Occurrence and physical setting of ferromanganese nodules beneath the Deep Western Boundary Current, Southwest Pacific Ocean

Abstract

An extensive ferromanganese nodule field adjacent to the Campbell Plateau in the Southwest Pacific Ocean forms beneath the Deep Western Boundary Current (DWBC) and Antarctic Circumpolar Current (ACC). West of c. 1748E, between 59 and 48°S, the field is inferred to be 300–500 km wide, but east of 174°E, where the currents impinge on the eastern slope of the Campbell Plateau, the field narrows from c. 200 km at 55°S to c. 120 km at 49°S. This coincides with deflection of current flow eastward, and consequent reduction in bottom‐current velocity and eddy kinetic energy. Based on seafloor photographs, dredge samples, and 3.5 kHz profile data, five principal nodule facies form broadly parallel zones eastwards from the lowermost Campbell escarpment. These are defined based on location, presumed nodule genesis, and seafloor nodule density: (1) slope hydrogenous, high‐density (SHH); (2) abyssal diagenetic/hydrogenous, low‐density (ADHL); (3) abyssal hydrogenous, high‐density (AHH); (4) abyssal diagenetic, high‐density (ADH); and (5) abyssal diagenetic, low‐density (ADL). Several nodule morphotypes are identified including distinctive discoidal forms with overgrowths possibly resulting from overcrowding and partial sediment burial. Seafloor abundance, surficial textures, and chemistry indicate a predominantly hydrogenous nodule growth and very low net sedimentation beneath the core of the DWBC. Increasing Mn, Ni, and Cu contents, and decreasing detrital silicate contents from core to rim, reflect intensifying abyssal DWBC circulation since c. 6 Ma. Because of the DWBC's high velocity, the nodules provide no record of an increasing terrigenous bedload from New Zealand since c. 3 Ma. However, in regions of reduced flow velocity eastwards from the main DWBC pathway, relatively higher sedimentation rates prevail, and nodules have a higher diagenetic component.