フィリピン海北部海山産マンガン団塊の地質学的意義

抄録

Manganese nodules from Komahashi-Daini Seamount, Amami Plateau, Daito Ridge, Okidaito Ridge and a seamount in a southern part of the Kyushu-Palau Ridge (SKPR) are similar in occurrence, morphology, surface texture, internal structure and chemical composition.
SEM observation revealed a presence of numerous natural casts of calcareous nannoplankton fossils within metal oxide layers of these manganese nodules. Fecal pellets, coccospheres and tests of agglutinated foraminifers, also preserved as natural cast, were discovered commonly in the inner layers. 27 species were identified as a whole. Younger species were systematically found outwards within each manganese nodule. These observations ensure the use of biostratigraphic method to determine the formation ages of these nodules.
A manganese nodule from SKPR grew on calcareous ooze in late Oligocene time, and was buried by pelagic clay of early Miocene in age. The rest of four nodules began to form on calcareous ooze in early to middle Pliocene with a rapid initial accretion followed by a slow growth (6.0-13.0cm to 0.6-1.7mm/Ma) which may have ceased in middle to late Pleistocene.
The formation of these four manganese nodules with respect to the geologic history of the Philippine Sea can be summarized as follows:
When the subsidence of the seamounts and ridges started during the late Mioce ne resulting from a differential deepening of this basin, pelagic condition became prevalent over the Philippine Sea region. However, the islands, i.e., the summits of seamounts and ridges, locally produced a surface condition favorable for planktonic organisms, resulting in the deposition of calcareous sediments during the Pliocene. As the subsidence continued, productivity in surface waters declined in early to middle Pliocene because of the reduction of the islands in scale. The resulted decreased sedimentation initiated the nodule formation with an initial rapid rate of accretion. As the islands continued to subside, the rate of nodule accretion decreased reflecting the lowered surface productivity. When the islands submerged completely below the sea level probably in a middle Pleistocene, the pelagic condition predominated this region. As the supply of calcareous material almost stopped, the growth of manganese nodules diminished substantially. A very local variation in nodule mineralogy and chemistry found on the Komahashi-Daini Smt. may be related to a depth dependent variation in marine environments during the susidence.
The formation of the one from SKPR is also explained by the same schema, although the subsidence and nodule formation occurred in late Oligocene.