B8
Manganese ore, as a clue for paleoceanography

Manganese nodules, which are balls with a diameter generally ranging from 1 mm to 20 cm, are widely found on the sea floor. Manganese crusts, which envelop rock masses with thicknesses generally thinner than 1 cm (maximum 15 cm), are also widely found on the sea floor. Manganese crusts and nodules are black and mainly include manganese and iron oxides. Their formation is estimated to be a few mm per a million years, however, the mechanism of chemical sedimentation of oxides is not fully understood. Manganese crusts and nodules are expected to be future resources, because they include diverse precious sub-components, including nickel, copper, cobalt, platinum, and rare earth elements.

The largest manganese deposit, Kalahari, formed in the ocean 2.4-2.2 billion years ago; thus a significant paleoceanographic record at that time of its formation. The display item is a manganese ore from the Hamayokokawa mine, Nagano, Japan. More than a thousand manganese mines once operated in Japan. The mine involved a bedded manganese deposit formed at the sea floor in the Paleozoic era. Many ore specimens from various manganese deposits have been collected in this museum. By studying the ore specimen, we can obtain the paleoceanographic information at the time when the deposit formed.　(Kaoru Kiyota)