Influence of geochemical and physical processes on the vertical distribution of manganese in Gulf of California waters

The vertical distribution of both dissolved and particulate manganese was determined in the Gulf of California (GC) during the MEGAMARCO expeditions conducted in summer 1996 and winter 1997. In addition, the total atmospheric deposition of manganese was recorded at a coastal station from September 1997 to September 1998. In general, the vertical distribution of dissolved manganese ( Mn d ) showed high surface concentrations, depletion in subsurface waters and a slight increase in deep waters associated with the oxygen minimum zone (OMZ). In contrast, particulate manganese ( Mn p ) exhibited lower values in the surface 60 m and increased with depth. Most Mn d profiles showed a surface maximum located within the first 50 m depth with values ranging from 1.7 to 6.9 nM, similar to the concentrations reported for the Pacific Ocean and the upwelling system off the coast of California. This surface maximum of Mn d can be partially explained by an atmospheric input of Mn d of 128 ± 92 nmol m - 2 d - 1 , with an increase possibly resulting from photoreduction of manganese oxides. Below surface waters, Mn d profiles showed a quite rapid decrease with increasing depth, and concentrations of < 2.0 nM were detected between 60 and 100 m, likely as a result of extensive oxidative removal and particle scavenging of Mn d . The residence time of Mn d in the surface layer of the GC was calculated to be 3.0 ± 2.3 years, suggesting that this element is removed from the water column of this system in a relatively short time span. mouth of the GC, the subsurface maximum of Mn d characteristic of regions with suboxic conditions was recorded, with a mean concentration of 3.8 ± 0.1 nM at 500 m depth. This concentration is similar to that reported for the subsurface Mn d maximum outside of the GC and appears to be maintained by a horizontal advective flux of Mn d from the adjacent northeastern Pacific. In contrast, south of the island region, the subsurface maximum appears to be influenced by fluxes of Mn d into the OMZ produced by resuspension of the sediments along the continental margin. Furthermore, the near-bottom waters of the island region showed a conspicuous enrichment of Mn d ( > 6 nM ) in summer and Mn p ( > 2.0 nM ) in winter, that was explained by an intense episodic diagenetic remobilization. In addition, the surface Mn d maximum was absent in this region under winter conditions, and the vertical profile was relatively homogeneous ( ∼ 1.8 nM ) in the upper 250 m, suggesting that the intense tidally driven vertical mixing taking place there also plays an important role in controlling the vertical distribution of Mn d . In general, the vertical distribution of Mn in this marginal sea is influenced by external sources, such as aeolian dust deposition and sediment input, and internal processes, such as vertical mixing, in situ reductive dissolution and particle scavenging. In conclusion, our data show that there is a clear superimposing of geochemical processes on the physical regime in the GC.