Constraining the magnitude of different iron sources to the ocean with a model of the stable isotopic composition of dissolved and particulate iron

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Iron is an essential micronutrient for ocean biota, and its distribution strongly affects the magnitude of phytoplankton primary productivity and thus the carbon uptake in the ocean. While we are slowly obtaining a picture of the present global distribution of dissolved iron in the ocean, there is still no consensus on the mechanisms behind this distribution and especially on the relative role of different external iron sources to the ocean (aeolian dust, marine sediments, hydrothermal and riverine inputs). Some of these (e.g. dust) are predicted to change strongly with ongoing climate change while others are not. The uncertainty of how relevant the specific sources are, is therefore severely hampering our ability to prognose how the oceanic iron cycle and primary production will react to climate change. A new tool to constrain the role of the different iron sources is the study of the stable isotopic composition of oceanic dissolved iron. As different external sources of iron have different isotopic ratios, this composition can in principle be used to infer the relative contribution of the different sources. However, processes that transform iron between different forms (dissolved and abiotic/biological particles, redox states) in the interior of the ocean can fractionate between its isotopes and physical processes (transport and diffusion) also affects the isotopic composition of iron. This makes it necessary to combine the interpretation of iron isotope data with a model of iron cycling in the ocean. In the proposed project, a global biogeochemical model of iron based on my previous work will be extended with an explicit representation of isotopic effects to disentangle the role of different processes affecting the isotopic composition of dissolved iron in the ocean. Together with the growing amount of iron isotope measurements obtained within the international GEOTRACES program, this will allow a better quantification of the magnitude and relative role of the main external iron sources, and thus allow more robust prognoses of the future oceanic primary production.