The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial-interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization--an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below--as invoked in some palaeoclimatic and future climate change scenarios--may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

Nature (London)

Chemical composition and interactions. Ionic interactions and processes; Exact sciences and technology; Earth, ocean, space; External geophysics; Climatology. Bioclimatology. Climate change; Physics of the oceans; Physical and chemical properties of sea water; Meteorology; Geography; Carbon - metabolism; Oceans and Seas; Carbon Dioxide - metabolism; Time Factors; Chlorophyll - analysis; Partial Pressure; Phytoplankton - metabolism; Atmosphere - chemistry; Seawater - chemistry; Diffusion; Iron - metabolism; Index Medicus; Sciences of the Universe; Ocean, Atmosphere; Kerguelen Plateau; Indian Ocean; fertilization; Carbon dioxide; Oceanographic survey; Biogeochemical cycle; carbon cycle; ocean-atmosphere interaction; Carbon sequestration; Climate variation; greenhouse gas; Dynamical climatology; climate modification; marine environment; Bloom; Antarctic Ocean; phytoplankton; Observation data; iron; plankton; primary productivity

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