Ferromanganese remobilization in recent sediments of the Gulf of Mexico

Abstract

Pore water Fe and Mn were measured in a suite of gravity cores from the northern Gulf of Mexico in a study of diagnostic remobilization in recent sediments. Other relevant parameters such as pH, oxidation potential, total dissolved CO₂, dissolved SO₄, and solid phase organic carbon were determined in order to evaluate oxidation-reduction conditions in the sediment. In addition, the greater than 62 μm fraction of the sediment was examined for the presence of Fe and Mn oxides, Fe sulfides and indications of sediment transport and accumulation rate. The midslope area of the Northwest Texas-Louisiana Slope was found to be the area of greatest Mn remobilization as shown by Fe-Mn coatings on sand and sized material, high concentrations of pore water Mn(up to 13 ppm), and by chemical leaching which showed the Mn to be in a mobile phase. The anturallu remobilized Fe and Mn are concentrated in dark brown to black laminae at the base of a surface oxidized zone which is buff or reddish brown in color. In the midslope area of greatest localization of remobilized Fe-Mn the total sediment was found to contain 4% reducible Mn and 1% reducible Fe, whereas the greater than 62 μm fraction was 5.6% reducible Mn and the silt fraction 7.4%. The continental Rise at the base of the Central Texas-Louisiana Slope had a pore water Mn maximum at 85 cm of 6 ppm and 1.5% leachable solid phase Mn at a depth of 35 cm..