World-Wide Distribution of Natural Radiocarbon

Abstract

The natural radiocarbon content of biological materials collected from locations widely scattered over the earth's surface have shown no evidence of the latitudinal variation which would be expected were the turnover time for the biosphere comparable with the mean life of radiocarbon. The strong dependence of the intensity of the cosmic-ray neutrons, which produce the natural radiocarbon, on latitude would be expected to cause this effect. Since it is expected, however, on geochemical evidence that the turnover time for the biosphere would be very short compared to the mean life of radiocarbon, the observed uniformity is not surprising.The observed mean assay of carbon in equilibrium with the biosphere is 16.1\ifmmode\pm\else\textpm\fi{}0.5 dpm/g C. Consideration of the amounts of the various types of carbon in equilibrium with the biosphere, of which the most abundant is the inorganic carbonate and bicarbonate ions dissolved in sea water, gives 8.3 grams per ${\mathrm{cm}}^{2}$ of the earth's surface. A direct comparison with observed cosmicray neutron intensity is made by taking the observed thermal intensity and correcting according to standard formulas for the epithermal production of radiocarbon, to give a total of 156\ifmmode\pm\else\textpm\fi{}30 neutrons/${\mathrm{cm}}^{2}$ of earth's surface/minute on the average. Therefore, one expects that the mean specific activity of carbon in equilibrium with the biosphere would be the ratio of 156 to 8.3, or 18.8\ifmmode\pm\else\textpm\fi{}5 dpm/g. The agreement between the predicted and observed specific activity is gratifying, but owing to the considerable uncertainties in our knowledge of the cosmic-ray intensity and the amount of diluting carbon, it may be that it is somewhat fortuitous. It does appear to indicate, however, that no considerable variation in cosmic-ray intensity over the past several thousand years has occurred. The fact that the principal part of the diluting reservoir is inorganic material in the sea water serves to make any large variation in the amount of carbon in this reservoir unlikely.