  Dissolved manganese (Mn) is a biologically essential element, and its
oxidised form is involved in the removal of trace elements from ocean waters.
Recently, a large number of highly accurate Mn measurements have been obtained
in the Atlantic, Indian and Arctic Oceans as part of the GEOTRACES programme.
The goal of this study is to combine these new observations with
state-of-the-art modelling to give new insights into the main sources and
redistribution of Mn throughout the ocean. To this end, we simulate the
distribution of dissolved Mn using a global-scale circulation model. This first
model includes simple parameterisations to account, realistically, for the
sources, processes and sinks of Mn in the ocean. Whereas oxidation and
(photo)reduction, as well as aggregation and settling are parameterised in the
model, biological uptake is not yet taken into account by the model. Our model
reproduces observations accurately and provides the following insights:
  - The high surface concentrations of manganese are caused by the combination
of photoreduction and sources to the upper ocean. The most important sources
are dust, then sediments, and, more locally, rivers.
  - Results show that surface Mn in the Atlantic Ocean moves downwards into the
North Atlantic Deep Water, but because of strong removal rates the Mn does not
propagate southwards.
  - There is a mostly homogeneous background concentration of dissolved Mn of
about 0.10 to 0.15 nM throughout most of the deep ocean. The model reproduces
this by means of a threshold on manganese oxides of 25 pM, suggesting that a
minimal concentration of Mn is needed before aggregation and removal become
efficient.
  - The observed sharp hydrothermal signals are produced by assuming both a
high source and a strong removal of Mn near hydrothermal vents.

Atmospheric and Oceanic Physics