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Growth rate of a vesicomyid clam from the 21° N East Pacific Rise hydrothermal area


Previously, we determined the growth rate of a vesicomyid clam from the Galápagos spreading centre by using natural radionuclide analyses of serially sampled shell layers1. The growth rate was 4 cm yr−1 measured along the 12 cm length of the axis of maximum growth, yielding an age for the clam of 3–4 yr, assuming this rapid rate for the entire age of the clam. We noted that the linear growth rate was actually highest in the last year of the life of the organism; this could indicate a change in food supply, which in turn, in this chemically driven ecosystem, would be an index of hydrothermal activity. We have now determined the growth history of a vesicomyid clam from the 21° N East Pacific Rise (EPR) site using 228Ra, 226Ra and 228Th measurements described earlier1. The 210Pb (half life 22.3 yr)–210Po (half life 138 days) couple is useful only for 2 yr, at which time secular equilibrium is attained, whereas 228Ra (half life 5.7 yr) and its progeny, 228Th (half life 1.9 yr), provide information on a longer time scale. The 228Th/228Ra activity ratio reaches the transient equilibrium value of 1.5 in 10yr and the 228Ra decay can be followed for 30 yr. We report that the average growth rate along the axis of maximum growth (10 cm) is 0.27 cm yr−1 resulting in an estimated age of 27 yr for the clam, about 10 times older than a clam of comparable size from the Galápagos spreading centre. There is evidence for changes in growth rate with time.

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Turekian, K., Cochran, J. & Bennett, J. Growth rate of a vesicomyid clam from the 21° N East Pacific Rise hydrothermal area. Nature 303, 55–56 (1983).

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