R O G E R R E V E L L E A N D H A N S E. S U E S S 22 New Zealand area: (RAFTER,1955). Pauna Shells 350 Limpet shells 360 Cockle shells 280 Pauna flesh 290 Limpet flesh 140 Seaweed 250 Seawater (East Cape area) 370 yrs. yrs. yrs. yrs. yrs. yrs. yrs. The average of the ages given here is 430 yrs. for the Atlantic samples and 290 yrs. for the New Zealand samples. The difference of 140 yrs. is due to the fact that the ages of the first group were calculated using wood grown in the 19th century as standard, whereas contemporaneous wood was used as standard for the New Zealand samples.’ Assuming that these apparent ages for marine surface materials are representative for the average age of marine carbon, or in other words, that mixing times of the oceans are short compared to the ages measured, an apparent age of about 400 years for marine carbon corresponds, according to Eq. (I), to an exchange time t (atm) of about 7 years. This lower limit for the exchange time of CO, between the atmosphere and the sea can now be compared with computations of this quantity from the observed effect of industrial fuel combustion on the specific C14 activity of wood. This second way, however, will lead to an upper limit for the exchange time if rapid mixing in the oceans is assumed. At present 9.1 x 1015grams of C14 free CO,, or a fraction of 3.9 x I O - ~ of the atmospheric CO,, is added per year to the atmosphere by artificial burning of fossil fuels. The total amount added during the IOO years prior to 1950 corresponds to about 12 % of the atmospheric carbon reservoir. Neglecting any effect of the industrial CO, on the rate constants k, and k, one obtains: as - = k, (it - s) at - k,s - for the amount of industrial CO, in the sea at the time t. As k, 60 k,, we may neglect k , as small compared to k,. We then obtain: (4) or Expressing i, s, and r in units of the atmospheric C O z ,we obtain with i = 0.25 % (corresponding G the value during the 1940’’s) and t=4o years, the following values of r for exchange times t kl Table 4 ~ / k= , t(atm) years 1 I 5 1.2 Empirical values for the decrease in the specific C14 activity I* were obtained by comparing C14 activities of wood samples grown in the 19th century with those grown more recently, taking into account isotope fractionation effects by CI3 measurements and correcting for the C14 decay by normalizing to equal age (SUESS1955). With the assumption that the total atmospheric carbon reservoir is only negligibly greater than the amount of C O , in the atmosphere, Y* will be equal to r. Table 5 Tree and integrated: Spruce, Alaska.. See FBRGUSSONand RAFTBR. These authors have now found that I I O years should be added to all of their previous age determinations to correct for the difference of the standards. I (atm) = - as listed: I Years of growth from annual rings .. . Y* % 1.77 White Pine, Massachusetts 1945-1950 1936-1946 1946-1953 3.40 2.90 Incense Cedar, California 1940-1944 195-1953 1.85 I .os