Iron, Biological Productivity, Carbon Drawdown, and Climate: Have we demonstrated the glacial/interglacial implications of the Martin Hypothesis?

Twenty-five years ago John Martin hypothesized that terrigenous iron transported to the oceans in aerosols could fertilize biological productivity in open ocean regions far from shore. During the 1980s Bob Duce's group did much to demonstrate how much iron was transported to remote regions, in what form, and how its soluble fraction was made available to phytoplankton. Since that time each decade has completed projects that have added to the evidence supporting key aspects of Martin's paradigm-shifting hypothesis. For example, early open ocean fertilization experiments in the 1990s showed that phytoplankton productivity was stimulated by the addition of soluble Fe to surface ocean waters. Most recently, results from EIFEX fertilization in the southern ocean conclusively demonstrated the transfer of organic matter from the surface to deep waters. Our recent studies of Quaternary sedimentation in the equatorial Pacific -- one of the critical areas highlighted by Martin in his earliest formulations of The Iron Hypothesis -- indicate that Fe in the sediment is dominantly from terrigenous non-volcanic sources and that terrigenous Fe in the sediments is closely correlated with the biogenic silica (diatom) in the sediments. Variations in Fe flux, paired with diatom response, are not related to glacial/interglacial periods in a simple fashion. Overall, nearly 30 years of work on the Iron Hypothesis appears to be converging toward an answer that at once demonstrates the complexity of the coupled Fe-biological system, yet also speaks to the overall accuracy of Martin's prediction.