MANGANESE NODULES FROM THE PACIFIC OCEAN: MIXTURES OF OXIC-HYDROGENETIC AND SUBOXIC-DIAGENETIC PRECIPITATES – WHY OXIC DIAGENESIS IS NEGLIGIBLE

Manganese nodules are a major type of marine mineral deposits that received increasing attention from both industry and academia over the last one or two decades. Despite they occur in water depths between 4000 and 6000 m, Mn nodules are considered to become a major resource of nickel, cobalt and other metals that are vital for today’s and future society. So far, it was generally accepted that Mn nodules are in principle formed by three different processes: hydrogenetic, oxic-diagenetic and suboxic-diagenetic precipitation. Nodules from the Cook Island EEZ are considered to be of pure hydrogenetic formation, nodules from the Clarion-Clipperton Zone in the equatorial Pacific are considered to form mainly as oxic-diagenetic precipitates and those from the Peru Basin in the southeast Pacific should mainly form under suboxic-diagenetic conditions.

Here we present results of high-resolution investigations of nodules from these three different Pacific regions using electron microprobes, laser-ablation ICPMS and X-ray photoelectron analysis as well as mineralogical and crystallographic methods. Our study indicates that the Peru Basin nodules were indeed formed under suboxic conditions with only a minor hydrogenetic fraction whereas the Cook Island nodules are generally hydrogenetic precipitates. In contrast to former studies, we propose that the CCZ nodules are only a mixture of oxic-hydrogenetic and suboxic-diagenetic layers, albeit with a higher hydrogenetic fraction than the Peru Basin nodules. The diagenetic layers of the CCZ nodules have mainly been formed under suboxic conditions similar to the environmental conditions currently prevailing in the Peru Basin. Furthermore, these results suggest that in the deep Pacific Ocean suboxic and oxic bottom waters alternated, probably due to recurrent shifts of surface water productivity and deep-ocean ventilation, on glacial-interglacial time scales.