Martin  suggests that intentional iron fertilization of nutrient-rich but iron-starved areas such as the southern oceans and equatorial Pacific might be used to stimulate organic productivity and thereby remove carbon dioxide from the atmosphere. Martin further proposes that low concentrations of atmospheric carbon dioxide during glacial times were partly the result of increased oceanic productivity stimulated by iron-bearing dust falling into oceans. Martin's work is very controversial both because the effectiveness of iron fertilization of the iron-starved parts of the oceans has been questioned [Peng and Broecker, 1991] and because proposed pilot-scale experiments to test the effects of iron addition on ocean ecology have met resistance from those who fear unanticipated and possibly harmful side effects. Of particular concern are possible long-term effects that would not necessarily be observed in proposed experiments. In this article, it is proposed that periodic additions of iron-bearing volcanic ash to the oceans represent natural iron-fertilization experiments that could be evaluated for their effects on ocean productivity.