IMPLICATIONS OF EPISODIC FERROMANGANESE NODULE GROWTH AND INTENSIFIED MANGANESE FLUX CAUSED BY CHANGING OCEANOGRAPHIC CONDITIONS

The Clarion-Clipperton-Fracture Zone located in the northern Central Pacific is a promising area for future Manganese nodules mining. Yet, nodule coverage varies within tens of meters within a prospective field. Thus, economic nodule mining requires a detailed knowledge of nodule distribution and its controlling factors.

In order to examine factors controlling occurrence and formation of ferromanganese nodules, selective leaching experiments of sediments from 37 box cores were carried out. Data was compared to the bulk sediment composition (XRF) and sedimentary components (EDX/SEM), grain size and organic carbon concentration.

Our data indicate that 80 % of the total Mn is bound as Mn⁴⁺-oxide in the near surface sediments decreasing with depth to 50 % on average. Increasing leachable Mn within top layers is associated with the occurrence of micronodules and siliceous microfossils, whereas carbonate layers at 13-28 cm bsf of box cores in the SE, that may correspond to the last glacial maximum (LGM) are devoid of micronodules and associated with well-preserved foraminifers. Calculated ages of micronodules vary from 13.5 to 27.5 kyrs within different sediment depth. Younger ages might be a result of recent hydrogenetic overprint and possible local mobilization of Mn. Bioturbation kept those micronodules on top, therefore younger and older ages found within the first cm. Different types of element concentration-depth profiles correlate to nodule abundance, size and coverage. Base metals such as Ni and Co are associated with Mn and following the same leaching pattern. Cu shows a different leaching pattern, with lower surface concentration towards the SE, that might indicate stronger Cu recycling. Mn-flux calculations indicate that hydrogenetic Mn accumulation since the last Mn depletion has not been high enough to explain Mn fluxes to the corresponding nodules. In fact it must have been at least as double as high in the past, indicating that diagenetic nodule growth during suboxic diagenesis played an important role within the CCFZ and is rather an episodic than constant process caused by changing climate and ocean water chemistry. Local variations in nodule coverage might be caused by hydrodynamic conditions, sediment grain size and accumulation or erosion as well as C_org concentration and its degradation.