Annual variability of dissolved manganese in Northeast Pacific along Line-P: 2010–2013

Abstract

The distribution of dissolved manganese (dMn) in the northeast Pacific across the Line-P transect was evaluated to investigate the mechanisms responsible for the spatial and temporal variability of dMn. A total of 299 filtered seawater samples (< 0.4 μm) collected in August at the five major stations each year from 2010 to 2013 were analyzed. Vertical profiles of dMn showed a clear distinction between onshore and offshore stations. Within the Summer Mixing layer (SML), we observed high dMn concentrations in all years driven by external sources, such as river water, coastal sediments or eolian dust, as well as photo-reduction. At the onshore stations, the absolute concentration of dMn at the surface was annually variable, depending on the strength of the Ekman transport. Within the subsurface layer, dMn decreased rapidly with depth down to 150 m due to particle scavenging. Within the Oxygen Minimum Zone (OMZ), near the continental margin, we observed elevated and annually variable dMn. The high dMn concentration is likely due to mixing of remobilized Mn from the continental margin, which varies year-to-year depending on the intensity of sedimentary Mn reduction process. At the offshore stations, dMn showed subsurface maxima, within the Winter Mixing Layer (WML) rather than surface maxima. We attribute this to the combined effect of biological drawdown of dMn in the surface during the spring and summer, where iron (Fe) is depleted in this High Nutrient and Low Chlorophyll (HNLC) region, and remnant dMn deeper in the WML, from earlier in the year. Using a simple one-dimensional advection and diffusion model, we identified that dMn found in North Pacific Intermediate Water (NPIW) at the western end of the transect can be advected as far as the outermost of the onshore stations in 2011 and 2013, while dMn in the central area of the transect is likely diluted by low dMn water from the south in 2010 and 2012. Within the OMZ, dMn showed elevated and annually variable concentrations, yet the enhancement of dMn of this study area was found to be less intense than in other regions. Lastly, an eddy found in 2010 enhanced dMn concentrations within the surface mixing layers.