Perspectives in stable isotopic analyses and paleoceanography of Quaternary shallow-water carbonates

Abstract

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, recent 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 post-depositional 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. Subsequently, the downcore δ¹⁸O change of planktic foraminifers in Pleistocene shallow-water carbonates (the Ryukyu Group) can be used for the oxygen isotope stratigraphy. By com- parison, whole-rock measurements provide information that records the diagenetic signals, such as negative δ¹³C spikes indicating horizons of subaerial exposure.