Meridional distribution of dissolved manganese in the tropical and equatorial pacific

The distribution of dissolved Mn (dMn) was studied in the Tropical and Equatorial Pacific along a meridional transect beginning at station ALOHA (22.45°N, 158°W) extending southwards to 7°S along 158°W. Results show that in the surface ocean (<100 m) dMn exhibited a southward decreasing gradient that appears to be controlled by atmospheric deposition and photo-chemical reduction as sources and particle scavenging as a sink. In the OMZ region (200–2000 m), no subsurface Mn maximum was observed from 16°N to 10°S, in contrast to earlier results in the North Pacific, where the subsurface Mn maximum appears related to the difference in AOU (apparent oxygen utilization) between the water in the OMZ layer (∼27.1 σ_θ) and the water above this layer (∼26.5 σ_θ,). Both remineralization of organic matter sinking from the euphotic zone and the transport of dMn along isopycnal surfaces from shelf and slope boundaries can be responsible for the subsurface maximum as long as the AOU difference between the depths exists. These two mechanisms can be studied integrally by using AOU (difference of AOU between depths) as a proxy for the subsurface Mn maximum. At 2000–3000 m, our results show that hydrothermal Mn is transported visually less further away than hydrothermal Fe from the EPR to the southeast, possibly due to lower dissolved Mn concentration than dissolved Fe in the hydrothermal source. In the deep waters below 800 m, dMn exhibits a linear relationship with AOU along the sampling transect. This dMn:AOU linear relationship is attributed to remineralization as the stations are far away (>6000 km) from the continents, which suggests that dMn in the deep open ocean is strongly affected by remineralization. The dMn:AOU linear relationship (n = 37, R = 0.87) that we found in this work is consistent with culture studies and can be applied to Mn models to reproduce the observed open-ocean Mn concentrations in deep waters.