Secular variations of radiocarbon in the atmosphere
Lundell, Leslie Lee
Adams, John A. S.
Master of Arts
As radiocarbon dating has become more accurate and available to more researchers it has become increasingly apparent that there are variations between the radiocarbon age and the absolute age. Tree-ring dating has provided a calibration scale for the radiocarbon age scale. Tree-ring dated wood of 7, years age gives a radiocarbon date of only 6, radiocarbon years. The cause of these variations has been the subject of much debate among researchers during the last decade. The Twelfth Nobel Symposium on "Radiocarbon Variations and Absolute Chronology" was concerned mainly with this problem. Many researchers believe that these variations are caused by variations in radiocarbon production due to variations in the dipole moment of the earth. This hypothesis is still questioned by many other authors. This thesis starts with the assumption that the radiocarbon production rate has remained constant and that the variations are due to variations in the specific activity of the atmosphere caused by changes In the carbon reservoir system. For the carbon reservoir system a six box model was adapted from the model of Bolin (197). Radiocarbon ages of the different carbon reservoirs of this model compare very well with those which have been measured. The only major problem is that the total radiocarbon in the different reservoirs does not add up to the total that there should be on earth based on the production rate. For this reason the excess radiocarbon was placed in the sixth box called the residual radiocarbon reservoir. The residual radiocarbon reservoir is Just a mathematical box which contains the additional radiocarbon on earth not found in the other five reservoirs. The exchange rate of carbon between the shallow sea and the deep sea has the greatest effect on the specific activity of the atmosphere. A lower exchange rate between these two reservoirs causes an accumulation of radiocarbon in the atmosphere. Therefore it is proposed that the young radiocarbon ages in the past may be due to a higher specific activity of the atmosphere caused by a slower exchange rate between the shallow and deep seas. An increase, in the specific activity of the atmosphere in the past causes an increase in the specific activity of the other reservoirs if the conditions of the reservoirs were the same in the past as they are at present. The total amount of radiocarbon on earth would be exceeded by the radiocarbon in the five reservoirs at the higher specific activities of the atmosphere. Therefore it is believed that the conditions of the reservoirs were not the same in the past. The exchange rate between the shallow and deep seas has a large effect on the amount of radiocarbon in the residual radiocarbon reservoir. With a slower exchange rate the age of the deep sea relative to the atmosphere is Increased causing both an increase in the specific activity of the atmosphere and an increase in the amount of radiocarbon in the residual radiocarbon reservoir. If the radiocarbon production is assumed to vary the same problems still exist. The exchange of carbon between the atmosphere and oceans is so rapid that any fluctuation in the production rate is damped by the oceans. The entire carbon reservoir of the atmosphere is turned over by the shallow sea in seven years. The carbon reservoir of the shallow sea is turned over once in about eleven years by exchange with the deep sea. These rates are fast enough to damp out any short term fluctuations due to changes in the production rates. It is believed therefore that the age variations are due to a slower exchange rate of carbon between the shallow and deep seas.