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

Iron is an essential nutrient that regulates productivity in similar to 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 CO2.

Automated summary

Iron is a vital nutrient for ocean productivity, and this study reveals the previously unknown mechanism of natural iron fertilization in the ocean. Shallow hydrothermal fluids along the Tonga volcanic arc significantly impact iron concentrations in the photic layer, stimulating biological activity and resulting in extensive chlorophyll blooms. This iron enrichment leads to higher diazotroph activity and carbon export fluxes, providing a regional hotspot sink for atmospheric CO2.