The geochemical behavior of metals during early diagenetic alteration of buried manganese nodules

Abstract

Marine polymetallic nodules and crusts represent an important resource for numerous metals such as Ni, Co, Cu, Mn and thus may be subject to deep sea mining operations in the future. Manganese nodules have a widespread occurrence on the seafloor of the Clarion-Clipperton Zone (CCZ) in the equatorial North Pacific and have been intensively studied in the past. They consist of hydrogenetic layers formed by precipitation from ambient oxic seawater and diagenetic layers sourced from suboxic pore water. While numerous studies have focused on nodules found on the sediment surface, only a few have dealt with nodules buried at greater sediment depth. In this study, we have thus systematically investigated the diagenetic processes altering the chemical and mineralogical composition of buried nodules in the eastern CCZ down to a sediment depth of around 10 m and assessed the influence of these processes on sediment geochemistry.Our results show that nodules subject to suboxic conditions are not simply reductively dissolved but react with the surrounding sediment and pore water. After burial, the hydrogenetic layers start to dissolve and the associated metals (Mn, Ni, Co, Cu, Mo) are released into the pore water. The remaining Fe oxides react with the surrounding sediment and pore water to form an authigenic Fe-Si-Al-rich layer consisting of nontronite and goethite. The diagenetic layers of buried nodules were found to be highly enriched in Co and Ba and strongly depleted in Mo, Zn and Li compared to surface nodules. The strong, up to 4-fold enrichment of Co during burial under suboxic conditions is noteworthy since the distribution of this element in nodules is commonly used as a proxy for oxic conditions in surface sediments. Here, the Co enrichment is interpreted to result from a redox reaction with Mn. When suboxic conditions in the sediment prevail long enough, the diagenetic Mn oxide phases are also reductively dissolved and only the Fe-oxide component of the former nodule remains.Under the changing redox conditions associated with burial, Mn nodules exert a strong control on the mobility of metals by either fixing them into the crystal lattice of the Mn mineral phases or releasing them into pore water during their reductive dissolution.