Impact of iron on ocean biogeochemistry
The continual increase of OMZs over recent decades due to human activities has the potential to affect the fertility of the oceans, by enhancing the recycling efficiency of bioavailable Fe from the seafloor. The aim of BICYCLE (Benthic iron cycling in oxygen minimum zones and implications for ocean biogeochemistry) project was to compare Fe release from sediments with past and present deoxygenation and investigate possible effects on marine biogeochemical cycles. Project partners studied Fe concentrations in sediment samples and pore waters (the water between grains of sediment) across the Peruvian OMZ. Results showed that the sediments are an important source of bioavailable Fe to the local water column. However, most of the Fe released is deposited close to the source rather than transported to the open ocean, where it plays an important role in primary production. A 15 metre long sediment core was used to track the release of Fe from seafloor sediments over the last 140 000 years, back to the penultimate interglacial. Results indicated that Fe release was more intense during periods of slightly enhanced oxygenation, such as under peak glacial conditions. Enhanced Fe release under higher oxygen levels may be due to lower concentrations of hydrogen sulphide in the surface sediment, which decreases Fe retention. The project also developed a general model of how ocean deoxygenation will affect Fe release from seafloor sediment in the future. Researchers compiled a database of seafloor Fe fluxes and used it to identify a connection between seafloor Fe flux, organic carbon rain rate and bottom water oxygenation. This information was used to conduct modelling experiments to determine how best to incorporate the seafloor Fe source into global Earth system biogeochemical models. In addition, researchers improved ways of characterising dissolved-particulate interactions in the water to determine the extent to which sediment-derived Fe is exported out of anoxic ocean regions. Particulate matter collected from the water column during a research cruise to the Peruvian continental margin revealed that most of the Fe in the anoxic water column is oxidised (and therefore demobilised) by nitrate rather than oxygen. BICYCLE provided fresh insights into the fate of sediment-derived Fe in oxygen-deficient ocean regions and how sedimentary Fe release was affected by environmental change in the past. Moreover, the models developed by the project directly contribute to EU efforts to predict how human-induced environmental change will affect the Earth and ocean system in the future.
