Bomb radiocarbon in annual tree rings from Thailand and Australia

We have examined the atmospheric C-14 excess in the tropics and the southern hemisphere temperate region in the bomb pulse period, using two sets of cross-dated tree rings. One set was from a medium-sized three-leaf pine (Pinus kesiya) grown in northwestern Thailand and the other was from a Huon pine (Lagarostrobos franklinii) grown in northwestern Tasmania, Australia. A total of 48 annual tree rings (24 pairs) from 1952 to 1975 AD were pretreated to alpha-cellulose, combusted to CO2 and converted to graphite for C-14 measurement in the tandem accelerator at AN-STO. Excellent agreement was found between our measured C-14 data from tree rings and atmospheric C-14 records at similar latitudes. A large depletion of atmospheric C-14 for Thailand in 1953-1954 AD was observed. This might be due to a combination of the Suess effect and upwelling in the tropical Indian Ocean. The results also showed the rise and decay of bomb C-14 peaks from north to south with a time delay of about 1.5 yr, and the effects of minor atmospheric nuclear tests in the late 1960s and early 1970s. A delay of at least one month for C-14 in tree cellulose of Huon pine compared with that in the atmosphere was also found. (C) 2000 Elsevier Science B.V. All rights reserved.