Sulphide and Metal Levels in Hydrothermal Plumes from Major Vents Along the North Mid-Atlantic Ridge

The formation of metal-sulphide nanoparticles has been suggested as a key mechanism for transporting micronutrient metals from sites of venting to the deep ocean. However, while sulphide levels in vent fluids are relatively well known, measurements of sulphide in hydrothermal plumes and their relationship to metals remain sparse. We report here on sulphide concentrations, their variability and links to a set of trace metals, measured in plumes of the major vents along the mid-Atlantic ridge (Menez Gwen, Lucky Strike, Rainbow, Lost City and Broken Spur) in January 2018 during the GA13 UK GEOTRACES cruise. Sulphide levels were measured on-board by voltammetry in unfiltered solutions. Presence of sulphide was always strongly linked to strong cathodic shifts from the in-situ ORP sensor on the rosette, similar to other reduced species like manganese. The relative ratio of sulphide to dissolved manganese showed variability due to different end member vent fluid compositions, plume age, sulphide removal rates but also possibly to signal from other species in addition to that of free sulphide. The latter was estimated at Broken Spur and found to be following a pseudo first order rate of 0.026 h-1, which was lower than previous measurements in hydrothermal plumes, but 3-4 times higher than expected in oxygenated seawater. Despite increased total metal concentrations in plume samples, for lead, cadmium and copper we found evidence for sulphide-mediated removal from the dissolved phase, which highlights the potential role of hydrothermal system as an ultimate sink for these elements.