Original oxygen isotopic composition of planktic foraminifers preserved in diagenetically altered Pleistocene shallow-marine carbonates

The original stable isotopic composition of low-Mg calcitic planktic foraminifer tests is preserved in Pleistocene shallow-marine carbonates (in the Ryukyu Group; Okinawa, Japan) that have been altered by meteoric diagenesis. Whole-rock analyses indicate depleted isotopic values for both δ¹³C (-1.9 to -5.4‰) and δ¹⁸O (-2.9 to -5.2‰), as well as carbonate mineralogy exclusively composed of low-Mg calcite. However, analysis of carefully-extracted planktic foraminifer tests (Globigerinoides sacculifer) that were separated from these whole-rock samples yield heavier δ¹³C values (-0.4 to 1.9‰) and δ¹⁸O values (-3.2 to -1.0‰). The foraminiferal values themselves and comparison of values of various components suggest that the low-Mg calcite tests preserve the original stable isotopic values. Subsequently, the downcore δ¹⁸O change of planktic foraminifers recorded in the Ryukyu Group results from middle Pleistocene glacial - interglacial change. By comparison, isotopic measurements based on whole-rock samples can be obtained diagenetic environmental signals, but misleading with regard to paleoclimatic inferences.