Distribution of manganese, iron and other trace elements in a core from the Northwest Atlantic

ABSTRACT An 838 cm oxidizing core consisting of red clay with negligible carbonate was taken at 6510 m from the northern wall of the Puerto Rican Trench. Concentrations of Mn, Fe, Zn, Ni, Co, and Cu were determined in the interstitial water and in the hydrogenous and non-hydrogenous fractions of the sediments. Anomalous concentrations of manganese found at depth in the interstitial water are probably caused by the release of Mn+2 during bacterial decay of organic matter and by the reduction of Mn+4 to Mn+2 in the microenvironment of the decaying organics. The observed enrichment of dissolved Mn, Fe, Ni, and Co at deeper levels with a corresponding depletion of solid phase hydrogenous Mn, Fe, Ni, and Co suggests dissolution of iron-manganese oxide at depth with ubsequent upward migration and reprecipitation; a process which may be important in the formation of the manganese nodules which have been reported from this general area. No evidence for a similar migration of Zn and Cu was found. The apparent remobilization of metals in an oxidizing core is difficult to explain and this report confirms the earlier view that in many cases the concentration of dissolved metals does not correlate with any measurable parameter such as Eh, pH, or lithology.