The phytoplankton of the upper ocean remove carbon dioxide from the atmosphere by photosynthesis. Their detritus or that of their grazers falls into the deeper ocean taking carbon with it. The ocean uptake of carbon dioxide is limited by the availability of nitrogen in the upper waters over much of the global ocean. This paper examines the cost of providing nitrogen to the upper ocean from a pilot plant with a capacity to sequester 2,000,000 tonnes of carbon dioxide per year. The plant would provide reactive nitrogen at the edge of the continental shelf and monitor the enhanced phytoplankton growth by satellite. The costs compare very favourably with other strategies of carbon dioxide capture and direct placement in carbon sinks. This comes about because the capture mechanism exploits solar energy and the large surface area of the ocean. The sequestration is shown to be permanent and not dependent on the overturning time of the ocean.