Abstract
The Zn isotopic compositions of ferromanganese nodules, sediment trap samples, sediments, and organic reference samples have been analyzed by multiple collector inductively coupled plasma-mass spectrometry (ICP-MS). The range of isotopic variations (1% for Zn-66/Zn-64) is significant with respect to the analytical precision (0.04% at the 95% confidence level). Marine argillaceous sediments show rather constant values consistent with those of continental shales and loess and with the value of a basalt from Reunion Island. We infer that the depletion of the light Zn isotopes in marine particles and in ferromanganese nodules (presumably equilibrated with seawater) results from biological activity in the upper water column. The delta Zn-66 values of sediment trap samples collected near the upwelling off the coast of Mauritania (central Atlantic) show a seasonal isotopic fluctuation consistent with biological pumping during the high-productivity period. Higher values of delta Zn-66 in nodules appear to be associated with the amplitude of seasonal variations rather than with the mean values of the biological productivity.