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Pleistocene dolomite and associated carbonates in south Okinawa, the Ryukyu Islands

Kenji Konishi, Kiyoshi Kaneshima, Koichi Nakagawa, Hitoshi Sakai
Geochemical Journal, Vol. 6, No. 1, P. 17-36, 1972

ABSTRACT

Pleistocene calciferous dolomite occurs in association with, besides pyrite and carbonaceous matter, either (1) magnesian calcite, calcite and aragonite, or (2) calcite with or without aragonite, in fossiliferous wackestone of the Naha Limestone in south Okinawa. This difference in mineralogical assemblages depends on the degree of leaching through subaerial weathering, which was witnessed by the systematic difference in Cl-, SO42- and organic carbon and pyrite contents of the carbonate rocks and by a negative shift of δ13C in the associated calcite. The dolomite has an isotopic composition δ13C =-15∼-36‰ and δ18O = +3∼+4‰ relative to the PDB standard. The conspicuous depletion of 13C, accompanied by enrichment of 18O in the dolomite is ascribed to dolomite formation from replacement of aragonite and magnesian calcite in contact with a hypersaline brine under a subtropical-restricted circulation where degradation of organic matter formed carbon dioxide enriched in 12C. A shallow subsurface environment, possibly together with the development of a back-reef lagoon, may be a plausible site of the dolomitization. This paper is the first to report the 13C-depleted dolomite from definitely shallow-water reefy sediments.

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