Southern Ocean deep-water carbon export enhanced by natural iron fertilization

The ocean's importance in storing carbon is widely recognized, as is the importance of iron as a limiting nutrient in much of the global ocean. But quantifying the increase in long-term carbon storage in response to the lifting of iron limitation has proved difficult. The CROZEX experiment, a cruise on-board the RRS Discovery, set out to test the hypothesis that the observed north–south gradient in phytoplankton concentrations near the Crozet Islands in the Southern Ocean is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. The data support the hypothesis: carbon export fluxes to the deep from the fertile waters were two to three times greater than those fluxes from an adjacent high-nutrient low-chlorophyll area not fertilized by iron. The efficiency of carbon export was somewhat greater than that reported in experiments where iron is added artificially, a possible consequence of large losses of the artificially added iron, but smaller compared to that reported from a naturally induced bloom, possibly related to the importance of horizontal iron supply.