The carbon and oxygen isotope compositions of carbonate sediments can be used to reconstruct sea-level changes and the diagenetic processes that have affected the sediments, primarily as isotope depth profiles through shallow-marine carbonate successions can identify stratigraphic horizons that underwent subaerial exposure-related meteoric diagenesis, even within intervals where sedimentary features diagnostic of subaerial exposure have been eroded away.
The Upper Pleistocene Ryukyu Group of Kikai Island, Kagoshima Prefecture, Japan, consists of reefal sediments that were deposited during high-amplitude and short-termed glacioeustatic sea-level changes prior to the last interglacial. These sediments are thought to have undergone repeated subaerial exposure, although no sedimentary evidence for this exposure has been observed. The carbon and oxygen isotope compositions of two borehole cores (STb-1 and STb-2) drilled in the northeastern part of Kikai Island were determined to identify subaerial exposure surfaces (SESs) within the Ryukyu Group, and to determine precisely the nature of Late Pleistocene sea-level changes in this area.
Correlations with adjacent outcrops indicate that the sediments within the STb-1 and STb-2 cores were deposited at approximately 62-52 ka. Carbon and oxygen isotope composition, lithology, mineral composition, and diagenetic textural data enabled the identification of three SESs in STb-1 and two in STb-2. The stratigraphic position of SESs within the sediments enabled the division of the STb-1 and STb-2 cores into four depositional sequences that were deposited during highstands corresponding to IS (Interstadial)-14-IS-17 stages of the ice core oxygen isotope curve. This study shows that carbon and oxygen isotope data can enable the identification of SESs and the reconstruction of high-resolution sea-level changes over intervals of a few thousand years. This method is useful for the construction of detailed sea-level change curves and the characterization of depositional sequences.