| Summary: | Iron limits phytoplankton growth and the biological carbon pump in large areas of the worlds oceans, including the climatically important Southern Ocean. Studies attempting to address how changes in iron supply impact the global carbon cycle rely on global models of ocean circulation and biogeochemistry that typically include dust and continental margins as their predominant iron sources. However, recent observational studies have highlighted the potential importance of an additional iron source from deep-ocean hydrothermalism. In light of this, our prior understanding of the oceanic iron cycle must be re-evaluated. Here we show that hydrothermal iron sources are important in governing the oceanic iron inventory and reproducing dissolved iron observations in a global model. Helium isotopes and iron data are compiled to parameterise the hydrothermal source of dissolved iron using prior observational studies as additional constraints. Importantly, new deep-ocean datasets from two distinct Southern Ocean basins are used to statistically evaluate model results. The deep ocean iron data collected during GEOTRACES cruises to the Southern Ocean suggest that hydrothermal Fe needs to be included in models in order to reproduce observations. Hydrothermalism can increase the biological carbon pump by 20-30% in the Southern Ocean and the impact of variability in the assumed hydrothermal flux is non-linear. We suggest a revised model of the marine iron cycle, particularly for the iron-limited Southern Ocean, with a role for different iron sources over distinct timescales. Due to its relative constancy at millennial timescales, hydrothermalism can buffer the oceanic dissolved iron inventory against shorter-term variability in other sources, such as dust deposition.
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