Identification of diagenetic artifacts in foraminiferal shells using carbon and oxygen isotopes

Abstract 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 δ 18 O and δ 13 C in foraminiferal shells provides a guideline for evaluation of diagenetic artifacts of deeply buried sediments that have been uplifted on land.