1. Rebello, A. & Wagener, K. Proc. int. Symp. on Environmental Biogeochemistry Vol. 1, (ed. Nriagu, J. O.) 13–23 (Ann Arbor, Michigan, 1976).
2. Stuiver, M. Science 199, 253–258 (1978).
3. Wilson, A. Nature 273, 40–41 (1978).
4. Freyer, H. D. Tellus 31, 124–137 (1979).
5. Pearman, G. I., Francey, R. J. & Fraser, P. J. B. Nature 260, 771–773 (1976).