Iron and manganese accumulation within the Eastern Tropical North Pacific oxygen deficient zone

The Eastern Tropical North Pacific (ETNP) contains the largest oxygen deficient zone (ODZ) in the modern ocean. We determined dissolved concentrations of Fe, Fe(II), and Mn from three cruises in the region. Similar to other reported ODZs, Fe(II) was highest in the depth range associated with the secondary nitrite maximum. The main source of this feature is likely lateral advection of water overlying reducing shelf sediments within a narrow density range centered on the potential density anomaly of 26.5 kg/m³. This density horizon is similar to the Eastern Tropical South Pacific (ETSP) and reflects the intersection of the same density range with a large fraction of the continental shelf bottom waters. We also observed subsurface maxima of dissolved Mn in this density range, in contrast to the ETSP. Deep waters were enriched in Fe within the ETNP, analogous to other eastern boundary upwelling systems as well as the Arabian Sea. We argue that in these systems, reducing conditions on the shelf and overlying water column facilitate a robust shelf to basin shuttle of Fe, moving Fe from the coastal margin to deep plumes. Mn is also transported offshore in the core of the ODZ, and the relationship between Fe(II), Mn, and nitrite is remarkably similar between the ETNP, ETSP, and Arabian Sea. The exception is that Mn supply from the Peruvian shelf is less pronounced than in the other two ODZs, potentially reflecting the absence of large rivers in the Peruvian system.