ABSTRACT

Pelagic manganese nodules were sampled at two sites. One was in the subtropical Pacific where the biological productivity in the surface water was low and Mn did not migrate in the surface sediment. The other was in the equatorial Pacific where manganese oxide in the sediment was reduced by organic material and dissolved Mn was lost from the sediment to the bottom water. It has been demonstrated that the difference in the sedimentary environment is reflected in manganese nodules. The bottom surface of the equatorial nodules contained larger amounts of transition metals soluble in weak acids than other sides in contact with sea water and the acid soluble metals decreased with depth from the bottom surface. On the bottom side of the equatorial nodule the oxide fractions of Cu and Ni increased with depth, indicating the trapping of metals which diffused through the pore waters of sediments and nodules. The bottom surface of the larger subtropical noudles contained much smaller amounts of ²³⁰Th than other surfaces, but the growth rate obtained from the depth profile of ²³⁰Th was similar to those of the other surfaces. The smaller subtropical nodules contained a normal amount of ²³⁰Th at the bottom surface. The concentration of ²³²Th decreased with depth at each surface, that is, with time, except for the bottom surface of the larger subtropical nodules, in which ²³²Th concentration is similar to that in the interior of nodules. These observations apparently lead to the conclusion that the bottom surface of the larger subtropical nodules are not growing at present, but the following possibility can not be excluded. The bottom surface is now growing with Th supplied chiefly from weathering debris rather than authigenic particles in sea water.