Distribution in ocean
Dissolved manganese has a modified, scavenged vertical profile.
Concentrations are high at the surface and they decrease with depth as dissolved
Mn2+ is sorbed onto sinking particles and removed to the sediment (data).
If low oxygen concentrations are present, as in the North Pacific, then a
subsurface maximum appears due to a slower removal rate (Johnson et al.,
1996). A major source of manganese to surface waters is dust
deposition and this produces great variability
in surface manganese.
Speciation
In the presence of oxygen, the stable form of Mn is Mn(IV), which forms an oxide
MnO2. However, the oxide is insoluble and most of the Mn
dissolved in seawater is Mn(II), present as the Mn2+ ion, which
persists due to slow oxidation kinetics (Johnson et al., 1996).
Residence time
The manganese residence time was calculated from the ocean standing stock
and measured vertical fluxes (Martin and Knauer, 1980).
Analysis
Dissolved Mn can be determined by preconcentration of Mn2+
using a chelating resin, such as Chelex-100, followed by graphite furnace atomic
absorption spectrophotometry. It can also be determined by flow injection
analysis with chemiluminescence or kinetic colorimetric detection (Chapin et a.,
1990).