VERTEX: the lateral transport of manganese in the northeast Pacific

Abstract

Vertical distributions (0 to 2000 m) of dissolved Mn were measured at 5 stations on a 3200-km east-west (California to Hawaii) transect during the VERTEX (Vertical Transport and Exchange) IV and V cruises. All profiles shared common features: surface maxima, subsurface minima, maxima associated with the oxygen minimum, and relatively low levels at depth. Particulate Mn fluxes, measured at four of the five stations using free-floating particle traps (∼ 100 to 2000 m), indicated that in situ particulate scavenging was not responsible for the formation of the subsurface dissolved Mn minimum nor was in situ particulate Mn regeneration responsible for the dissolved Mn maximum associated with the oxygen minimum. Thus, these dissolved Mn extrema result primarily from lateral advective transport processes. The Mn minimum is associated with the shallow salinity minimum, a water mass that sinks away from the surface in the North Pacific (∼ 47°N), and spreads to the south and east in our study area. Additional evidence of the onshore flow of open-ocean, near-surface water is provided by the high Pb concentrations (∼ 50 pmol kg −1 associated with this feature. Waters in the oxygen minimum/ Mn maximum appear to have northerly and offshore flow in our study area. Box model estimates indicate that offshore lateral advective velocities must approach 0.4 cm s −1 to balance vertical diffusive losses from the oxygen minimum/dissolved Mn maximum. Maintenance of the subsurface Mn minimum requires onshore lateral transport of water with low Mn content at velocities on the order of 0.9 cm s −1 offshore, increasing to 4.4 cm s −1 inshore. These velocities represent maximum estimates since north-south Mn distribution data were not available.