Geochemical and sedimentological studies of a box core from the western sector of the Peru Basin

Abstract

The distribution of sediments, micronodules and manganese nodules which has been studied in a number of cores from the Peru Basin is clearly related to the proximity of the basin to the C.C.D. and to the equatorial high-productivity zone. One core from the Peru Basin has been looked at in considerable detail. In this core an extensively bioturbated calcareous, siliceous-rich upper sedimentary unit is underlain by an “overconsolidated” dark brown clay. Characteristic differences in mineralogy and chemistry are observed in the composition of these sediment types with the dark brown clays having higher contents of Mn, Fe, Co, Ni, Cu, Zn and P than the sediments of the upper horizon. In addition, these dark brown clays contain montmorillonite as the principal clay mineral whereas the sediments of the upper-sediment horizon consist of montmorillonite, kaolinite and illite. Micronodules are much more abundant in the coarse fraction (> 40 μm) of the dark brown clay horizon than in the upper sedimentary unit and are characterized by higher Mn Fe ratios and higher Ni and Cu contents than those from the upper-sediment horizon. This reflects the fact that these “overconsolidated” sediments are well oxidized and that element remobilization from the micronodules has not taken place within this sediment type. In the upper sedimentary unit there is a sharp drop in the Mn Fe ratio and Ni and Cu contents of the micronodules immediately beneath the sediment surface, reflecting the selective migration of Mn and other transition elements such as Ni and Cu from the micronodules. Buried managanese nodules are found at two horizons within the sediment column. These nodules show characteristic differences in their surface texture compared to surface nodules, but no marked difference in composition. The buried nodules are not associated with any well-defined feature such as a sedimentary hiatus. It is suggested that nodules are maintained at the sediment surface by bioturbation. Buried nodules are simply those nodules that have failed to be maintained at the sediment—water interface.