Controls on the chemical composition of ferromanganese nodules in the Clarion-Clipperton Fracture Zone, eastern equatorial Pacific

Abstract

Ferromanganese nodules have long been considered an attractive source for Mn, Co, Ni and Cu and, potentially, for the rare earth elements and yttrium (REY). Sixteen exploration contracts have recently been granted for the Clarion-Clipperton Fracture Zone (CCFZ) in the eastern equatorial Pacific and, because of concern about the environmental impacts of seafloor mining, some parts of the CCFZ have been designated ‘Areas of Particular Environmental Interest’ (APEI) from which mining will be excluded. As the APEIs were selected based on surface ocean characteristics and seafloor topography, estimated from satellite altimetry, the abundance and composition of nodules in these areas are unknown, and as a result their relevance as an intended baseline for mining disturbance has not been demonstrated.To fill this gap, we have undertaken a detailed study of the chemical composition of nodules and seafloor sediments in the UK Contract area in the CCFZ and its closest APEI, APEI-6. There are distinct differences between the two sites: nodules from the UK Contract area are larger (10–15 cm in diameter) compared to those from APEI-6 (2–4 cm diameter), and they have faster growth rates (~11 vs ~3 mm/Ma). Nodules from APEI-6 have, on average, higher contents of Fe (7.2 vs. 5.7 wt%), Co (0.28 vs. 0.13 wt%) and total REY (1120 vs. 715 ppm) relative to those from the UK Contract area. Analyses of individual growth layers reveal that nodules from the UK Contract area contain a greater proportion of Mn-rich phases that are also enriched in Ni and Cu. Although pore waters in the uppermost sediments are oxygen replete today, these layers likely precipitated from suboxic sediment pore waters indicating that the supply of organic material to the sediments in this area has been higher in the past. Sediments from the UK Contract area, in turn, have higher Mn/Al ratios compared to sediments from APEI-6 and have a higher proportion of Mn in easily-reducible phases. Nodules from APEI-6 have a greater proportion of Fe-rich layers that are also enriched in Co and the REY, and likely precipitated from seawater. These differences in the chemical composition of nodules from the UK Contract area and APEI-6, as well as differences in the oxygen penetration depth in the sediments, mean that APEI-6 does not provide a relevant geochemical baseline for mining disturbance in the UK Contract area, or for many other contract areas within the CCFZ.