The distribution of dissolved manganese in the tropical–subtropical North Atlantic during US GEOTRACES 2010 and 2011 cruises

Abstract

Dissolved manganese (Mn) concentration was determined in seawater samples collected along a section across the North Atlantic between 17°N and 40°N during US GEOTRACES North Atlantic 2010 and 2011 cruises (GT GA03). Dissolved Mn exhibits elevated concentrations in the surface, at the mid-ocean ridge and near oceanic boundaries, and low concentrations in the deep water of the gyre center. At both western and eastern boundaries, dissolved Mn concentration in the surface mixed layer (30–40m) and at depths below 200m decreases with increasing distance from shore, in contrast to the seaward increase of dissolved Mn concentration at depth between 80 and 120m. Simple model calculation suggests that in the North Atlantic gyre center, dissolved Mn concentration is controlled primarily by eolian Mn(II) deposition and in-situ photochemical reduction of MnO2 in the surface mixed layer (0–40m), and by the intensity of sunlight available for in-situ MnO2 photochemical reduction in the euphotic zone below the mixed layer (40–200m). At depth between 200 and 700m, dissolved Mn concentration is determined by regeneration, preformed Mn in the source water and lateral inputs from hydrothermal and sedimentary sources. Below 700m, dissolved Mn concentration is controlled predominantly by lateral inputs from hydrothermal and sedimentary sources. Accounting for the observed dissolved Mn concentration by lateral mixing at this depth range requires low rates of Mn(II) oxidation.