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 delta C-13 ( - 1.9 to - 5.4%) and delta O-18 (- 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 delta C-13 values (-0.4 to 1.97%) and delta O-18 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 delta O-18 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. (C) 2001 Elsevier Science B.V. All rights reserved.