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We have analyzed carbon and oxygen isotopes of two groups of planktonic foraminiferal shells from sediments near the Pliocene/Pleistocene (P/P) and the Miocene/Pliocene (M/P) boundaries, in the Lower Gutingkeng Formation in southwestern Taiwan. The P/P shells display heavier isotopic compositions ( δ 18 O =−1.5‰, δ 13 C =1.5‰ ) over a narrow range, suggesting a pristine environmental signature. On the other hand, the M/P shells are much lighter ( δ 18 O =−3 to −7‰, δ 13 C =1 to −1.5‰ ), and more scattered, associated with secondary carbonate diagenesis. Our detailed SEM examination confirms this proposition. The P/P shells display a clean micro-texture and are free from carbonate recrystallization. However, there are abundant secondary calcite crystals in the M/P shells. Therefore, the extreme shift of δ18O and δ13C in foraminiferal shells provides a guideline for evaluation of diagenetic artifacts of deeply buried sediments that have been uplifted on land. |
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