Reduction of manganese micronodule distribution in the South Pacific during the last three million years

Abstract

Analysis of the accumulation rate of manganese micronodules in twenty-three cores from the Bellinghausen Basin in the South Pacific reveals an increase in rates in successively older time periods. The lowest mean accumulation rate occurs in the Brunhes (0−0.7 m.y.) with a narrow zone of highest rates which is restricted to an area previously described as a zone of high bottom current activity. Accumulation rates increased during the Matuyama (0.7−2.43 m.y.) and the zone of highest rates at approximately 62°S widened along an east—west axis. Highest manganese micronodule accumulation rates occurred during the Gauss (2.43−3.32 m.y.) when the entire Bellinghausen Basin was within the zone of highest rates. We interpret the reduction of this zone of high manganese micronodule rates as evidence of waning bottom current activity during the last three million years. Alternative explanations of the down-core increase in manganese micronodule concentration may be a remobilization of manganese during diagenesis in a reducing environment below the seafloor or a change in manganese availability. Our results, however, are inconsistent with these mechanisms since we observe synchronous marked decreases in accumulation rates in widely spaced cores with widely different sedimentation rates suggesting a time control rather than depth of burial, or diagenic, control. These synchronous decreases occur in the mid-Matuyama and the Early Brunhes which are interpreted as times of major decreases in bottom current activity superimposed on a trend of decreasing currents over the past three million years.