Radiocarbon distribution

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. 

Variability in Radiocarbon Distribution. One of the early major promises of the radiocarbon method was its potential to provide direct... 

The variations in C seen in the deep oceans of the world (Fig. 11-6) essentially show features created by radioactive decay. The radiocarbon distribution is an important tool for determining the replacement times of the deep oceans. Great care has to be taken when interpreting the C distribution to take into... 

The oceans mediate some important carbon fluxes. The exchange of carbon dioxide between ocean and atmosphere has been studied extensively, since the prevailing view is that fossil fuel derived CO2 not remaining in the atmosphere has entered the oceans. To appraise the ocean-atmosphere exchange, we make use of the radiocarbon distribution in the oceans. All C is produced in the atmosphere hence all radiocarbon in the oceans must have entered through the air-sea interface. Under a steady-state assumption, the net influx of C must be balanced by the total decay within the oceans. Using our knowledge of the C distribution in the oceans (Fig. 

Up to this point the discussion has been limited to changes in radiocarbon distribution due to oceanic uptake of bomb-produced radiocarbon. Many radiocarbon applications, however, require not the change but the distribution of either bomb or natural radiocarbon. Ocean water measurements give the total of natural plus bomb-produced Since... 

Comparison of results between GEOSECS and TTO/SAVE shows that the bomb radiocarbon inventory has increased by 36% for the region north of 10° N, by 69% for the equatorial region and by 71% for the region south of 10° S. These data reflect the radiocarbon uptake for the Atlantic Ocean between 1973 (GEOSECS) and 1985 (TTO/SAVE). Along with global bomb radiocarbon distribution, this information provides crucial constraints for the carbon cycle in the ocean. Preliminary results from CGC-91, one of the WOCE cruises, show that the observed increase in bomb radiocarbon inventory from 1974 to 1991 in the northern Pacific Ocean is consistent with the first-order prediction from a box-diffusion ocean model. 

The content of the material in a carbon reservoir is a measure of that reservoir s direct or indirect exchange rate with the atmosphere, although variations in solar also create variations in atmospheric content activity (Stuiver and Quay, 1980, 1981). Geologically important reservoirs (i.e., carbonate rocks and fossil carbon) contain no radiocarbon because the turnover times of these reservoirs are much longer than the isotope s half-life. The distribution of is used in studies of ocean circulation, soil sciences, and studies of the terrestrial biosphere. 

Craig, H. (1957b). The natural distribution of radiocarbon and the exchange time of carbon dioxide between atmosphere and sea. Tellus 9,1-17. 

Neff U, Bollhofer A, Frank N, Mangini A (1999) Explaining discrepant depth profiles of " U/ U and °Thexc in Mn-crasts. Geochim Cosmochim Acta 63(15) 2211-2218 Nozaki Y, Cochran JK, Turekian KK, Keller G (1977) Radiocarbon and °Pb distribution in submersible-taken deep-sea cores from Project Famous. Earth Planet Sci Lett 34 167-173 Nozaki Y, Horibe Y, Tsubota H (1981) The water coluirm distributions of thorium isotopes in the western North Pacific. EarthPlanet Sci Lett 54 203-216... 

The carbon dioxide molecules including a radiocarbon atom are chemically undistinguishable from those of ordinary carbon dioxide, with which it mixes, and eventually, carbon dioxide, including a radiocarbon atom, is homogeneously distributed throughout the earth s atmosphere and hydrosphere. Thus there is a state of constant production, distribution, and decay of radiocarbon, which results in the relative amount of radiocarbon in the atmosphere and hydrosphere remaining constant. In this homogeneously distributed condition, radiocarbon enters the carbon cycle - as the... 

Radiocarbon is uniformly distributed in the carbon exchange reservoir. 

Figure 16.6 Calibration of the radiocarbon ages of the Cortona and Santa Croce frocks the software used[83] is OxCal v.3.10. Radiocarbon age is represented on the y axis as a random variable normally distributed experimental error of radiocarbon age is taken as the sigma of the Gaussian distribution. Calibration of the radiocarbon agegivesa distribution of probability that can no longer be described by a well defined mathematical form it is displayed in the graph as a dark area on the x axis...

The atmospheric 14C/C ratio during the last 50,000 y was sufficiently constant to make radiocarbon a remarkably reliable dating tool. Evidence for fluctuations of the 14C/C ratio could be found by high precision measurements on samples of known age. These fluctuations can be attributed to variations of processes in the solar system (solar activity) and on earth (fluctuations of C02 distribution among the atmospheric, oceanic, and biospheric reservoirs). Both fluctuations of solar activity and of the atmospheric C02 content may have contributed to past climatic changes. 

All of the above particulate investigations were based on mini-radiocarbon measurement techniques, with sample masses typically in the range of 5-10 mg-carbon. This constituted a major advantage, because it was practicable to select special samples (given region, source impact, sediment depth) and to further subject such samples to physical (size) or chemical separation before 14C measurement. This type of "serial selectivity" provides maximum information content about the samples and in fact it is essential when information is sought for the sources or atmospheric distributions of pure chemical species, such as methane or elemental carbon. 

Toggweiler, J.R., K. Dixon, and K. Bryan. 1989. Simulations of radiocarbon in a coarse-resolution world ocean model 1. Steady state prebomb distributions. Journal of Geophysical Research 94(C6) 8217-8242. 

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. 

Peng T.-H. (1986) Uptake of anthropogenic CO2 by lateral transport models of the ocean based on the distribution of bomb-produced 14C. Radiocarbon 28, 363-375. 

Bauer, J.E., Wolgast, D.W., Druffel, E.R.M., Griffin, S., and Masiello, C.A. (1998) Distributions of dissolved organic and inorganic carbon and radiocarbon in the eastern North Pacific continental margin. Deep Sea Res. n. 45, 689-714. 

Nozaki, Y., Cochran, J.K., Turekian, K.K., and Keller, G. (1977) Radiocarbon and 210Pb distribution in submersible-taken deep-sea cores from project FAMOUS. Earth Planet. Sci. Lett. 34, 167-173. 

The natural distribution of radiocarbon Mixing rates in the sea and residence times of carbon and water. In Earth Science and Meteoritics, S. 103—114. Ed. Geiss, J. and E. D. Goldberg. Amsterdam North-Hol-land Publ. Comp. 1963. 

Over the past quarter century, the basis of the technique has been discussed in great detail and needs little explanation (4,5). Figure 1 sketches in diagrammatic form some of the basic elements of the radiocarbon model. The natural production of radiocarbon is a secondary effect of cosmic ray bombardment of the upper atmosphere. As C02, radiocarbon is distributed differentially into various atmospheric, biospheric, and hydrospheric reservoirs. Metabolic processes maintain the radiocarbon content of living organisms at an essentially constant level. 

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