Manganese Carbonate Bands as an Indicator of Hemipelagic Sedimentary Environments

Two contrasting patterns of Mn distribution emerged from geochemical examination of marine cores in several regions. Uniformly abundant Mn is contained in sediments from pelagic open oceans, while in hemipelagic regions, Mn is often concentrated in several specific horizons and is associated with$CO_{2}$. Mn carbonate layers, designated as "manganese bands" in this paper, occur typically in hemipelagic sediments; carbon isotopic data show that the possible source of the carbonate is organic material supplied to hemipelagic regions from nearby lands. Ancient analogs of the present-day manganese bands were found to occur in shale and chert within the Paleozoic-Mesozoic Mino terrane of central Japan. Manganese bands, consisting largely of rhodochrosite, are characterized by higher contents of P, Ba, and some heavy metals and a lower ratio for Zn/Co, compared with the ambient rocks. This suggests that manganese bands formed under suboxic conditions where organic materials are oxidized to supply$CO_{2}$necessary for the carbonate formation. Some manganese bands, particularly those hosted by chert, are characterized by extremely light carbon isotopes; this may result from methane-derived$CO_{2}$. Furthermore, manganiferous schists, such as piemontite-quartz schist in the Sambagawa terrane, can be correlated to the chert with manganese bands in the Mino terrane. On the basis of the several lines of evidence obtained here, forearc and back arc basins like the Sea of Japan and rifted continental margins with narrow basins such as the Gulf of California are possible tectonic settings that are modern analogs to the Paleozoic-Mesozoic complex in the Japanese Islands.