A seasonal cycle in δ13C, varying between −22‰ in early spring and −28‰ in late fall, has been observed in maple leaves growing under natural conditions. A similar δ13C cycle, varying between −25‰ and −30‰ is indicated for grass growing in the same location as the maple leaves. The seasonal cycle appears to be analogous to the diurnal δ13C cycle.14C/12C ratios of various plants from different parts of Canada show that the 14C produced by thermonuclear bombs has approximately a half-life of eleven years in the atmosphere over Canada. When due allowance is made for the isotope fractionation effect produced by plants, no significant variations in the 14C content in plants from different localities across Canada can be detected. The reported 14C peak observed in atmospheric CO2 in late summer also shows up in maple leaves but with reduced intensity. 14C levels show clearly that the first leaves in spring are composed primarily of carbon assimilated in the previous fall.
Both the 2-L counter, described in GSC I, and the 5-L counter (GSC IV) were operated routinely during the past year. Approximately one-half of the determinations reported here were obtained from each counter. The 5-L counter was operated mainly at 1 atm.For more than a year, age calculations have been carried out monthly by an I.B.M. 1620 computer. If the background, standard, and sample counts during a month conform to statistical laws, they are entered on sheets together with their respective counting times and sample identifications and sent to the computing center for processing.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.