Original oxygen isotopic composition of planktic foraminifers preserved in diagenetically altered Pleistocene shallow-marine carbonates
✍ Scribed by S. Sakai; A. Kano
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 553 KB
- Volume
- 172
- Category
- Article
- ISSN
- 0025-3227
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✦ Synopsis
The original stable isotopic composition of low-Mg calcitic planktic foraminifer tests is preserved in Pleistocene shallowmarine carbonates (in the Ryukyu Group; Okinawa, Japan) that have been altered by meteoric diagenesis. Whole-rock analyses indicate depleted isotopic values for both d 13 C (Ϫ1.9 to Ϫ5.4‰) and d 18 O (Ϫ2.9 to Ϫ5.2‰), 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 d 13 C values (Ϫ0.4 to 1.9‰) and d 18 O values (Ϫ3.2 to Ϫ1.0‰). 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 d 18 O 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.