson and Libby, 1951 Kamen, 1963 Ralph, 1971 Libby, 1982). 

Suess. H.E. (1968) Climatic changes, solar activity and the cosmic ray production rate of radiocarbon. Meteorol. Monogr. 8, 146-150. 

The significance of these eflFects to archaeologists are limited. Except when combined with variations in the production rate, they make it difficult to distinguish radiocarbon concentrations in organic samples over the past several hundred years and force laboratories to use 100 or 150 years as the minimum age which can be cited. This aflFects the accuracy of values for late prehistoric or protohistoric archaeological contexts for several areas of the world. It also explains why radiocarbon laboratories cannot use modern wood as a contemporary reference standard. Rather, artifical standards must be used to provide what the value of the activity in the atmosphere would have been without human intervention. 

Graf T, Marti K, Wiens RC (1996) The Ne production rate in a Si target at mormtain altitudes. Radiocarbon 38 155-156... 

The method of choice in routine productivity studies throughout the world nowadays is the radiocarbon method, introduced by Steemann Nielsen (1952). This method requires the addition of a smaU amount of NaH COa to a sea-water sample with a ae, and the measurement of the amount of C taken up by the cells after a certain incubation time. The C method is very sensitive and it can measure far smaller primary production rates than the O2 method, and as such is preferable for measuring the relatively low primary production of the oceans. Measurements can be made in situ, incubating the samples at the same depths from where they were drawn, with the ship at anchor at a certain locality during the incubation period. Unfortunately there is no universally accepted procedure for measuring primary productivity, but the technique for analysis of plankton is described in detail by Strickland and Parsons (1972). 

The distribution of radiocarbon in the ocean is controlled by the production rate in the atmosphere, the spatial variability and magnitude of flux... 

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 %. 

The generation process for radiocarbon in the atmosphere makes C02 which enters the biosphere because of the long lifetime the mixing is essentially perfect. We assumed the rate of production to be constant which turns out to be somewhat incorrect. Variations of about 10 percent can be seen back in time to early Egyptian periods and before. The earth s magnetic field was apparently weaker then as the cosmic rays delivered to the surface and the atmosphere were more intense. 

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. 

In contrast to the primary production, which is estimated by a wide use of radiocarbon techniques, the methods for calculation of the secondary production are based either on the data for the rate of weight growth at different stages of their lifecycle and their abundance, or on the use of physiological characteristics of the organism such as the daily ration, the proportion of the assimilated food in the ration, and the respiration. The data obtained with these methods allow estimation of the values of production of the main species of copepods and other animals [69]. The mean annual values of the... 

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... 

Net production has been measured directly by radiocarbon incubation experiments, whereby water samples are spiked with radiocarbon-labeled bicarbonate, and the net rate of transfer of the radioisotope into organic matter phases determined by comparison of light versus dark incubations. Global maps of net productivity have been constructed on the basis of such measurements, and current... 

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