“To Fe or not to Fe?” Iron fertilisation in the Southern Ocean

Abstract

As public concern about global warming grows, and the need to reduce greenhouse gas emissions is becoming clear; lawmakers, businesses, the public and investors are being presented with a number of new ideas for how to achieve these goals. Recently one such approach, ‘iron fertilization’ of the oceans - the process of ‘seeding’ some parts of the ocean with the essential micronutrient iron in order to promote plankton growth and thus remove atmospheric carbon (in the form of CO2) and store it in the oceans - has been promoted in the hope that iron fertilization could go some way to sequester carbon emissions1 . However, this process raises a number of questions, including its effectiveness as a market-based sequestration system as well as the possible negative effects on the ocean and other environmental systems. Fertilizers of various forms have become a common means toward improving plant growth. Imagine a fertilizer so powerful it could increase a yield by over 2500%. This is what scientists observed when they ‘fertilized’ a patch of ocean in 1995 with Iron Sulfate as part of the IronEX II experiment. But why would this chemical have had such a profound effect on phytoplankton growth? Over 20% of the world’s oceans are nutrient rich but iron poor 2 , which is the limiting nutrient for phytoplankton growth. By adding sufficient levels of iron to the surface ocean water, a phytoplankton bloom can be induced, and the effect can be quite dramatic. During the 1995 experiment, 450 Kg of iron was spread over a 100 km2 patch of the ocean, producing a phytoplankton bloom which consumed over 2500 tons of carbon dioxide from the surface ocean waters3 . Whether iron fertilization has been a viable mechanism controlling climate in the past, and whether it could be useful in the future is a topic of current debate. What is clear from fertilization experiments to date is that they have been effective tools allowing us to question the role of iron in controlling phytoplankton growth, nutrient cycling and the flux of carbon from the atmosphere to the deep sea.