Oxic bottom water dominates polymetallic nodule formation around the Caiwei Guyot, northwestern Pacific Ocean

Abstract

Polymetallic nodules are enriched in critical metals and possess high economic values. The authors observed, for the first time, high abundance of spherical polymetallic nodules on the abyssal basin around the Caiwei Guyot in the northwestern Pacific Ocean. In order to reveal their metal resource potential and long-term metallogenic process, the authors carried out high-resolution scanning electron microscopy (SEM), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray diffraction (XRD), transmission electron microscopy (TEM) of laminas, and marine chemical analysis. The polymetallic nodules are composed of columnar or lamina growth patterns with low reflectance, which can be subdivided into four types of laminas based on microtexture. The Mn/Fe ratios of the laminas increased in the order type 1.2 < type 1.3 < type 2 < type 1.1. The nodule laminas yielded low Mn/Fe ratios (<4.01), Ni (<12000 ppm) and MgO (<3.74%) contents but high contents of Co (average 8516 ppm), Pt (average 416 ppb), and rare earth elements and Y (REY, average 2887 ppm), relative to other polymetallic nodules in the global ocean. XRD and TEM analyses indicated that the polymetallic nodules are composed of Fe-vernadite. Evidences from micro-texture, geochemistry and mineralogy imply that the polymetallic nodules around the Caiwei Guyot have been forming from oxic bottom water since initial growth, due to oxygen-rich bottom water flowing-through and low surface productivity. The dissolved Mn and Fe elements in the bottom water are continuously precipitated into ferromanganese oxides under oxic conditions. The ferromanganese oxides absorb high contents of multivalent metals such as Co, Pt, and Ce, owing to surface oxidation of the manganate octahedral layer during precipitation. The northwest Pacific Ocean shows deep depth, slow sedimentary rates, moderate-to-high contents of dissolved oxygen, and low summer surface productivity. Based on this, the authors propose that the northwest Pacific Ocean is an ideal metallogenic belt for polymetallic nodules with abundant Co, Pt, and REY resources.