We thank Lisa Woolgar for her help with the helium–tritium age analysis. The optimal interpolation routines were kindly provided by Lynne Talley. Support was provided by the Natural Environment Research Council under Grant NER/A/S/2000/00438 (E. L. M. and H. L. B.) and under the core strategic research projects Ocean Variability and Climate (B. A. K., S. A. C, and R. M.) and Biophysical Interactions and Controls over Export Production (R. J. S). Comments by two anonymous reviewers helped clarify the analysis and conclusions.
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