Regional variation of manganese nodule facies on the Wake-Tahiti transect: Morphological, chemical and mineralogical study

Abstract

During the GH80-1 cruise of RV “Hakurei-Maru” (January to March 1980) conducted by the Geological Survey of Japan, a significant regional variation of manganese nodule facies was found on the 4000 km long transect from Wake to Tahiti. More than 190 bottom samplings and 120 sea-bed photography using box corer, free-fall grab, and piston corer, revealed three characteristic nodule facies, each extending over hundreds of kilometers. In the northern Central Pacific Basin and the Mid-Pacific Mountains area (zeolitic/pelagic clay surface sediment), discoidal to irregular shaped smooth surface nodules are dominant. They consist of inner fragmented older nodules composed of δ-MnO 2 and surrounding 2–5 mm thick layers of δ-MnO 2 and 10 Å manganate. Mn Fe and Cu+Ni are low (averages: 1.4 and 0.85 wt.%) consistent with mineralogy. In the central to southern Central Pacific Basin (siliceous clay or ooze surface sediment) spherical and rough surface nodules with concentric structure predominate. Each concentric layer consists of 10 Å manganate, yielding high Mn Fe and Cu+Ni (averages: 4.1 and 2.41 wt.%). In the Penrhyn Basin (zeolitic clay surface sediment), spherical to discoidal shaped smooth surface nodules of simple internal structure are distributed. They consist mostly of δ-MnO 2 and consequently Mn Fe and Cu+Ni are low (averages: 1.3 and 0.85 wt.%). The Manihiki Plateau covered with thick calcareous sediments is a barren area. These regional variation in surface and internal structures and chemical characteristics are principally due to preferential growths of the two ferromanganese minerals. The mode of regional variation of the minerals and its close relationship to surface sediments variability are consistent with the two supply route model of these minerals. The 10 Å manganate forming rough surface dominantly developed within surface sediments as a result of diagenesis of siliceous sediments in the central and southern Central Pacific Basin, whereas δ-MnO 2 develops through direct supply of metal elements from the overlying bottom waters forming smooth surface nodules in the northern Central Pacific Basin and Penrhyn Basin.