The oxygen and carbon isotopic composition of shallow-water carbonates and their fossil components can be important tools for understanding Quaternary paleoceanographic conditions of marginal seas, including coral reef regions. However, most previous paleoceanographic studies using isotopes have been based on pelagic or hemipelagic sediments because the original δ18O and δ13C values of shallow-water carbonates are normally altered by postdepositional diagenesis. This study shows that marginal to open ocean isotopic signals can be preserved in carefully cleaned low-Mg calcitic planktic foraminifers of shallow-water carbonates, even when the isotopic values of the carbonate host rock have been altered by meteoric fluids and subaerial exposure. The two studied core sections were collected from the mid-Quaternary Ryukyu Group (southwestern Japan), which consists mainly of bioclastic grainstone-packstone, rhodolith floatstone-rudstone, and large foraminiferal floatstone corresponding to open shelf to shelf slope facies, from the southwest Ryukyu Islands, Japan. Down core δ18O records of planktic foraminifers in the Ryukyu Group imply that sea surface temperatures (SST) in the northwestern Pacific increased ~2.0¿C at the beginning of oxygen isotope stage 19 (post mid-Pleistocene revolution (MPR)). This warming may have triggered the expansion of reef complex deposits as large as those now in the Ryukyu Islands and was approximately coeval with the lower section of the Australian Great Barrier Reef. SST warming due to the expansion of the Western Pacific Warm Pool (WPWP) toward the north and south subtropical regions is a possible mechanism for reef expansion and initiation of the coral reefs of the Western Pacific region.