The geochemistry of manganese in the northeast Pacific Ocean off Washington1,2

Water collected along a transect normal to the coast of Washington was analyzed for dissolved (co.4 pm) and total dissolvable manganese (TDM). The vertical profiles exhibit the same general features observed elsewhere in the Pacific, but a horizontal section of the transect shows that concentrations increase markedly toward the continental margin. A surface maximum, ranging from 0.57 nmol kg-’ in offshore waters to 5.24 over the continental slope, is probably due to fluvial inputs, especially from the Columbia River. Another possible source is the upwelling of manganeseenriched bottom water over the shelf. Horizontal gradients are also observed in the oxygen minimum as manganese concentrations decrease from 4.75 nmol kg-l near the continental slope to 0.7 1 in open ocean water. Porewater and solid phase data suggest that manganese is being actively remobilized under reducing conditions in the slope sediments and is diffusing into the overlying seawater at a rate sufficient to balance losses by vertical mixing and particulate scavenging. Two stations on opposite sides of the Juan de Fuca Ridge have deep concentration maxima that are almost certainly due to injections of hydrothermal vent fluid along the ridge,

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