Ocean Iron Fertilization: Carbon Sink or Nutrient Redistribution Artifact?

Recent Southern Ocean experiments reignited the debate over iron fertilization as a geoengineering tool. The LOHAFEX and SOFeX trials demonstrated phytoplankton blooms following iron addition, but carbon export below 1000m remained negligible in most cases. Proponents point to natural iron inputs near the Kerguelen Plateau showing sustained export. Critics counter that experimental blooms are grazed rapidly by zooplankton, recycling carbon in the upper ocean rather than sequestering it.

The core tension: satellite-observed chlorophyll increases do not translate to verified deep carbon flux. Sediment trap data from artificial fertilization consistently underperform models calibrated on natural systems. This discrepancy suggests either fundamental differences between pulsed versus continuous iron supply, or that natural systems benefit from co-limiting nutrient resupply that experiments fail to replicate.

What resolves this — longer-duration experiments with continuous iron delivery, or higher-resolution deep flux measurements that capture episodic export events current traps miss?