Insights into hydrothermal iron flux variability at slow-spreading ridges from deep drilling data and reaction-transport modeling

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Recent discoveries in chemical oceanography point to hydrothermal iron as an important modulator of global scale biogeochemical ocean cycles, as iron availability often limits phytoplankton growth and thereby the biological carbon pump. Interestingly, these new results from the GEOTRACES program re-confirm the preceding realization within the mid-ocean ridge community that hydrothermal activity and thereby the hydrothermal export flux along the Mid-Atlantic Ridge (MAR) is disproportionally higher than its slow-spreading rate would suggest. Yet, the controls of hydrothermal iron export and how those vary between mafic and ultramafic geological settings remain poorly understood.Here, we will combine reaction-transport models with IODP data to identify the critical combination of parameters that results in the venting of the most saline and iron-rich fluids at the TAG and Rainbow hydrothermal fields. When seawater is heated while cycling through a hydrothermal system, it may split into a dense and saline brine phase and a lighter low salinity vapor phase upon intersecting with the two-phase boundary, with the metals accumulating in the brine phase. Tracking the brine phase throughout the circulation system that feeds the vent sites is therefore one key to an improved assessment. Quantifying the re-deposition of Fe in hydrothermal minerals around the upflow zone is another. We propose to return to the unique ODP 158 data on the basaltic-hosted fault-controlled TAG hydrothermal system and to integrate it with reaction-transport models to provide new insights into brine formation and iron transport. This work will be complemented by a study on the ultramafic-hosted Rainbow hydrothermal field, for which an international effort of submitting an IODP drilling proposal is on the way. From these two case studies, we anticipate fundamental new insights into the key controlling factors of brine formation and hydrothermal iron export at slow-spreading ridges, which is now realized to be so important for global biogeochemical cycles.