In contrast, pMn showed no gravitational settling behavior (Figures 1, 4b). Instead, peak 116 pMn concentrations remained close in depth to dMn and 3 He xs and followed density surfaces, 117 indicative of isopycnal mixing. Thus, while most pMn and pFe are removed exponentially from (Figures 1, S1). 159The physicochemical form of hydrothermal dissolved iron -A key finding of this study, 160 not discussed previously 8 but critical to the fate of Fe in the SEPR hydrothermal plume, is that 161 the maximum dissolved Fe also deepens by ~350 m by Sta. 36, mimicking pFe (Figures 1, 4a). is not isotopically resolvable in the near-field plume (dFe >5 nM). would not apply to dMn because dMn is minimally complexed by organic matter in natural 233 waters 2 , and negligible Mn was observed in the colloidal fraction ( Figure S4).

Operando, spatiotemporal resolved synchrotron X-ray fluorescence mapping measurements on a custom aqueous Zn/α-MnO2 cell provided direct, quantitative evidence of a Mn dissolution-deposition faradaic mechanism that governs the electrochemistry.