Distribution and stability of Mn complexes in the ocean: Influence of hydrothermal plumes and weather events

We measured the speciation of dissolved Mn from the surface to just above the hydrothermal vents at 9°50′N East Pacific Rise in the open ocean of the Pacific over a 3‐week period. Total dissolved Mn concentrations ranged from 2.2 to 135 nmol L−1 with a significant contribution of dissolved Mn(III) bound to humic acid in one third of our samples representing up to 64% of the total dissolved Mn. These humic complexes were mostly detected in the hydrothermal vent plume and at the redox boundaries of the oxygen minimum zone in the water column. In the hydrothermal plume, the Mn(III)‐humic acid stabilized the manganese in solution up to a ~ 10,000‐fold dilution of the venting water. In the upper water column, Mn(III)‐humic acid was only detected after a squall and rain event, which indicates that it is a transient species, persistent over days to weeks. This temporal variability highlights the importance of non‐steady‐state processes in the open ocean, which may help to explain previous observations of a dissolved Mn maximum within oceanic oxygen minimum zones.