Sea-level and Climate Changes Recorded in Carbonate Diagenesis

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Shallow-marine carbonate sediments are subjected to various types of diagenesis in near-surface environments because of their chemistry. Therefore, environmental changes, especially sealevel and climate changes, do not only influence the sedimentation of biogenic carbonates, but also control their diagenesis. Consequently, diagenetic features recorded in shallow-water carbonates are good indicators of environmental changes.The objectives of this research are to clarify the time scale and mechanism of early carbonate diagenesis and to recognize sealevel and climate changes recorded in the Holocene to Pleistocene carbonate sediments of the Ryukyu Islands on the basis of sedimentary petrographical observations and carbon and oxygen stable isotopic compositions.As a result of examinations of the Late Quaternary carbonate sediments of the Ryukyu Islands, diagenetic alteration on mineral and carbon and oxygen isotopic compositions are within about 4,000 years after the deposition, and serious changes of isotopic and mineral compositions from primary values occur within about a few ten thousand years by early diagenesis. Especially, it is pointed out that isotopic compositions are drastically changed by meteoric diagenesis related to subaerial exposure associated with relative sealevel changes. The study on diagenesis of the Plio-Pleistocene Daito Formation, Minaimi-Daito-jima Island, made clear that primary sediments of the formation were dolmiteized twice associated with relative sealevel changes, and that the dolomitizaing fluids were marine or near-marine water. Further, a preliminary research on a stalagmite, which is a typical terrestrial carbonate sediment, suggests a possibility that δ^<13>C and δ^<18>O of a stalagmite record climate changes on land, such as water temperature and so on, and that it is useful for precise reconstruction of paleoclimate within past a few ten thousand.