The impact of Mid-Ocean Ridges on the Ocean's Iron cycle

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Photosynthesis by marine phytoplankton provides energy to higher trophic levels (such as fish and marine mammals), as well as contributing to the partitioning of carbon dioxide between the atmosphere and the ocean. Iron is essential for phytoplankton growth as it is required for a number of important enzymes that participate in both photosynthesis and respiration. In contrast with terrestrial systems, iron is present at very low concentrations (less than 1 iron atom to every billion water molecules) in the open ocean. Thus phytoplankton photosynthesis is limited by iron over large parts of the ocean. This iron deficiency has important ramifications for the earth system since phytoplankton photosynthesis is an important means by which the ocean regulates global climate. Mid-ocean ridges are an important source of iron with estimates suggesting that ridge-derived iron makes up 25-75% of global ocean iron stocks. These mid-ocean ridges are the deep-sea mountain ranges that form a single global mid-ocean ridge system throughout the world's ocean, making it the longest mountain range in the world. At these ridges, new magma mixes with seawater and is exhaled as a high temperature fluid. While this ridge fluid has been noted to be a large source of iron to the deep-sea, the far field influence of this iron depends on its retention in dissolved forms by ocean chemistry. Our recent work shows that iron from mid-ocean ridges appears to have a much longer lifetime than previously thought and be exported up to 4000km away from the ridge. Despite the emerging role for ridge-derived iron, we do not understand its impact on deep ocean iron stocks, as well as how iron is mixed into surface waters to drive biological activity. We have highlighted that understanding the fate of ridge-derived iron and its ultimate influence on the ocean requires more information on the quantity and chemical form of iron supplied by ridges (e.g. dissolved or particles) and how these change with distance from source. To do this we need to appraise the role of small organic molecules called ligands and so-called iron nanoparticles, which have been invoked to control the lifetime of ridge-derived iron. Accounting for the specificity of iron within hydrothermal systems is key to constraining its wider impact. In addition, recent work by our colleagues has shown that interactions between the deep ocean tide and the ridge itself can elevate rates of physical mixing. If increased vertical mixing typifies mid-ocean ridges it implies that these regions may also exhibit efficient transfer of iron to surface waters. Given the ubiquity of mid-ocean ridges, the synergistic combination of these phenomena may be key to the large-scale supply of iron to surface waters.Sampling and measurement of iron at very low concentrations in seawater is challenging and the applicants are among the few research groups in the world who are able to do this reliably. Our group is at the forefront of representing the role of iron is global ocean models, which are crucial tools for assessing larger scale impacts on biological productivity. This project will participate in a NERC funded research cruise where scientists with expertise in measuring mixing and other macronutrients will be studying the nutrient and carbon pump over mid-ocean ridges. This proposal will therefore benefit from these measurements and will add value to this cruise by determining the associated role for iron.Overall, this project will provide state of the art observational and modelling constraints on two important aspects of the ocean iron cycle: 1) How does the ocean ridge impact physical mixing of iron to the surface and 2) what chemical processes control the large scale influence of the iron directly supplied by mid-ocean ridges. Ultimately we will be able to address the broader question of how the amount and chemical form of iron from mid-ocean ridges can influence phytoplankton growth in the open ocean.