Using an Ocean Model to Examine the Role of Different Iron Sources and Internal Cycling in Controlling the Distribution of Iron and Iron Isotopes in the Global Ocean

Iron is a key limiting nutrient for net primary production over much of the ocean, but despite large advances in data coverage thanks to the GEOTRACES programme, there is still substantial uncertainty regarding the importance of different sources and internal cycling processes. Iron isotopes have the potential to contribute to reducing this uncertainty, especially if field data can be combined with biogeochemical modelling experiments. Here we present results using a new version of the biogeochemical global ocean model PISCES, adapted to include iron isotope tracers, to improve our understanding of the major ocean iron sources and internal transformation processes. Our model experiments focussed on examining how different assumptions regarding the model iron and iron isotope cycling affect the reproduction of both dissolved and particulate iron isotope and concentration data from GEOTRACES Atlantic and Pacific basin transects. Via new parameterisations of sediment iron input from reductive or non-reductive iron dissolution, we were able to assess the importance of these processes in both regionally (e.g. on Eastern and Western margins) and globally, as well as the relevance of iron input from aerosol deposition and hydrothermal systems. Moreover, we conducted specific experiments aimed at evaluating the impact of anthropogenic aerosols, with much lighter iron isotope values, on iron and iron isotope distributions. Finally, we examined how the isotopic fractionation during complexation of iron by ligands may operate in combination with distinct iron source end members in driving the iron isotope values in the deep ocean and their variation within and between the Atlantic and Pacific basins.