Sea-level and Climate Changes Recorded in Carbonate Diagenesis

浅海碳酸盐沉积由于其化学性质，在近地表环境中经历了各种类型的成岩作用。因此，环境变化，特别是海平面和气候变化，不仅影响着生物碳酸盐的沉积，而且控制着它们的成岩作用。因此，浅水碳酸盐岩记录的成岩特征是环境变化的良好标志。本研究的目的是在沉积岩相学观察和碳氧稳定同位素组成的基础上，阐明早期碳酸盐成岩作用的时间尺度和机制，识别琉球群岛全新世至更新世碳酸盐沉积中记录的海平面和气候变化。通过对琉球群岛晚第四纪碳酸盐沉积的研究，矿物成岩作用和碳、氧同位素组成在沉积后约4000年内发生变化，早期成岩作用使同位素和矿物组成从原生值发生了约1万年的严重变化。特别指出，与相对海平面变化有关的大气成岩作用与陆上暴露有关，使同位素组成发生了显著变化。对米奈米-岱托-硫马岛上更新世代托组的成岩作用研究表明，该组原生沉积物在相对海平面变化的作用下经历了两次白云石化作用，白云石化流体为海相或近海水。此外，对典型的陆相碳酸盐沉积石笋的初步研究表明，石笋的δ^&lt；13&gt；C和δ^&lt；18&gt；O可能记录了陆地上的气候变化，如水温等，这对精确重建过去几万年的古气候是有用的。

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.