Natural iron fertilization by shallow hydrothermal sources fuels diazotroph blooms in the ocean

Abstract

Iron is an essential nutrient that regulates productivity in ~30% of the ocean. Compared with deep (>2000 meter) hydrothermal activity at mid-ocean ridges that provide iron to the ocean’s interior, shallow (<500 meter) hydrothermal fluids are likely to influence the surface’s ecosystem. However, their effect is unknown. In this work, we show that fluids emitted along the Tonga volcanic arc (South Pacific) have a substantial impact on iron concentrations in the photic layer through vertical diffusion. This enrichment stimulates biological activity, resulting in an extensive patch of chlorophyll (360,000 square kilometers). Diazotroph activity is two to eight times higher and carbon export fluxes are two to three times higher in iron-enriched waters than in adjacent unfertilized waters. Such findings reveal a previously undescribed mechanism of natural iron fertilization in the ocean that fuels regional hotspot sinks for atmospheric CO ₂ .

Editor’s summary

Large areas of the world’s surface ocean contain limited concentrations of iron, most of which is supplied by upwelling of deeper, nutrient-rich water or atmospheric deposition. Bonnet et al . found that shallow hydrothermal sources also can supply iron to the surface ocean. They show that fluids emitted along the Tonga volcanic Arc in the South Pacific supply enough iron to the photic layer to stimulate biological activity well above that in adjacent unfertilized waters. —H. Jesse Smith

Abstract

Shallow submarine hydrothermal vents can supply the iron needed to fuel phytoplankton blooms.