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^-, SO₄^2- 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.