The Contrasting Geochemistry of Manganese and Chromium in the Eastern Tropical Pacific Ocean

  • James W. Murray
  • Berry Spell
  • Barbara Paul
Part of the NATO Conference Series book series (NATOCS, volume 9)


The study of the different oxidation states of trace metals can put limits on the oxidation state of seawater. In addition, the changes in oxidation state can influence input and removal in the ocean. Manganese and chromium form a contrasting pair with opposite tendencies. Under oxidizing conditions Mn(IV) is scavenged as MnO2 while Cr(VI) is soluble as CrO4 2−. Under reducing conditions Mn(II) is soluble as Mn2+ while Cr(III) is removed as Cr(OH)3. Thus the distributions of manganese and chromium can be influenced by the oxygen minimum zone.

These chemical transformations as well as the influence of the continental margin, can be demonstrated using data from the Eastern Tropical Pacific. Vertical profiles of manganese in the Guatemala Basin exhibit pronounced features that all intensify approaching the continental margin. There is a surface maximum (up to 9 nM), a maximum in the oxygen minimum (up to 6 nM) and a strong increase towards the sediments in the bottom 1000 m (up to 9 nM). The surface maximum is probably due to atmospheric or riverine input or remobilization out of shelf sediments. The maximum in the oxygen minimum is due to in situ reduction or remobilization out of slope sediments followed by horizontal mixing. The partitioning between dissolved and acid soluble manganese tends to support the latter explanation. The increase in the bottom water is mostly in the acid soluble fraction and increases as the continental margin is approached. The regional distribution favors a sediment rather than a hydrothermal source.

Chromium marine geochemistry is equally complicated. Important features can be seen in data from a line of stations normal to the coast of Baja at 22°N. Along this section the oxygen minimum intensifies to the east. Total chromium is between 3.0 and 4.0 nM at the surface and then decreases sharply to a minimum of less than 2.5 nM at the top of the oxygen minimum. The concen?trations then increase to about 5.0 nM in the deep water. In general, the deep profiles are similar to those of silica. Coincident with the minimum in total chromium are high values in Cr(III) and particulate chromium. A logical explanation is that Cr(VI) is reduced to Cr(III), much of which is rapidly adsorbed and scavenged by the vertical particle flux.


Continental Margin Oxygen Minimum Total Chromium Oxygen Minimum Zone Eastern Tropical 
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Copyright information

© Springer Science+Business Media New York 1983

Authors and Affiliations

  • James W. Murray
    • 1
  • Berry Spell
    • 1
  • Barbara Paul
    • 1
  1. 1.School of OceanographyUniversity of WashingtonSeattleUSA

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