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Hydrothermal activity occurs in all ocean basins, releasing high concentrations of key trace elements and isotopes (TEIs) into the oceans. Importantly, the calculated rate of entrainment of the entire ocean volume through turbulently mixing buoyant hydrothermal plumes is so vigorous as to be comparable to that of deep-ocean thermohaline circulation. Consequently, biogeochemical processes active within deep-ocean hydrothermal plumes have long been known to have the potential to impact global-scale biogeochemical cycles. More recently, new results from GEOTRACES have revealed that plumes rich in dissolved Fe, an important micronutrient that is limiting to productivity in some areas, are widespread above mid-ocean ridges and extend out into the deep-ocean interior. While Fe is only one element among the full suite of TEIs of interest to GEOTRACES, these preliminary results are important because they illustrate how inputs from seafloor venting might impact the global biogeochemical budgets of many other TEIs. To determine the global impact of seafloor venting, however, requires two key questions to be addressed: (i) What processes are active close to vent sites that regulate the initial high-temperature hydrothermal fluxes for the full suite of TEIs that are dispersed through non-buoyant hydrothermal plumes? (ii) How do those processes vary, globally, in response to changing geologic settings at the seafloor and/or the geochemistry of the overlying ocean water? In this paper, we review key findings from recent work in this realm, highlight a series of key hypotheses arising from that research and propose a series of new GEOTRACES modelling, section and process studies that could be implemented, nationally and internationally, to address these issues.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

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Dissolved iron supply controls half of ocean primary productivity. Resupply by remineralization of sinking particles, and subsequent vertical mixing, largely sustains 2 this productivity. However, our understanding of the drivers of dissolved iron resupply, and their influence on its vertical distribution across the oceans, is still limited due to sparse observations. There is a lack of empirical evidence for what controls subsurface iron remineralization due to difficulties in studying mesopelagic biogeochemistry. Here, we present estimates of particulate transformations to dissolved iron, concurrent oxygen consumption and iron-binding ligand replenishment based on in situ mesopelagic experiments. Dissolved iron regeneration efficiencies (i.e., replenishment/oxygen consumption) were ten-to one hundred-fold higher in low-dust Subantarctic waters relative to higher-dust Mediterranean sites. Regeneration efficiencies are heavily influenced by particle composition. Their make-up dictates ligand release, controls scavenging, modulates ballasting, and may lead to differential remineralization of biogenic versus lithogenic iron. At high-dust sites these processes together increase the iron remineralization length-scale. Modelling reveals that in oceanic regions near deserts, enhanced lithogenic fluxes deepen the ferricline, which alter vertical patterns of dissolved iron replenishment, and set its redistribution at the global scale. Such wideranging regeneration efficiencies drive different vertical patterns in dissolved iron replenishment across oceanic provinces. Globally, the productivity of major phytoplankton groups, including diatoms and diazotrophs, is set by dissolved iron (DFe) supply 1. Twenty years of research has revealed diverse modes of DFe supply, from dust to hydrothermal vents, and their regional influences 2,3. Iron biogeochemistry is a rapidly evolving field, driving the development of global modelling initiatives 4. However, iron cycling in the oceans' interior, a fundamental component of iron biogeochemistry 5,6 , represents a major unknown, and critically is hindering model development 7 .